Magma rock naked eye identification method

I. Identification content and method

Ultrabasic rocks: peridotite, pyroxenite, amphibolite, Kimberlite

Basic rock: gabbro, diabase , basalt

Intermediate rocks: diorite, andesite, syenite, trachyte

Acid rock: granite , rhyolite

Pulse rock: lamprophyre, fine-grained rock

In view of the main identification characteristics of the listed magmatic rocks, the main mineral components, structural structures and other characteristics of different rock types were observed under the naked eye by means of a magnifying glass and a knife.

Second, the magma rock limit identification method and steps

Observing the magma rock hand specimens, it is common to observe the color, structure, structure, mineral composition and content of the rock, and finally determine the rock name.

1) Color: mainly describes the color of the fresh surface of the rock, but also pay attention to the color after the weathering.

Directly describe the overall color of the rock, such as purple, green, red, brown, gray and other colors. If some colors are somewhere in between, use compound names such as off-white, yellow-green, and magenta.

The color of the magmatic rock is reflected in the relative content of dark minerals and light minerals. A ship with a dark mineral content >60% is called dark rock; 60-30% is called medium color rock; <30% is called light color rock.

2) Structure: According to the degree of crystallization of each component in the rock, it can be divided into all-crystalline, semi-crystalline, vitreous and other structures.

Description content and method of magmatic rock structure:

3) Structure: The intrusive rock is often a massive structure, and the minerals in the rock are not oriented; the ejected rock often has a stomatal, almond-shaped and rhyolitic structure. Pay attention to the size and shape of the stomata, the filling of the almonds, the stomata, and the orientation of the almonds.

4) Mineral composition: Mineral composition and its content are important basis for the name of magmatic rock. Any minerals in the rock that can be identified with the naked eye are described. First, the composition, shape, size, physical properties and relative contents of the main minerals should be described. Secondly, the secondary minerals should be briefly described.

5) Secondary changes: After the consolidation of magmatic rocks, they are subjected to hydrothermal action and surface weathering during the magma period, which often causes all or part of the minerals in the rock to undergo secondary changes. If the change is strong, it should be described as etched. What kind of minerals. Such as olive stone, pyroxene easy to form serpentine, hornblende, biotite often become chlorite, while feldspar becomes sericite, kaolinite and so on.

6) Rock name: The rock name is determined based on the observation and description of the meat limit.

Color + structure + basic rock name, such as light gray coarse grain granite; gray black medium gabbro

The classification of magmatic rocks is named, and beginners can be carried out as follows:

(1) View color, initial class: The color of rock reflects the mineral composition and its content, which is an intuitive basis for rock classification and naming. However, it should be pointed out that when estimating the dark mineral content, it is easy to cause visual errors. When a light-colored mineral is coated on a dark mineral, it is easy to regard it as a dark mineral because of its transparency, so the estimation of dark mineral content is often high. Also pay attention to the influence of the color of the secondary changes.

(2) Demonstration of minerals: according to the color is divided into three parts, according to the mineral species, content and symbiotic combination characteristics, the rock is divided into (1) ultrabasic rocks, (2) basic rocks, (3) Neutral (calcium-alkaline) rock, (4) acid rock, (5) alkaline rock and other five categories can determine which type of rock belongs to.

Method: The indicated minerals are divided into two ends, the dark minerals are divided into the middle, and the symbiotic minerals are tested. Quartz >20% is acid rock; olivine (+ pyroxene, or amphibole)>90% is ultrabasic rock; medium and basic rock are plagioclase+color dark mineral; medium and basic rock In addition to the color removal rate, there are two main rules: 1 dark mineral species: neutral rocks are mainly amphibole, basic rocks are pyroxene; 2 symbiotic minerals: basic rocks and ultrabasic rocks A small amount of olivine can be found; the neutral rock is adjacent to the acid rock, and a small amount of quartz and meat red potassium feldspar can be found. The color of acid rock and alkaline rock are near flesh red, and the distinction between the two is mainly based on the fact that the quartz and plagioclase (off-white) content of alkaline rock are very small.

For the effusion rock and the diagenetic rock with porphyry structure, the matrix is ​​cryptocrystalline, and it is difficult for the naked eye to identify its constituents, which are mainly named by phenocrysts. Because phenocrysts are generally composed of the main minerals in the rock, plaque minerals can also be named.

For plaque-free cryptocrystalline rocks, it is only roughly judged by the color of the rock and the degree of compactness. Acidic cryptocrystalline rocks with higher SiO ₂ tend to have greater hardness.

(3) Look at the structure (structure), push the environment (production): the same rock composition is the same, but each type is divided into three types: deep diagenesis, superficial diagenesis and effusion rock according to different occurrences, giving different rock species names. The rock occurrence is the rock formation environment, which is mainly reflected in the structure and structure.

There are many types of rocks in nature, and there are many types of transitions between categories. For example, a rock is dominated by hornblende and plagioclase, and the minor minerals are quartz (up to 5-20%), potassium feldspar (up to 20%), biotite, etc., and the rock should be between medium and acidic. It is designated as granodiorite; some is between the eruptive and superficial rocks, called ultra-shallow diagenesis.

(4) According to the color of the rock, the main, minor mineral content and structural structure are named in detail.

For intrusive rocks: color + structure + basic name such as: black gray medium gabbro

For ejected rock: color + structure + basic name such as: black ventral basalt

System classification table for magmatic rocks:

Example of identification of magmatic rock meat and identification: specimen n

Black and gray, the weathered surface is slightly blackish green, and the grain structure is equal. The particles are generally in the shape of l-1.5 mm. The main minerals are plagioclase and pyroxene, each accounting for 55% and 40%. The plagioclase is grayish white, columnar or granular. When the cleavage surface is sparkling, the glass is shiny, the pyroxene is black, short columnar, glass luster, and some cleavage surfaces are clear. The rocks are fresher and the last time they change. According to the various characteristics of the n-type specimen rock described above, it can be defined as basic and deep diagenesis, and it is named: black-gray medium-gypsum gabbro.

According to the coating characteristics