The class of carbonate rocks includes limestones, dolomites, marls and sidirite rocks. Between the first two types there is a relatively small number of transitional rocks.
The classification of rocks transitional between pure limestone and dolomite is based on the content of calcite and dolomite in them. The group of limestones or dolomites includes rocks composed of more than 50% of one of these minerals.
Among rocks transitional between pure limestones and dolomites, dolomitic and dolomitic limestones, calcareous and calcareous dolomites are distinguished.
In carbonate rocks, a significant admixture of sand and clay particles is usually observed. Pure limestones and dolomites contain an admixture of other minerals in an amount of not more than 5%.
Some dolomites contain a significant admixture of gypsum and anhydrite. Such rocks are commonly referred to as sulphate-dolomitic. There are also transitions between carbonate and siliceous rocks.
Rocks intermediate between clays and pure carbonate rocks are called marls.
The classification scheme for carbonate-argillaceous rocks according to S.G. Vishnyakov is illustrated in the figure.
Clays: 1- non-carbonate, 2- calcareous-dolomite (or dolomite-calcareous).
Clay marls: 3 - clay marl, 4 - dolomitic clay marl, 5 - calcareous-dolomitic clay marl, 6 - dolomitic clay marl.
Marls: 7 - typical, 8 - dolomite, 9 - calcareous-dolomite, 10 - dolomitic.
Limestones: 11 - clayey, 12 - dolomitic-clayey, 13 - dolomitic-clayey, 14 - pure, 15 - dolomitic, 16 - dolomitic.
Dolomites: 17 - calcareous-clayey, 18 - calcareous-clayey, 19 - clayey, 20 - calcareous, 21 - calcareous, 22 - clean.
Mineralogical and chemical composition
The main minerals that make up carbonate rocks are: calcite, which crystallizes in a trigonal syngony, aragonite, a rhombic variety of CaCO3, and dolomite, which is a double carbon dioxide salt of calcium and magnesium (CaCO 3 * MgCO 3). Recent sediments also contain powdered and colloidal varieties of calcite (druite or nadsonite, bugleite, etc.).
The determination of the mineral and chemical composition of carbonate rocks is carried out in thin sections, as well as using thermal and chemical analyzes and according to the Shcherbina method.
AT field conditions determined by reaction with dilute HCl. Dolomites boil only in powder.
The theoretical chemical composition of calcite and limestone ~ CaO - 56%, CO 2 - 44%, in dolomites - 22-30% CaO and 14-21% MgO.
Naturally, if clastic material is present in the rocks, then the content of SiO 2 will sharply increase (sometimes up to 26%).
Main rock types
Limestones - the color of limestones is diverse and is determined, first of all, by the nature of impurities. Pure limestones are colored white, yellowish, gray, dark gray, and sometimes black.
An important feature of limestones is their fracture, the nature of which is determined by the structure of the rock. Very fine-grained calcareous rocks with a weak cohesion of grains (for example, chalk) have an earthy fracture. Coarse-crystalline - have a sparkling fracture, m / z rocks - a sugar-like fracture, etc.
For limestones, the following main types of structures can be distinguished:
Crystalline granular structure, among which several varieties are distinguished depending on the diameter of the grains: coarse-grained (grain size in diameter 0.5 mm), medium-grained (from 0.5 to 0.1 mm), fine-grained (from 0.10 to 0.05 mm), fine-grained (from 0.05 to 0.01 mm) and micro-grained (less than 0.01 mm) structures.
Organogenic structure, in which the three most significant varieties are distinguished:
a). actually organogenic, when the rock consists of calcareous organic residues (without signs of their transfer), interspersed in t/z carbonate material;
b). organogenic-detrital, when crushed and often rounded organic remains are present in the rock, located among the t / s of carbonate material;
in). detritus, when the rock is composed only of crushed organic remains without a noticeable amount of heavy carbonate particles.
The clastic structure is observed in limestones formed by the accumulation of fragments arising from the destruction of older carbonate rocks. Here, as well as in some organic limestones, in addition to fragments, a calcareous cementing mass is clearly visible.
The oolitic structure is characterized by the presence of concentrically folded oolites, usually clastic grains are often present.
Sometimes oolites acquire a radially radiant structure.
Inlay and crustification structures are also observed. In the first case, the presence of crusts of a concentric structure is characteristic, filling the former large voids. In the second case, growths of elongated carbonate crystals are observed, located radially relative to the fragments or organic remains that make up the rock.
In the process of transition from sediment to rock and petrification, many limestones undergo significant changes. These changes are manifested, in particular, in recrystallization, petrification, dolomitization, ferruginization and partial dissolution with the formation of stillolites.
Varieties of limestone
Organic limestones
This is one of the most widely used varieties. They are composed of shells of benthic crinoids, algae, corals and other benthic organisms. Much less often, limestones form due to the accumulation of shells of planktonic forms.
Typical representatives of organogenic limestones are reef (biohermic), limestones, consisting largely of the remains of reef-forming organisms and living in a community of other forms.
Writing chalk.
It is one of the very peculiar representatives of calcareous rocks, which stand out sharply in their appearance. It is characterized by white color, uniform structure, low hardness and fine grain. Complicated - mainly calcium carbonate (no dolomite) with a slight admixture of clay and sand particles.
Organic residues make up most of the chalk. Among them, the remains of coccolithophorids are especially common - unicellular calcareous algae that make up 10-75% of chalk and chalk-like marls in the form of small (0.002-0.005 mm) plates, disks and tubes. Foraminifera are found in chalk, usually in an amount of 5-6% (sometimes up to 40%). There are also shells of mollusks (mainly inocerams, less often oysters and pectinids) and a few belemnites, and in places also ammonite shells. Remains of bryozoans, sea lilies, urchins, corals and tube worms, although observed, do not serve as rock-forming elements of the chalk.
Limestones of chemical origin.
This type of limestone is conditionally separated from other types, because. in most limestones there is always some amount of calcite precipitated from the water by a purely chemical means. You can easily and quickly buy a suitcase in Moscow on the website caseplus.ru. Also here you will find many different bags and backpacks, various leather goods and just the necessary accessories.
