When observing rocks, we can observe the color, crystalline state, the size of various crystals and the structure of rocks, such as pore structure, flow structure and bedding structure of sedimentary rocks.
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Magmatic rocks are mainly composed of silicate minerals, in addition, they often contain trace minerals such as magnetite. According to the content of SiO2 in rocks, magmatic rocks can be divided into four categories: ultrabasic rocks: SiO 2 < 45%; Basic rock: SiO 2 = 45 ~ 52%; Neutral and alkaline rocks: SiO 2 = 52 ~ 65%; Acid rocks: silica > 65%.
The alkalinity of rock refers to the saturation of alkali in rock, and the alkalinity of rock has a certain relationship with alkali content. Generally, the sum of the weight percentages of Na2O+K2O is called the total alkali content. The higher the Na2O+K2O content, the greater the alkalinity of the rock. A.Rittmann 1957 considers the relationship between SiO2 _ 2 and Na2O+K2O, and puts forward a commonly used combination index (σ) for determining rock alkalinity. The greater the σ value, the stronger the alkalinity of the rock. According to the degree of alkalinity, each rock can be divided into three types: calc-alkaline, alkaline and hyperalkaline rocks. σ& lt； 3.3 is calc-alkaline rock; When σ= 3.3-9.0, it is alkaline rock; σ& gt； At 9 o'clock, it is alkaline rock.
In addition to petrochemical composition, mineral composition is also one of the bases for classification of magmatic rocks. The composition and content of some common minerals in magmatic rocks change regularly due to different rock types. If feldspar is white or flesh-colored, it is called light-colored mineral; Olivine, pyroxene, amphibole and mica are dark green and dark brown, which are called dark minerals. Ultra-basic rocks usually do not contain chronological and a small amount of feldspar, and are mainly composed of dark minerals. However, there are few dark minerals in acid rocks, which are mainly composed of light minerals; The mineral composition of basic rocks and intermediate rocks is between the two, and light-colored minerals and dark-colored minerals each account for a certain proportion.
Magmatic rocks can be divided into intrusive rocks and extrusive rocks according to their occurrence, that is, according to whether the rocks invade the ground or eject the surface. According to the different formation depths, intrusive rocks can be divided into plutonic rocks and shallow rocks. The chemical composition of intrusive rocks and extrusive rocks in each type is the same, that is to say, the magma composition is similar, but their structures and structures are obviously different due to the different forming environments. The plutonic rocks are located in the deep underground, the magma condensation speed is slow, the rocks are mostly pure, the mineral crystal particles are large, and large porphyritic crystals are often formed; Shallow diagenetic rocks are close to the surface and often have fine-grained structure and porphyritic structure; However, due to the rapid solidification speed of ejected rocks, minerals have no time to crystallize, often forming aphanitic and glassy rocks.
According to the above principles, magmatic rocks are first divided into four types according to acidity, and then each type of rocks is divided into several rocks according to alkalinity, which are the main members of magmatic rock family. For example, ultrabasic rocks: calc-alkaline rocks are peridotite-picrite; Alkaline rock is kimberlite containing diamond; The total alkali rocks are aegirine rocks and carbonate rocks. Basic rocks: the calc-alkaline series rocks are gabbro-basalt; The corresponding alkaline rocks are alkaline gabbro and alkaline basalt. Intermediate rocks: the calc-alkaline series is amphibolite; The alkaline series is syenite-trachyte; The total alkali rock is nepheline syenite-acoustic rock. Acid rocks: mainly granite rhyolite of calc-alkaline series.
The report time is 20: 00 on July 9, 2008.
