Minerals And Rocks

Crystal Info
Crystal Knowledge For Minerals And Rocks

Crystal is a sub-category of minerals and rocks. By understanding minerals and rocks, we can better understand crystals.

Table of Contents


Minerals, or free and uncombined natural elements, such as gold, silver, copper, etc.; or compounds of elements, such as feldspar, limestone, amphibole, and mica. Except for a few minerals such as mercury and opal, most minerals in nature are solid.

Mineral formation

Minerals are generally formed in the hydrothermal fluid circulating in the crustal fissures.



Mineral veins are fault zones where rock formations are dislocated and displaced, or fractured zones where rock formations are not dislocated or displaced. The veins are rich in mineral resources.

Metamorphic rock

The minerals formed in metamorphic rocks either reorganize the chemical composition of the original rock by the action of temperature and pressure, or add new components to the minerals in the circulating flow of chemically active fluids, such as garnet, mica and blue. Spar and so on.

Magmatic rock

The minerals formed in magmatic rocks are formed by cooling and crystallizing magma (melted rock underground) or lava (rock that melts out of the surface). When the magma temperature is higher, the density of minerals formed, such as olivine, pyroxene, etc.; when the temperature of magma is lower, the density of minerals formed, such as feldspar, quartz, etc., is lower.

Sedimentary rock

Minerals formed in sedimentary rocks, such as hematite, bauxite, etc., are formed by low-temperature hydrothermal fluids close to the surface.

Mineral composition

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The composition of minerals can be represented by chemical formulas. For example, the chemical formula of fluorite is CaF2, which means that calcium atoms (Ca) and fluorine atoms (F) are combined together, and the number two means that fluorine atoms are twice as much as calcium atoms. Minerals can be divided into natural elements, halides, oxides and hydroxides, sulfides, carbonates, sulfates, phosphates, silicates, etc. according to their chemical composition and crystal structure.

Natural elements

Natural elements are free and uncombined elements. There are few minerals of this kind, about 50 kinds, some of which have commercial value, such as gold and silver.


Halides are minerals containing halogen elements fluorine, chlorine, bromine, and iodine. They are generally atomized with metals to synthesize minerals, for example, rock salt is composed of sodium and chlorine, and fluorite is composed of calcium and fluorine. The number of halides is relatively small, there are about 100 kinds.

Oxide and hydroxide

Oxide is a compound formed by the oxidation of one or two metal elements. Hydroxide is a mineral formed by combining one metal element with water and hydroxyl groups. There are about 250 such minerals.


Sulfides are minerals formed by the combination of sulfur and metal and semi-metal elements. They are the most common, such as pyrite and realgar. There are about 300 species.


Carbonate is formed by the combination of one or more metal elements and carbonate, there are about 200 kinds, and calcite is the most common carbonate, which is formed by the combination of calcium and carbonate.


Sulfate is formed by combining one or more metal elements with sulfate radicals.


Phosphate is synthesized by one or more metal elements and phosphate. Such minerals are generally bright in color and are generally associated with arsenate and vanadate.


Silicates are formed by the combination of metal elements and single or connected silicon-oxygen tetrahedrons. This is an important and common type of mineral, with about 500 species.

The properties of the mineral

To test the properties of minerals, it is necessary to measure the color, luster, aggregate shape of the mineral, as well as cleavage, fracture, hardness, specific gravity and streaks.


Crystal system

According to the symmetry and geometric shape of mineral crystals, it can be divided into six crystal systems, including monoclinic system, equiaxed system, triclinic system, tetragonal system, hexagonal system and trigonal system. Each crystal system has many different morphologies, and all morphologies in the same crystal system should be related to the symmetry of the crystal system.


Aggregate is the external feature of crystals, which determines the main forms of minerals, including dendritic, blade-shaped, columnar, needle-shaped, massive, kidney-shaped, etc.

Twin crystal

Twin crystals refer to two or more crystals of the same mineral that are regularly connected to each other, which can be divided into contact twin crystals, interspersed twin crystals, and poly-sheet twin crystals. The contact twin crystals appear as radial blocks in appearance; the interspersed twin crystals are two crystals connected together; the convergent twin crystals are two or more crystals connected together in parallel and repeatedly.