Typical limestones of chemical origin are microgranular, devoid of organic residues and occur in the form of layers, and sometimes accumulations of concretions. Often they contain a system of small calcite veins, which form, with a decrease in the volume, initially colloidal sediments. Often there are geodes with large and well-formed calcite crystals.
Clastic limestones.
This type of limestone contains a significant admixture of quartz grains and is usually associated with sandy rocks. Clastic limestones are characterized by oblique bedding.
Clastic limestones are composed of carbonate grains different size, the diameter of which is measured in tenths of a millimeter, less often several millimeters. There are also conglomerate-like limestones, consisting of large fragments. Clastic carbonate grains are usually well rounded and similar in size.
Secondary limestones.
This group includes limestones occurring in the upper part of salt domes, and limestones arising in the process of transformation of dolomites during their weathering (razdolomitization or dedolomiticization).
Broken rocks are medium- or coarse-grained limestones, dense, but sometimes porous or cavernous. They lie in the form of solid masses. In some cases, they contain lenticular inclusions of fine-grained and fine-grained dolomites, sometimes loose and soiling fingers. More rarely, they form inclusions and branching veins in the thickness of dolomites.
Dolomites
They are carbonate rocks, consisting mainly of the mineral - dolomite. Pure dolomite corresponds to the formula CaMg(CO 3) 2 and contains 30.4% CaO, 21.8% MgO and 47.8% CO 2 or 54.3% CaCO 3 and 45.7% MgCO 3 . The weight ratio of CaO:Mg is 1.39.
Dolomites usually contain less clastic impurities than limestones. Also characteristic is the presence of minerals that precipitated purely chemically during the formation of the sediment or that arose during its diagenesis (calcite, gypsum, anhydrite, celestite, rhodochrosite, magnesite, iron oxides, less often silica in the form of opal and chalcedony, organic matter, etc.). In some cases, the presence of pseudomorphs along the crystals of various salts is observed.
In appearance, many dolomites are very similar to limestones, with which they are similar in color and inability to distinguish calcite from dolomite in a finely crystalline state with the naked eye.
Among the dolomites, there are completely homogeneous varieties from micro-grained (porcelain-like), sometimes soiling the hands and having a conchoidal fracture, to fine- and coarse-grained varieties, composed of dolomite rhomboids of approximately the same size (usually 0.25-0.05 mm). The leached varieties of these rocks are somewhat reminiscent of sandstones in appearance.
Dolomites are sometimes characterized by vugginess, in particular due to leaching of shells, porosity (especially in natural outcrops) and fracturing. Some dolomites have the ability to spontaneous cracking. Well-preserved organic remains in dolomites are rare. Dolomites are mostly colored in light shades of yellowish, pinkish, reddish, greenish and other tones. Some dolomites are somewhat reminiscent of mother-of-pearl in their color and brilliance.
Dolomites are characterized by a crystalline granular (mosaic) structure, which is also common for limestones, and various kinds of relict structures caused by the replacement of calcareous organic residues, oolites or carbonate fragments during dolomitization. There is sometimes an oolitic, as well as an incrustation structure formed as a result of various cavities, usually in reef massifs.
For rocks transitional from limestones to dolomites, a porphyritic structure is typical, when individual large rhombohedrons of dolomite are present against the background of a finely crystalline calcite mass.
Varieties of dolomites
By origin, dolomites are divided into primary sedimentary, syngenetic, diagenetic and epigenetic. The first three types are often grouped under the name of primary dolomites, while epigenetic dolomites are also called secondary.
Primary sedimentary dolomites.
These dolomites arose in sea bays and lagoons with high salinity water due to the direct precipitation of dolomite from the water. These rocks lie in the form of well-seasoned layers, within which thin bedding is sometimes clearly expressed. Primary vugginess and porosity, as well as organic residues, are absent. Interlayering of such dolomites with gypsum is often observed. The contacts of the layers are even, slightly wavy, or gradual. Sometimes there are inclusions of gypsum or anhydrite.
The structure of primary sedimentary dolomites is uniformly microgranular. The predominant grain size is ~0.01 mm. Calcite occurs only as a minor admixture. Sometimes there is petrification, sometimes intense.
Syngenetic and diagenetic dolomites.
Among them is the predominant part of the dolomites. It is not always possible to distinguish between them. They arise due to the transformation of lime sludge.
These dolomites occur in the form of layers and lenticular deposits. They are strong rocks with uneven, rough fractures, usually with unclear layering. The structure of syngenetic dolomites is often uniformly microgranular. For diagenetic, unevenly granular is more typical (their grain diameters vary from 0.1 to 0.01 mm). Characteristic of diagenetic dolomites is also an irregularly rhombohedral or oval shape of dolomite grains, often having a concentric zonal structure. In the central part of the grains there are dark dust-like accumulations.
In some cases, gypsuming of the rock occurs. At the same time, the most permeable for solutions areas of carbonate rock (in particular, organic remains), as well as accumulations of pelitomorphic dolomite, were most easily replaced by gypsum.
Secondary (epigenetic) dolomites.
This type of dolomite is formed in the process of replacement with the help of solutions of already solid limestones, fully formed as rocks. Epigenetic dolomites usually occur in the form of lenses among unchanged limestones or contain areas of residual limestone.
Epigenetic dolomites are characterized by massiveness or indistinct layering, uneven-grained and heterogeneous structure. They are coarse and inhomogeneously porous. Near areas completely dolomitized, there are areas almost unaffected by this process. The boundary between such areas is sinuous, uneven, and sometimes passes in the middle of the shells.
Mergeli
Marl refers to rocks that are transitional between carbonate and clay, containing 25-95% CaCO 3 . Their most carbonate varieties (75-95% CaCO 3), in the case of significant compaction of the rock, are called clayey limestones.