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Igneous rock is also called magmatic rock. Under different geological conditions, rocks formed by the condensation and consolidation of molten materials from the interior of the earth. When molten magma overflows the surface of a volcano and solidifies, it is called extrusive rock or volcanic rock. Common volcanic rocks are basalt, andesite and rhyolite. When lava rises below the surface and condenses at a certain depth in the earth's crust, it is called intrusive rock, which can be divided into deep diagenesis and shallow diagenesis according to different intrusion sites. Granite, gabbro and diorite are typical plutonic rocks. Granite porphyry, gabbro porphyry and diorite porphyry are common shallow rocks. According to chemical composition, igneous rocks can be divided into ultrabasic rocks (SiO _ 2 content is less than 45%), basic rocks (SiO _ 2 content is 45% ~ 52%), intermediate rocks (SiO _ 2 content is 52% ~ 65%), acidic rocks (SiO _ 2 content is more than 65%) and alkaline rocks (containing special alkaline minerals, SiO _ 2)
② Sedimentary rocks. Layered rocks formed by transportation, deposition and diagenesis of weathered materials, pyroclastic materials, organic matter and a small amount of cosmic materials at normal temperature and pressure on the surface. According to the genesis, it can be divided into clastic rocks, clay rocks and chemical rocks (including biochemical rocks). Common sedimentary rocks are sandstone, tuffaceous sandstone, conglomerate, clay rock, shale, limestone, dolomite, siliceous rock, iron ore, phosphorite and so on. Sedimentary rocks occupy 7.9% of the crust volume, but they are widely distributed in the surface layer of the crust, accounting for about 75% of the land area, and the seabed is almost completely covered by sediments.
Sedimentary rocks have two prominent characteristics: one is bedding, which is called bedding structure. The interface between layers is called bedding plane, and usually the rocks below are older than the rocks above. Second, many sedimentary rocks contain "stony" remains of ancient creatures or traces of their existence and activities-fossils, which are precious materials for judging geological age and studying paleogeographic environment.
③ Metamorphic rocks. Rock formed by metamorphism of original rock. According to the types of metamorphism, metamorphic rocks can be divided into five categories: dynamic metamorphic rocks, contact metamorphic rocks, regional metamorphic rocks, migmatites and metasomatic metamorphic rocks. Common metamorphic rocks include mylonite, cataclastic rock, amphibole, slate, phyllite, schist, gneiss, marble, quartzite, amphibole, schist, eclogite and migmatite. Metamorphic rocks occupy 27.4% of the crust volume.
Rock has specific physical properties such as specific gravity, porosity, compressive strength, tensile strength, etc. It is a factor to be considered in construction, drilling, excavation and other projects, and also a carrier of various mineral resources. Different kinds of rocks contain different minerals. Taking igneous rocks as an example, basic ultrabasic rocks are related to iron-loving elements, such as chromium, nickel, platinum group elements, titanium, vanadium and iron. Acidic rocks are related to ishihara-loving elements, such as tungsten, tin, molybdenum, beryllium, lithium, niobium, tantalum and uranium. Diamond only occurs in kimberlite and K-Mg lamprophyre; Chromite is mostly produced in pure peridotite; The early Yanshanian granite in South China is rich in tungsten-tin deposits. Independent tin deposits and niobium, tantalum and beryllium deposits are often formed in granite in the late Yanshan period. Oil and coal exist only in sedimentary rocks. Iron ore in Precambrian metamorphic rocks is worldwide. Many stones are also important industrial raw materials, such as white marble in Beijing, which is a well-known building decoration material at home and abroad. Rainflower Stone in Nanjing, Shoushan Stone in Fujian and Qingtian Stone in Zhejiang are all good arts and crafts stones, and even river sand and pebbles that are not noticed by people are very useful building materials. Many rocks are also important raw materials of traditional Chinese medicine, such as medical stone (a kind of intermediate-acid dike rock), which is a very popular medicinal rock. Rock is also an important factor in tourism resources. The famous mountains, rivers and grottoes in the world are all related to rocks. Our ancestors have been using rocks since the Stone Age. In today's highly developed science and technology, people can't live without stones without clothing, food, shelter, transportation and medical care. Studying rocks, using rocks, hiding rocks, playing with rocks and loving rocks are no longer the patents of scientists, but gradually become an integral part of the lives of the broad masses.