Cleavage refers to the way of cracking along the weak surfaces of minerals. These surfaces are generally in the direction of the weakest atomic chemical bond or between atomic layers. Although the cleavage surface of the mineral is not as smooth as the crystal surface, it can still reflect light evenly. Cleavage can be described as complete, clear, unclear and uncleavable.


When the mineral is hit with a geological hammer, it will crack and expose a rough surface. This is the cross section, which can be described as jagged, conchoidal, jagged, and lobed.


Hardness refers to the ability of a mineral to resist scratching. We generally use the hardness standard invented by Moss to judge the hardness of minerals, namely Mohs hardness, which is divided into 10 levels: talc (level 1), gypsum (level 2), calcite (level 3), fluorite (level 4) , Apatite (grade 5), orthoclase (grade 6), quartz (grade 7), topaz (grade 8), corundum (grade 9), diamond (grade 10). Among them, talc (grade 1) has the lowest hardness, and diamond (grade 10) has the highest hardness. In the Mohs hardness, high-grade minerals can be used to mark low-grade minerals. For example, calcite can mark gypsum but not fluorite.

In addition to the Mohs hardness, the hardness of minerals can also be measured with daily necessities, such as using a coin first, then a knife, glass or quartz.

Now there is a special tester for testing the hardness of minerals, that is, the hardness tester. The measurement range is 3-10, and the hardness of the minerals can be easily measured.


The specific gravity of a mineral refers to the ratio of the weight of the mineral to the weight of the same volume of water, which can be represented by numbers. For example, a specific gravity of 2.5 means that the weight of the mineral is 2.5 times the weight of the same volume of water.


The color of a mineral refers to the color that appears under natural light. It can help us identify minerals with bright colors, but we cannot rely on this feature completely, because some minerals have multiple colors, and some minerals are white or colorless. Therefore, the identification of minerals requires a combination of various factors to arrive at Accurate identification conclusions.


The streak refers to the color of the mineral powder. Generally, the mineral powder can be obtained by carving on the white unglazed porcelain plate, but if the mineral is harder, you can use a geological hammer to break a small part or rub it with a hard surface to obtain the mineral powder. Because the streak of minerals is more stable than the color of minerals, it is an important feature of mineral identification.


The transparency of the mineral refers to the degree to which the mineral can transmit visible light, which is related to the way the atoms are connected in the crystal structure of the mineral. When the mineral is cut into 0.03 mm thin slices, if it can clearly see through other objects, it is transparent, if it can pass light but cannot see other objects clearly, it is translucent, and if light cannot pass through at all, it is opaque.


Luster is the reflectivity of the mineral surface to light. It is mainly determined by the surface properties and reflectivity of the mineral. It is often described in terms such as dim, metal, pearl, glass, grease, and silk.


Rock is a collection of several minerals. It is an important part of the earth’s crust. According to its genesis, it can be divided into three categories: magmatic rock, metamorphic rock and sedimentary rock.

Crystal Specimen

Rock formation

The crustal movement inside the earth never ceases, so new rocks are constantly being formed.

The magma inside the earth slowly rises through the movement of the earth’s crust, and magmatic rocks are formed during the ascent and cooling process. Later, the movement of the earth caused part of the magmatic rock to rise to the surface. Under the erosion of glaciers, running water and wind, the rock was broken into particles, which were then carried by glaciers, rivers and wind, and gradually deposited in lakes, deltas and deserts to form sedimentary rocks. In addition, in large-scale orogeny, some magmatic rocks and sedimentary rocks become metamorphic rocks through the action of high temperature and pressure.

The properties of magmatic rock

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Magmatic rock is formed by the crystallization of liquid magma and lava. The original composition of magma, the way of intruding into the crust, and the cooling rate all affect its composition and properties.


There are two main types of magmatic rocks: intrusion and eruption. Intrusive magmatic rock is formed by slow cooling of liquid magma inside the crust, while magmatic rock of eruption origin is formed by natural overflow of liquid magma and rapid cooling of the ejected surface.


Occurrence refers to the form of lava cooling and solidification. For example, plutonic rocks are large and deep intrusive rocks that can stretch for thousands of meters. The dykes are elongated and irregular slab-shaped rock masses, and the rock beds are integrated sheets.