Marls are divided into three main groups:
1. Actually marls, with a CaCO 3 content of 50-70%,
2. Lime marls, in which the content of CaCO 3 varies within 75-95%,
3. Clay marls with CaCO 3 content from 25 to 50%.
Typical marl is a rock of very fine texture, homogeneous in structure, consisting of a mixture of clay and carbonate particles and often exhibiting a certain plasticity when wet. Usually marls are painted in light colors, but there are also brightly colored varieties - red, brown, purple (especially in red-colored strata). Thin layering is not typical for marls, but many of them occur in the form of thin layers. Some marls form regular rhythmic interlayers with thin clayey and sandy layers.
As an impurity, marls contain organic residues, detrital grains of quartz and other minerals, sulfates, iron oxides, glauconite, etc.
Siderite rocks
The chemical formula of siderite is FeCO 3 , with iron containing 48.2%. The name of the mineral itself comes from the Greek "sideros" - iron.
Siderite rocks are an accumulation of granular or earthy aggregates, dense, sometimes representing spherical concretions (spherosiderite).
Their color is brownish-yellow, brown. Siderite easily decomposes in HCl, while the drop turns yellow due to the formation of FeCl 3 .
Origin.
1. Hydrothermal - occurs in polymetallic deposits as a vein mineral. 2. When replacing limestone, it forms metasomatic deposits. 3. Siderites can also be of sedimentary origin; as a rule, they have an oolitic structure. 4. There is siderite of metamorphic origin, formed during the metamorphism of sedimentary iron deposits. In the oxidation zone, it easily decomposes and passes into iron oxide hydrates, forming iron hats.
ORGANIC ROCKS (from the Greek organon - organ and -genes - giving birth, born, biogenic rocks * a. organogenic rocks, biogenic rocks; and. organogene Gesteine; f. roches organogenes, roches biogenes; I. rocas organogenicas) - sedimentary rocks consisting of the remains of animals and plants and their metabolic products. Organisms have the ability to concentrate certain substances that do not reach saturation in natural waters, forming skeletons or tissues that are preserved in a fossil state.
According to the material composition, carbonate, siliceous, and some phosphate rocks, as well as coals (see), oil shale, oil, and solid bitumen, can be distinguished among organogenic rocks. Organogenic carbonate rocks () consist of shells of foraminifers, corals, bryozoans, brachiopods, mollusks, algae and other organisms.
Their peculiar representatives are reef limestones that make up atolls, barrier reefs and others, as well as writing chalk. Siliceous organogenic rocks include: diatomite, spongolite, radiolarite, etc. Diatomites consist of opal skeletons of diatoms, as well as spicules of flint sponges and radiolarians. Spongolites are rocks containing usually more than 50% spicules of flint sponges. Their cement is siliceous, of opal rounded bodies, or clayey, slightly calcareous, often including secondary chalcedony. Radiolarites are siliceous rocks, more than 50% consisting of radiolarian skeletons, which form radiolarian silt in modern oceans. In addition to radiolarians, they include sponge spicules, rare diatom shells, coccolithophores, and opal and clay particles. Many jaspers have a base of radiolarians.
Phosphate organogenic rocks are not widely distributed. These include shell rocks from phosphate shells of Silurian brachiopods - obolid, accumulations of bones of fossil vertebrates (bone breccias), known in sediments different ages, as well as guano. Organogenic carbonaceous rocks - fossil coals and oil shale - are common, but their mass in the earth's crust is small compared to carbonate rocks. Oil and solid bitumen are peculiar rocks, the main material for the formation of which was phytoplankton.
According to the conditions of formation (mainly in relation to carbonate rocks), bioherms can be distinguished - the accumulation of the remains of organisms in their lifetime, thanato- and taphrocenoses - the joint burial of dead organisms that lived here or were carried by waves and currents; rocks that have arisen from planktonic organisms are called planktonic (for example, diatomite, chalk, foraminiferal limestone).
If organic remains are crushed as a result of the action of waves and surf, organogenic-detrital rocks are formed, consisting of fragments (detritus) of shells and skeletons held together by some mineral matter(for example, ).
Stones of organic origin - a selection of stones, photos, properties, originStones born of life
They say about the stone "cold", "dead", "lifeless". But life on Earth is not much younger than the planet itself, and many terrestrial minerals are formed by living organisms. Oil, according to modern concepts, is a visible trace of the existence of microscopic unicellular plants and animals of the distant past. Coal was considered by ancient naturalists to be the brother of oil. Chalk, limestone, marble are the life products of sea creatures...This is where the list of minerals of biogenic origin that comes to mind to the average person usually ends. However, a knowledgeable mineralogist could go on and on with the list of rocks that appeared on Earth solely due to the existence of life.
Even gemology, the science of precious stones, is ready to present an impressive list of gems, each of which was once alive. The champion of popularity among jewelry of biological nature is pearls!
Mother of pearl - half brother of pearls
It just didn't come out in shape. If a pearl is a spherical formation (or close to a sphere in shape), then it is only deposited on the walls of the shell.
The demand for mother-of-pearl has always exceeded the demand for pearls due to the low price and wide availability of the material. Pearls are rare, and there are tons of mother-of-pearl in any river. Mollusk shells, covered with a thick layer of mother-of-pearl, have been used to make buttons, combs, handles and other consumer goods for many centuries. Today there is no type of plastic that would be used as widely and actively as mother-of-pearl in the recent past.
Once palm trees grew everywhere
...because it was warm and humid. The petrified palm stem can be found in coal deposits, in shale, and in quartz deposits. It is silicates that make palm wood an aesthetically expressive stone.
It should be noted that in its botanical essence, the palm tree, although tree-like, herbaceous plant. You can't find annual rings on palm trees! On the other hand, the longitudinal vessels, through which the nutrient juices circulated throughout the plant, are very clearly visible. They - both on the transverse and on the longitudinal cut of petrified palm wood - make up the beauty of the stone.
The soft starchy core of the palm trunk is not rich in vessels, and therefore is replaced during fossilization by a homogeneous siliceous material.
Various silicas, impregnating the trunks of flooded, covered, drowned trees in swamps, often turn unremarkable wood into a precious gem. Silicates, colored with a variety of mineral impurities, acquire an iridescent color. A chip, saw cut, and even better, a thin section often amazes with the richness of the natural palette of colors.
In this case, the layered wood structure remains, as a rule, well distinguishable. That just adds to the decor. beautiful stone biological origin.
Stromatolite jaspers
Jasper Rock Mary Ellen is located in the state of Minnesota (USA). It is famous for the fact that the main masses of the rocks that make up the mountain - red jasper and silver hematite - are intertwined in unimaginable clubs and twists.
Red and black is an advantageous color combination for any artistic subject. However, stromatolites, formed from layered colonies of cyanobacteria two billion years ago, rarely turn red. Only on the American continent were found traces of the first steps of life on the planet, made by red jasper on black iron ore...
petrified corals
A polished petrified one makes you want to blow off dust particles from it - the jewelry work of nature is so fine. Cellular frameworks of marine organisms of the distant past are delicately arranged and skillfully "executed". The resemblance of fossil coral to the work of a skilled craftsman is endless!
Quartz and calcite, replacing organic tissue in fossilized corals, make jewelry durable. However, the bright colors characteristic of modern corals are not found in fossil polyps. Fiery red or transparent yellow earrings made of petrified corals are the product of handicraft "improvement".
"Sand Dollar"
"Sand dollar" in both Americas is called the skeleton of a sea urchin, classified as incorrect (such is zoological terminology). Correct hedgehogs- round echinoderms, irregular - flat. They have been living on Earth for a long time, and in some places they inhabit the shelf bottom so densely that they lie on the sand like scales on the body of a crucian carp - or even in two layers.
Wrong hedgehogs they have a very conditional needle protection, and therefore everyone who is not lazy feeds on them. Nevertheless, many of the animals that are flat as a toy saucer manage to grow a decent thickness of the skeleton, live to a natural death and please people with the sight of their skeleton - the "sand dollar". Especially highly valued are dollars "issued" millions of years ago...
Ammonites
Anyone who has been interested in the history of evolution knows about the ammonites. They - sometimes quite modest in size, sometimes under two meters in diameter - are twisted into a flat spiral, like the horns of the god Amun in one of his earthly incarnations. Ammonites are easy to find in natural screes. In some European countries, they have long been called "golden snails".
Ammonite "gold" is a layer of petrified mother-of-pearl in sealed shell chambers. The most beautiful ammonites are mined in the Canadian province of Alberta. The iridescent radiance of the polished walls of the shells surpasses the play of color in opal and labradorite.
dinosaur bone
The process of bone petrification is extremely lengthy, because each molecule of calcium phosphate (of which, in fact, bones are composed) must be replaced by a molecule of silicon dioxide. It takes at least two million years for a medium-sized dinosaur skeleton to turn into a precious gem!
Fortunately, something, but dinosaur bones have enough time with a large margin. For 65 million years separating us from the last animal lizards of the Earth, many tons of bones turned into colored quartz. Moreover, a large part of the quartz took on impurities, which allowed the hitherto unattractive natural material to acquire both the look, and the pattern, and the texture at a good jewelry level. Dinosaur bone cabochons are often extremely attractive!
Ivory is younger than dinosaur bones. Today, under the name of "ivory" distinguish the tusks of African and Indian elephants, fossil mammoths, walrus fangs, hippo and sperm whale teeth.
The main thing is its luxurious appearance. However, the manufacturability of the material is also important. Last but not least, artisans fell in love with ivory because of its ability to become plastic, and then harden again.
Ivory color varies. The white and blue tooth of a hippopotamus, warm shades (up to red-brown) of mammoth tusk, translucent whiteness of the tusk of a young elephant are valued.
The list of stones of biological origin can go on and on. The gallery of precious gems is replenished by the efforts of geologists, researchers, pioneers of remote areas of the planet.
Like the glow of dawn
The first pearls people found in search of food. Oysters producing this gem are still loved by gourmets. For thousands of years, people have been admiring the radiance of pearls that have grown by the will of nature - and for several decades now we have been forcing mollusks to envelop seed grains of sand in multi-colored layers.
Today's pearls are all colors of the rainbow and even the colors of the night! But, as in the old days, this is a stone in which at least half of the mass falls on organic tissue. We looked at pearls in more detail in the article, and you can be sure that this stone of biological origin has been in favor with fashionistas for a reason for the fifth millennium in a row!
Frozen sunshine...
... poetically called amber. Both honey-transparent and the most “foggy” forms of the stone really give the impression of clots of luminous substance. There are countless varieties of amber! The color range of this natural jewel ranges from milky white through all shades of yellow and red to blue and green. There are amber and black!
Every amber is a piece of fossilized resin of a tree that grew millions of years ago. There are ambers born in pine groves, and ambers that originated from the resin of tropical trees. We talked about amber in the articles: and. Now the time has come to pay attention to the trees that grew hundreds of millions of years ago, and by our time have turned into "precious stones".
"Peanut" wood
Wood with a clear structuring of the array during fossilization can also give an unexpected visual effect. Particularly interesting are the fossilized wood remains that have spent many years under water. The point, in fact, is not in the water, but in the mollusks that inhabit the reservoirs of the planet. Some of them feed on rotting wood, and in the process of obtaining food they go deep into the flooded logs, gnawing through numerous passages.
The subsequent mineralization of organics led to a striking result. The cavities gnawed (more precisely, machined) by the mealybug were filled with white quartz. The fabrics of the tree remained colored. Minerologists dubbed this kind of petrified wood "peanut forest" - for the similarity of the stone pattern with sprouting peanuts is almost one hundred percent.
Jet
However, not all plant remains of the distant past are so lucky. Jet, a mineral related to coal, is recognized as the same prehistoric wood that survived flooding in the silt layers two hundred million years ago.
Unattractive in its raw form, polished jet shines like silk velvet. The best varieties stones are distinguished by a mirror gloss and are used to make jewelry. In the recent past, a lot of haberdashery trifles were made from jet - like buttons, beads, beads. served its owners no worse than mother-of-pearl.
corals
Most of the bottom marine sediments are formed by the calcareous remains of organisms that lived in ancient times. However, corals, having won a warm place five hundred million years ago, thrive to this day.