Mineral composition

Rocks are aggregates of minerals. Minerals such as feldspar, mica, quartz, and iron-magnesium are all components of rock, and the composition of minerals determines the chemical properties of the rock.

Particle size

Generally speaking, plutonic rocks in magmatic rocks have coarse grains, while extruded rocks have fine grains. Coarse-grained magmatic rocks such as gabbro have crystals of more than 5 mm in diameter. Neutral basalt has crystals of 0.5 to 5 mm. Fine-grained basalt has crystals. The diameter is less than 0.5 mm.

Crystal shape

The slow cooling of magma causes the mineral crystals to develop into perfect euhedral crystals, and the rapid cooling of magma will form inferior crystals.


Structure refers to the arrangement of mineral particles or crystals.


The color of a mineral is an accurate indicator of the chemical properties of the mineral, which can reflect the content of a certain mineral. Acidic rocks are light-colored, basic rocks are dark-colored, and neutral rocks are in between.

Chemical composition

According to the chemical composition, magmatic rocks can be divided into the following categories: acid rock, containing more than 65% silicate and more than 10% quartz; neutral rock, containing 55% to 65 percent silicate; basic rock , Containing 45% to 55% silicate and less than 10% quartz; ultrabasic rock, silicate content is less than 45%.

The nature of sedimentary rocks

Sedimentary rocks have two distinctive features, which can be easily distinguished from magmatic rocks and metamorphic rocks: one is that sedimentary rocks are produced in layers and can usually be stripped along the layers; the other is that sedimentary rocks generally contain biological fossils, and magmatic rocks never Containing fossils, metamorphic rocks are also relatively rare.



Rock particles are transported by wind, flowing water, glaciers, etc., and deposited on land, rivers, lakes, and oceans, and are mainly formed on or near the surface.


A large number of fossils of animals and plants are preserved in sedimentary rocks. These fossils can promote the research of paleontology, geology and other disciplines. For example, fossils of marine organisms can indicate that rocks were formed in the marine environment.

Particle size

The grain size of sedimentary rocks is usually expressed in terms of coarse grain, medium grain and fine grain. The clastics of coarse-grained rocks, such as conglomerate, breccia and sandstone, can be seen with the naked eye; the particles of medium-grained rock can be distinguished by portable magnifying glass, such as sandstone; the particles of fine-grained rock can be observed by microscope, including shale, claystone and sandstone. Mudstone.

Particle shape

The particle shape of sedimentary rock depends on its transportation method. For example, wind erosion will form round particles, and flowing water will form angular sand-gravel particles.


Sedimentary rocks can be divided into clastic rocks, biological rocks and chemical rocks according to the source of their rock particles. Clastic rocks contain pre-diagenetic particles, biological rocks contain crusts or other chemical clastics, and chemical rocks are the product of chemical precipitation.

Type of deterioration

Red and Blue Gemstone Pyramid R-L-A

Metamorphic rocks are magmatic rocks and sedimentary rocks formed under the action of high temperature and high pressure.

Regional metamorphism

Regional metamorphism is the effect of temperature and pressure in the vicinity of the mountain belt. The metamorphism is the strongest, and the range of metamorphism can reach several thousand square kilometers, forming regional metamorphic rocks. The following examples illustrate how shale forms different metamorphic rocks under different pressures.

  • No pressure: Shale is a fine-grained sedimentary rock, containing clay minerals, quartz and fossils. When there is no pressure, it will not deteriorate.
  • Low pressure: Under low pressure, shale will be distorted or damaged to form slate.
  • Medium pressure: Under medium pressure, shale forms medium-grain schist.
  • High pressure: In a geological environment with high pressure and high temperature hydrothermal activity, shale will become coarse-grained gneiss.

Contact metamorphism

Contact metamorphism refers to the effects of temperature and pressure around magma intrusions or near lava flows. Contact metamorphic rocks are formed under the direct action of temperature and pressure, and the range of metamorphic zones is related to the temperature of magma or lava or the size of the intrusions . The high temperature not only changed the minerals in the original rock, causing recrystallization, but also caused the fossils contained in it to disappear. For example, the dark coarse-grained basalt at the bottom of the cliff forms lighter keratinite under the action of the intrusive black shale heat flow, while the sandstone becomes crystalline and non-porous quartzite under the action of temperature and pressure.