Calcareous skeletons of corals have three and a half hundred variants of natural coloration. Polished coral is an excellent material for making jewelry. However, the user must remember: the thicker the color of the coral, the more organic matter it contains, and the more careful the subject should be treated.
Modern views corals are different from the polyps that inhabited the earth's seas in past geological epochs. However, we can say with confidence: petrified corals are extremely beautiful and interesting!
Compressed carcasses of sea lilies
Crinoid sea lilies once so abundantly inhabited the shallow bottom of the warm seas that their calcareous cores - mostly tubular, divided into short segments - became a rock-forming element. Many of the most interesting specimens of these Proterozoic pufferfish were obtained during the construction of the Moscow metro.
However, crinoidal limestone, formed by the remains of flower-like animals three hundred million years ago, is not found under (literally) Moscow. Although this mineral is widely distributed.
Distinguishable remains of crinoids, “soldered” into the thickness of a translucent mineral, are sometimes very decorative. Such stones become a worthy decoration.
Under the sonorous name lies a beautiful mineral with an unusual history. In fact, turritella terebra is the name of a marine mollusk with a helical shell. They say that it was turitella shells that prompted the legendary Archimedes to construct a water-lifting propeller.
Turitella agate is, in fact, a scattering of shells of a mollusk of this species, which are in varying degrees of preservation, filled with hardened silicate. Many of the real turitell agates include sand, water, air bubbles.
Take a closer look at the appearance of the gem! Under the name of agate-turitella, any petrified garbage is often sold. If you do not see distinctly preserved elements of cone-spiral shells, this is a fake!
Origin and classification of rocks
Any natural stone is "a rock, a natural formation consisting of individual minerals and their associations." The study of the composition, origin and physical properties rocks are engaged in petrography. According to her, all breeds by origin last for three main groups:
1. Igneous ("primary" rocks)
- formed directly from magma - a molten mass of predominantly silicate composition, as a result of its cooling and solidification. Depending on the conditions of solidification, deep and outflowing are distinguished.
deep
arose as a result of the gradual cooling of magma at high pressure inside the earth's crust. Under these conditions, the constituents of the magma crystallized, due to which massive dense rocks with a fully crystalline structure were formed: granite, syenite, labradorite and gabbro.
poured out
formed as a result of a volcanic eruption of magma, which quickly cooled on the surface at low temperature and pressure. There was not enough time for the formation of crystals, so the rocks of this group have a latent or finely crystalline structure with an abundance of amorphous glass with high porosity: porphyries, basalts, travertine, volcanic tuffs, ashes and pumice.
Granite (from the Latin granum, grain) is the most common rock. Granite has a pronounced granular-crystalline structure and consists mainly of feldspars, quartz, mica and other minerals.
According to the size of the grains, 3 structures of granite are distinguished: fine-grained, medium-grained, coarse-grained. The color of granite can be very different. The most common gray granite, ranging from light to dark with different shades, there are also pink, orange, red, bluish-gray and sometimes bluish-green granite. Exceptionally rare granite with blue quartz. In decorative terms, the most valuable are fine-grained light gray with blue tint, rich dark red and greenish-blue varieties of granites.
2. Sedimentary (or "secondary" rocks)
They are called secondary, as they were formed as a result of the destruction of igneous rocks or from the waste products of plants and animal organisms.
They can be in the form of chemical precipitation, which are formed during the drying of lakes and bays, when various compounds precipitate. Over time, they turn into limestone tuffs, dolomite. General feature of these rocks - porosity, fracturing, solubility in water.
There are also clastic sedimentary rocks. These include cemented sandstones, breccias, conglomerates and loose: sands, clays, gravel and crushed stone. Cemented deposits were formed from loose deposits as a result of natural bonding, cementing. For example, sandstone is made from quartz sand with lime cement, breccia is made from cemented crushed stone, and conglomerate is made from pebbles.
Rocks of organic origin are also known, these are limestones and chalk. They are formed as a result of the vital activity of animals and plants.
Sandstone
For geologists and petrographers - clastic rock, consisting of cemented sand. There are gray, green, red, yellow, brown and brown. Siliceous sandstones are considered the most durable.
Basically, sandstones are not able to acquire a polished texture, so they usually use a chipped or sawn texture, and sometimes polished. Sandstones lend themselves well to hexing and diamond processing.
Fine-grained red, chocolate brown and green varieties of sandstone are considered decorative, which are successfully used for outer cladding. In Moscow and St. Petersburg architectural monuments built in the 19th and early 20th centuries, cladding made of Polish sandstone of gray-green, yellow and shades of pink. Assumption Square of the Kremlin is lined with Lyubertsy sandstone.
Sandstone is a rather porous material, so it is undesirable to use it for finishing elements in contact with water. It is also not recommended to use it on basement structures.
3. Metamorphic (modified rocks)
- formed by the transformation of igneous and sedimentary rocks into a new type of stone under the influence of high temperature, pressure and chemical processes.
Among the metamorphic rocks, massive (granular) are distinguished, these include marble and quartzite, as well as schistous ones - gneisses and shales.
Marble
The name "marble" comes from the Greek marmaros, shiny. This is a granular-crystalline rock, which was formed in the bowels of the Earth as a result of recrystallization of limestone and dolomite under the influence of high temperatures and pressure. In construction, not only this stone, but also other dense transitional carbonate rocks are often called marble. These are, first of all, marble-like or marbled limestones and dolomites.
Quartzite
These are fine-grained rocks that were formed during the recrystallization of siliceous sandstones and consist mainly of quartz. Quartzite comes in gray, pink, yellow, crimson red, dark cherry, and sometimes white.
Quartzite is considered a highly decorative stone, especially crimson red and dark cherry. The texture of the "rock" significantly brightens the general background of this stone, which is often used by combining such products with polished contrasting colors.
Quartzite has a very high hardness and is a difficult material to cut, but it accepts a very high quality polish.
Often used in the construction of unique structures. It was used in the construction of the Church of the Savior on Blood. It has also been used as a ritual stone for centuries. The sarcophagi of Napoleon and Alexander II are made from it, top part mausoleum of Lenin.
Slate
A dense and hard rock that was formed from highly compacted clay, partially recrystallized under high and one-sided pressure (from top to bottom, for example). It is characterized by an oriented arrangement of rock-forming minerals and the ability to split into thin plates. The color of shales is most often dark gray, black, gray-brown, red-brown.
Slate - durable material, it can be processed (stratified into thin plates), some types also accept polishing. However, more often it is used without processing at all, since the split surface is very decorative.
Slate is used in both exterior and interior cladding. This stone was widely used in well-known architectural monuments (the floors of St. Isaac's Cathedral in St. Petersburg are partly made of slate).
4. Semi-precious stones.
These can include, mainly rocks, called "decorative and ornamental stones." These are jasper, onyx, opal, malachite, lapis lazuli. They are much rarer than ordinary stone and are valued more. However, it is expensive to veneer large areas with them, therefore, most often small elements are trimmed with these stones: details of columns, window sills, bathrooms ...
One of the most common decorative and ornamental stones is onyx (“nail” in Greek). Onyx has a layered or radical-radiant structure. Onyx color - white, light yellow, yellow, brown, dark brown, pale green. Striped pattern - alternation of stripes different shades. Most marble onyxes are translucent, sometimes to a depth of 30…40 mm. Onyx is well processed by cutting and grinding tools and adopts high quality polishing.
Throughout its existence, the Earth has gone through a long series of continuous changes. They are caused by processes different in speed, scale and energy sources. These processes of the movement of matter, modifying the earth's crust and surface of the Earth, are called geological or geodynamic.
endogenous processes called such geological processes, the origin of which is associated with the deep bowels of the Earth. In the bowels of the Earth under its outer shells, complex physical-mechanical and physico-chemical transformations of matter occur, as a result of which powerful forces arise that act on the earth's crust, due to which they transform it. Endogenous processes fundamentally change the nature of the earth's crust and, in particular, its surface; they lead to the creation of the main forms of the Earth's surface relief - mountainous countries and individual hills, huge depressions - receptacles of ocean and sea water, etc. The main internal sources of the Earth's energy are: gravitational differentiation, rotational (rotational) forces, radioactive decay, chemical and phase transformations occurring in the depths. The processes caused by these energy sources are called endogenous or processes of internal dynamics. These include:
1. tectonic movements (oscillatory and mountain building);
2. magmatism;
3. metamorphism;
4. earthquakes;
The second group of processes is caused by external energy sources and manifests itself on the surface of the Earth and they are called exogenous. These are solar energy and gravity, the movement of water and air masses, the influence of various plant and animal organisms, their impact on rocks and minerals. Such processes are called exogenous or processes of external dynamics. These include:
1. weathering;
2. influence of flowing surface and ground waters;
3. influence of glaciers and water-glacial flows;
4. processes in the frozen zone of the lithosphere;
5. influence of the seas and oceans, lakes and swamps;
6. gravitational processes;
7. human activity (technogenesis).
Endogenous and exogenous processes operate simultaneously and are closely related to each other (Fig. 2.5)
Rocks - a natural collection of minerals of a more or less constant mineralogical composition, forming an independent body in the earth's crust
Rocks are formed during various processes occurring both in the bowels of the Earth and on its surface, forming alloys, mechanical mixtures, consisting of one (marble) or several minerals (granite) (Fig. 2.5).
Rice. 2.5. Origin of rocks.
Rocks are classified by origin (by genesis) and chemical composition. Distinguished by origin igneous, sedimentary and metamorphic rocks (Fig. 2.6).
Figure 2.6. Classification of rocks by type of formation
Igneous and metamorphic rocks make up about 90% of the volume of the earth's crust, however, on the surface of the continents, their areas of distribution are relatively small. The remaining 10% are sedimentary rocks, which occupy 75% of the earth's surface area.
Igneous rocks subdivided into intrusive- deep and effusive- poured out.
intrusive rocks formed in the interior of the Earth under conditions high pressures and very slow cooling. Magma at a depth of several tens of kilometers from the Earth's surface is under a very large all-round hydrostatic pressure, reaching several thousand atmospheres, and has a high temperature. When magma intrudes into the overlying layers of the Earth, the physical environment changes: magma meets with solid and relatively cold rocks and begins to solidify and crystallize. However, the release of heat from magma to the environment is very slow, since the thermal conductivity of rocks is low. The temperature of the magma drops gradually over millions of years. The following observation can serve as an example: in the North Caucasus, in the area of Pyatigorsk, magma intrusion occurred at the end of the Paleogene period (~30 million years ago). However, even at present, heated masses of magma exist at a relatively shallow depth, as indicated by hot springs emerging on the surface of the earth.
With the slow cooling of magma, a gradual and successive separate crystallization of the chemical compounds included in its composition occurs, each of which turns into a crystal of a mineral. Due to the slow growth, crystals can reach relatively large sizes, therefore, many intrusive rocks are characterized by a coarse crystalline structure. As a result of the slow cooling of the magma, complete crystallization of all its matter occurs, and no amorphous areas remain in the resulting rock.
The minerals formed during crystallization fall out of the melt in a certain time sequence. This sequence determines the degree of refractoriness of minerals, as well as the chemical composition of magma. An important role in the process of crystallization is played by volatile vaporous and gaseous substances, which contribute to and often determine the order and rate of crystallization of minerals.
Let us explain this by the example of a magma of granitic composition, as a result of which crystallization at a depth a rock is formed - granite. The composition of granite includes such rock-forming minerals as feldspars, quartz, from dark-colored silicates - and less often hornblende (Table 2.4). The melting temperature of biotite and hornblende is very high (at 600 MPa, 620–270 o C), so their crystals form even in liquid magma.