Dynamic metamorphism

Dynamic metamorphism refers to the compression in the crust, especially near faults, when large-scale crustal movement occurs. At this time, the big rocks are squeezed together, and the places where they touch each other are ground and crushed to form mylonite.

The properties of metamorphic rocks


One of the typical characteristics of metamorphic rocks is that the minerals that make up the rock are crystalline. The arrangement direction of the crystals is determined by temperature and pressure, and the particle size of the crystals directly reflects the temperature and pressure strength they are subjected to. Therefore, we can determine its origin by observing the crystals in metamorphic rocks.


Structure refers to the arrangement and distribution of minerals in the rock. For example, the crystalline structure of contact metamorphic rock has irregular crystal arrangement, while regional metamorphic rock has a schistose structure, and pressure makes certain minerals line up in a straight line.

Particle size

The temperature and pressure conditions of rock formation can be judged from the particle size. Generally, the higher the pressure and the higher the temperature, the larger the rock particles will be. Therefore, the slate formed under low pressure is fine-grained, the schist formed under medium pressure is medium-grained, and the gneiss formed under high temperature and high pressure is coarse-grained.

Temperature and pressure

The temperature generated by metamorphism is 250-800 degrees Celsius. Below this temperature, the rock cannot produce metamorphism. Above this temperature, the rock will melt into magma or lava; the pressure generated by metamorphism is 2000-10000 kPa, which is lower than This pressure cannot produce metamorphism, and above this pressure, the rock is powdery.

Mineral content

The unique minerals in metamorphic rocks are very helpful for identification. For example, garnet and kyanite are found in schist and gneiss, pyrite crystals are commonly found in the cleavage plane of slate, and brucite is found in marble.

Rock identification

Rock identification is mainly divided into four steps: the first step is to determine whether the rock is magmatic rock, sedimentary rock or metamorphic rock; the second step is to determine the particle size, which is divided into coarse-grained, medium-grained, and fine-grained; and the third step is to consider other rocks Features such as color, structure, mineral combination, etc. The specific identification steps are as follows:

The first step is to determine whether the rock is magmatic rock, sedimentary rock or metamorphic rock. The magmatic rock has a crystalline structure, which is formed by the interconnection and aggregation of mineral crystals. The crystals in the rock may gather irregularly or show a certain directionality. The magmatic rock does not have the bedding structure of sedimentary rock, nor the schist structure of metamorphic rock. Some lava is also full of pores. Does not contain fossils.

Sedimentary rocks have obvious bedding, and their mineral particles are loosely connected and can be rubbed off with your fingers. In addition, the most important thing is that sedimentary rocks contain fossils, which can be distinguished from magmatic rocks and metamorphic rocks.

Metamorphic rocks are divided into two types, one is regional metamorphic rocks, which have a unique schistal structure and are wavy, not as flat as the bedding surface of sedimentary rocks; the other is contact metamorphic rocks, and their crystals are more irregularly arranged .

The second step is to determine the particle size.

After determining the type of rock, it is necessary to determine the size of the rock particles, which can be divided into fine-grained, medium-grained, and coarse-grained. Pay special attention here: the particle size refers to the size of the particles that make up the rock, not the size of the individual crystals embedded in it.

The third step is to consider other characteristics of the rock, such as color, structure, and mineral combination.

After judging the rock type and rock particle size according to the first two steps, the other characteristics are combined to make an accurate judgment.

If it is magmatic rock, the next step is to observe the color. Acidic rock is rich in light-colored silicate with low density and light color; basic rock and ultra-basic rock are rich in dense ferro-magnesia The color is deep; the neutral rock has a mineral content between the first two types, and the color is also in the middle.

If it is a sedimentary rock, it is necessary to observe its mineral composition. Sedimentary rocks can be divided into four categories according to their mineral composition: rocks mainly containing rock clastics; rocks mainly containing quartz clastics. Quartz is usually gray and hard and easy to identify; rocks mainly containing calcium carbonate and rocks containing calcium carbonate Not only the color is light, but also when it interacts with dilute acid salts, it will foam; finally, the rocks contain other minerals.

If it is a metamorphic rock, it should be observed whether it has a schistal structure, which is one of the most prominent features of a metamorphic rock, that is, the directional arrangement of certain minerals under the action of temperature and pressure.

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