In the second phase of crystallization, feldspar crystals appear, the melting point of which is lower than that of dark silicates (at 10 5 Pa 1120 - 1250 o C). In contrast to the conditions of the first phase, during the crystallization of feldspars, solid crystals of dark-colored silicates already exist in the liquid mass of magma. As a result, feldspar crystals can "overgrow" biotite or hornblende crystals and include them.
After crystallization of dark and light silicates, the rock will be formed by 75-80% of the volume. Silica, which is contained in excess in granitic magma, will begin to pass into a solid crystalline state last, turning into quartz. Its crystals occupy the free space between the previously formed crystals of biotite, hornblende and feldspar and take the form of grains of irregular shape, although the internal structure of their crystal lattice is quite correct. As a result, complete crystallization of magma will occur, all its substance will take on a crystalline structure. The resulting rock structure is called full-crystalline. The full-crystalline structure provides information about deep, or abyssal, the conditions for solidification of magma.
At great depths under conditions of all-round pressure, the orientation of the axes and planes of growing crystals is not controlled by anything, and their location in the rock is random. A similar texture of the rock is called massive, non-oriented; it is typical mainly for deep rocks.
During magmatic intrusion, a viscous mass of magma may flow, although within limited limits. In this case, crystals with elongated shapes, such as columns of hornblende and mica leaves, are oriented with their long axes parallel to the direction of flows in the magma. The so-called fluid texture. While occurring in intrusive rocks, it is, however, more typical of effusive rocks.
Effusive rocks formed when molten magma erupts onto the earth's surface. When effusion is almost instantaneous, the temperature changes environment and pressure decreasing from several thousand atm. up to 1 atm. As a result of this, a rapid release of gases dissolved in magma begins at first, accompanied by explosions. The lava coming out of the vent of the volcano splashes, throwing up spray. The gases released from the lava can froth it, forming numerous bubbles that persist even when the substance solidifies. This creates a bubble texture. A breed of similar build is called pumice. Its density is so low that pumice floats in water.
The sharply decreasing temperature creates conditions under which many minerals crystallize simultaneously. However, the very rapid solidification of the substance leads to the formation of small rudimentary forms of crystals, which can only be detected under a microscope. A significant part of the rock turns into an amorphous or glassy mass. This rock structure is called cryptocrystalline. With a very rapid cooling of the lava, the crystallization process may not begin at all, in which case the rock will entirely consist of volcanic glass. This breed is named obsidian. This is a black, dark gray or dark brown rock with a conchoidal fracture, similar to a block of glass. The cavities of gas bubbles are often filled with minerals, which are formed a second time - as a result of their crystallization from hot water solutions that have penetrated into the solidified lava. At the same time, against the background of a dark gray rock with a cryptocrystalline structure, rounded light spots of such inclusions stand out. Usually they are represented by minerals such as calcite and amorphous silica - opal and chalcedony.
The process of volcanic eruptions is also associated with the formation of a group of rocks, which are commonly called pyroplastic. The gases released from the magma often accumulate inside the vent of the volcano in such large quantities and under such great pressure that powerful explosions occur, ejecting huge masses of lava, consisting of particles of various sizes, high into the atmosphere. They cool in the air and fall to the ground in the form of hard dust particles, peas and larger debris. They are called volcanic ash. Masses of this volcanic material cover the surroundings of an erupting volcano with a thick loose layer. Rains wet it, and it sets in motion, forming streams of volcanic mud. Drying, the mud turns into a light porous and hard rock called tuff. A similar rock formed at the bottom of the sea or lake is called tuffite.
Classification of intrusive and effusive rocks are built on the basis of the above features of the structure and texture, as well as their chemical and mineralogical composition. According to the chemical composition, igneous rocks are divided depending on the content of silicon oxide SiO 2 in them (Table 2.5). Acid rocks are more often light, sometimes white. As the silica content decreases, the color of the rock changes from gray to dark gray. Ultrabasic rocks are characterized by a black or dark green color, depending on the increase in the content of dark-colored minerals rich in iron and magnesium oxides.
Table 2.5. Classification of igneous rocks according to the content of silicon oxide.
| Group name | Rocks (examples) | |
| Low and non-silica | pellets | |
| ultrabasic | dunite, peridotite, pyroxenite, kimberlite, olivinite | |
| Main | gabbro, labrodarite, basalt, diabase, trachyte | |
| Medium | syenite, diorite, trachyte, andesite, feldspar, porphyrite | |
| Sour (acidic) | granite, liparite, quartz porphyry | |
| Ultra-acid | pegmatite, alaskite, pumice, volcanic glass |
In table. 2.6. given a brief description of basic igneous rocks.
Table 2.6. Characteristics of the main igneous rocks.
|
Rock |
Mineralogical |
Structure |
|
intrusive rocks |
||
| Granite red, pink, light gray | Quartz, feldspars (orthoclase, microcline), hornblende, micas | |
| Syenite | Full-crystalline, even-grained and porphyritic | |
| Gabbro | Plagioclases (from labradorite to anorthite), olivine | Full-crystalline, even-grained and porphyritic |
|
effusive rocks |
||
| Pumice | Foamy, highly bubbly | |
| Volcanic tuff | From various minerals enriched with silicon | blistered |
| Volcanic glass (obsidian) | Quartz | glass wool |
| Liparite (effusive analog of granite) | Quartz, feldspars (orthoclase, microcline) | Porphyry |
| Trachyte (effusive analogue of syenite) | Orthoclase, microcline, hornblende, biotite | Porphyritic, fine bubble |
| Basalt (effusive analogue of gabbro) | Plagioclase, olivine, augite | Dense, finely crystalline, cryptocrystalline |
| Andesite | Plagioclases, feldspars, hornblende, biotite | Partially crystalline porphyritic, fine-grained |
The most widespread in the earth's crust are granites (intrusive rocks), andesites and basalts (effusive rocks).
Granites make up ~30% of the mass of the earth's crust. Granites are composed primarily of three minerals: quartz, feldspar, and mica (or hornblende).
Andesites, rocks interspersed with feldspars (albite, anorthite), hornblende, micas, and pyroxene, make up ~25% of the mass of the Earth's crust.
Basalts make up ~ 20% of the mass of the earth's crust; they mainly include feldspars, pyroxene, and olivine. The rest is accounted for by all other rocks.
Sedimentary rocks are formed during the mechanical and chemical destruction of igneous rocks under the action of water, air and organic matter.
According to their origin, they are divided into three groups: clastic, chemical and organic.
Clastic rocks are formed in the processes of destruction, transfer and deposition of rock fragments. These are most often scree, pebbles, sands, loams, clays and loess. Clastic rocks are divided by size:
coarse clastic (> 2 mm); acute-angled fragments - gruss, crushed stone, cemented by shale, form breccias, and rounded - gravel, pebbles - conglomerates);
medium clastic (from 2 to 0.5 mm) - form sands;
Fine clastic, or silty - form loess;
fine clastic or clayey (< 0,001 мм) – при уплотнении превращаются в глинистые сланцы.
Sedimentary rocks of chemical origin – salts and deposits formed from saturated aqueous solutions. They have a layered structure, consist of halide, sulfate and carbonate minerals. These include rock salt, gypsum, carnallite, flasks, marl, phosphorites, iron-manganese nodules, etc. (Table 2.4). They can be formed in a mixture with detrital and organic deposits.
Marl formed by leaching of calcium carbonate from limestone, contains clay particles, dense, light.
Iron-manganese nodules are formed from colloidal solutions and under the influence of microorganisms and create spherical deposits of iron ores. Phosphorites are formed in the form of cone-shaped concretions of irregular shape, at the confluence of which phosphorite slabs appear - deposits of gray and brownish phosphorite ores.
Rocks of organic origin are widely distributed in nature - these are the remains of animals and plants: corals, limestones, shell rocks, radiolarians, diatoms and various black organic silts, peat, black and brown coals, oil.
The sedimentary layer of the earth's crust is formed under the influence of climate, glaciers, runoff, soil formation, vital activity of organisms, and it is inherent in zoning: zonal bottom silts in the World Ocean and continental deposits on land (glacial and water-glacial in the polar regions, peat in the taiga, salt in the desert, etc.). Sedimentary strata accumulated over many millions of years. During this time, the zoning pattern changed many times due to changes in the position of the Earth's rotation axis and other astronomical reasons. For each specific geological epoch, it is possible to restore the system of zones with the differentiation of sedimentation processes corresponding to it. The structure of the modern sedimentary shell is the result of the overlap of many zonal systems at different times.
In most of the world, soil formation takes place on sedimentary rocks. In the northern part of Asia, Europe and America, vast expanses are occupied by rocks deposited by glaciers. Quaternary period(moraine) and products of their erosion by melted glacial waters.
Moraine loams and sandy loams. These rocks are characterized by a heterogeneous composition: they are a combination of clay, sand and boulders of various sizes. Sandy loamy soils contain more Si0 2 and less other oxides. The color is mostly red-brown, sometimes pale-yellow or light brown; the build is tight. A more favorable environment for plants is represented by moraine deposits containing boulders of calcareous rocks.
Covering clays and loams - boulderless, fine-earth rocks. Consist predominantly of particles smaller than 0.05 mm in diameter. The color is brownish-yellow, for the most part they have fine porosity. They contain more nutrients than the sands described above.
Loess-like loams and loesses - boulderless, fine earth, carbonate, fawn and yellow-fawn, finely porous rocks. Typical loess is characterized by the predominance of particles with a diameter of 0.05-0.01 mm. There are also varieties with a predominance of particles with a diameter of less than 0.01 mm. The content of calcium carbonate ranges from 10 to 50%. The upper layers of loess-like loams are often freed from calcium carbonate. The non-carbonate part is dominated by quartz, feldspars, and clay minerals.
Red-colored weathering bark. In countries with tropical and subtropical climate Fine-earth deposits of the Tertiary age are widespread. They are distinguished by a reddish color, highly enriched in aluminum and iron, and depleted in other elements.
Indigenous breeds. In large areas, marine and continental rocks of pre-Quaternary age come to the surface, united under the name "bedrocks". These breeds are especially common in the Volga region, as well as in the foothills and mountainous countries. Among the bedrocks, carbonate and marl loams and clays, limestones, and sandy deposits are widespread. It should be noted that many sandy bedrocks are enriched in nutrients. In addition to quartz, these sands contain significant amounts of other minerals: micas, feldspars, some silicates, etc. As a parent rock, they differ sharply from ancient alluvial quartz sands. The composition of the bedrocks is very diverse and insufficiently studied.
metamorphic rocks are igneous and sedimentary rocks altered by temperature, pressure and chemically active substances. Metamorphosis of rocks occurs under the influence of the following factors:
Pressure arising from mountain building processes;
Temperature increase caused by magma penetrating into the lithosphere, hot aqueous solutions and gases carrying new chemically active compounds;
Pressure of overlying rocks.
One of the latest classifications of metamorphism is given in Table. 2.6.
Table 2.6. Classification of metamorphism of rocks
| Type of metamorphism | Factors of metamorphism |
| Immersion metamorphism | Increase in pressure, circulation of aqueous solutions |
| Heating Metamorphism | temperature rise |
| Hydration metamorphism | Interaction of rocks with aqueous solutions |
| Dislocation metamorphism | Tectonic deformations |
| Impact (shock) metamorphism | The fall of large meteorites, powerful endogenous explosions |
For example, during the accumulation of sedimentary rocks with a thickness of 10 - 14 km, their lower layers experience enormous pressure, accompanied by an increase in temperature and recrystallization of the entire material. As a result of this process, first shales are formed from clays, and then gneisses, resembling granite in composition. The composition of gneisses is different. From sands in the presence of iron compounds, first sandstones are formed, which crumble very easily with little effort, and then quartzites, i.e. crystalline rock. Quartzites and gneisses retain the layered structure characteristic of sedimentary rocks. Limestones recrystallize to form marble.
Thus, the processes of metamorphism, as it were, conclude a cycle of changes that occur with rocks.
