Field identification of major rock-forming minerals in magmatic rocks

I. Field identification of major rock-forming minerals in magmatic rocks

1. Quartz Quartz is the main mineral of granite rocks. Its shape is worm-like in the granitic granite and hexagonal porphyry in the diagenetic and eruptive rocks. In most cases, it is a granular crystal. The color ranges from colorless to smoky. The crystal face is glassy, ​​but the grease luster on the fracture surface is common. Potassium feldspar, plagioclase acid, biotite symbiosis. It has strong anti-weathering ability and often shows obvious bulging on rock weathering surface. The difference with feldspar is that there is no cleavage, no double crystals, grease luster and no weathering products.

2, potassium feldspar potassium feldspar includes feldspar, micro plagioclase, striped feldspar, translucent stone and so on. The main types of intrusive rocks are feldspar and micro-plagioclase, which may be perlite in the diagenetic and effusion rocks. Stripe feldspar is the product of feldspar or micro-plagioclase and sodium feldspar, in which feldspar or micro-plagioclase is more than albite. Color is an important indicator for the identification of potassium feldspar. Potassium feldspar is usually fleshy red, but it is also purplish red, white, grayish white, or even grayish black. Potassium feldspar changes color during weathering, meat red can turn grayish white, grayish white can also change to meat red. And acid plagioclase is often meat red. Therefore, color cannot be used as a characteristic marker in the identification of K-feldspar. Potassium feldspar and micro-plagioclase, which are produced in deep diagenes, often have granular crystals. When potassium feldspar forms phenocrysts in plaque-like, plaque-like rocks, it is often a plate-like, slab-like self-shaped crystal. In the field identification of Changshi, special attention should be paid to the observation of twin crystal. When the specimen is rotated, it is found that half of the reflection on the feldspar cleavage surface, and half of it is not reflective, this is the Casbah twin crystal; when the phase reflection occurs, it is the polycrystalline twin crystal. Potash feldspar often has Casbah twin crystals, while plagioclase often has polycrystalline twin crystals. This is the most important sign distinguishing between K-feldspar and plagioclase. If on the large K-feldspar crystals, there are fine veins with roots and roots, and the color of the veins is lighter, it is striped feldspar. When potassium feldspar is weathered, white earthy kaolinite is often formed.

3. The plagioclase plagioclase is widely found in various types of magmatic rocks. The type and content of plagioclase is critical for the classification and identification of magmatic rocks. The plagioclase can be in different shades. Generally, the basic plagioclase is darker in color, dark gray to grayish white; the acid plagioclase is lighter in color and can be grayish white and fleshy red. The basic plagioclase is subjected to sodium zoi petrochemical, and its altered products often have a green hue; while the acidic plagioclase is easy to mica, its weathering products are mostly grayish white. In basic superficial diagenes or eruptive rocks (such as diabase and basalt ), plagioclase is difficult to identify because of its dark color and fine crystals. At this time, semi-weathered specimens can be collected for observation, and the color of the plagioclase becomes lighter, which is easily distinguished from dark minerals. Polycrystalline twin crystal is an important identification mark of plagioclase. Rotate the specimen in different directions until a set of parallel lines of light and dark lines or fold lines appear on the crystal face or cleavage surface with a field or magnifying glass. This is the double crystal grain. In general, the acid plagioclase double crystal grain is dense and straight, while the basic plagioclase double crystal grain is thin and not flat enough.

4. Ordinary hornblende Ordinary hornblende is a mineral often found in diorite and syenite , and is also often found in granite. Ordinary hornblende is mostly black, dark green, and sometimes brown. The hornblende in the intrusive rocks is mostly long columnar crystals, but in some granites and granodiorites, the crystal habit of the amphibole's long-term elongation is not significant. Amphibole is similar to pyroxene in cleavage, gloss and hardness, so it is easy to be confused with pyroxene. In the field identification, it can be distinguished according to the angle of cleavage. The specific method is: see a set of well-reflected stepped reflective surface (cleavage surface) under the illumination light, and then rotate the specimen under the gaze of the eye until the second set of reflective surfaces is observed, and the rotation angle is the cleavage clip. angle. It is estimated that this angle is pyroxene if it is nearly 90 degrees; if it is a sharper or obtuse angle, it is amphibole. In addition, ordinary hornblende is often symbiotic with quartz, potassium feldspar, and biotite; and pyroxene is often symbiotic with olivine and basic plagioclase. In granite, ordinary hornblende and biotite are sometimes confused, the difference is that the hornblende hardness is greater than the knives, the knives can only be obtained with the knives, the cleavage surface is the glass luster; and the biotite The hardness is less than the knives, and the knives can be used to pick thin slices and cleave the pearl of the mask. In addition, the amphibole subjected to weathering often has a green hue, while the biotite often has a brown hue after weathering.

5. Pyroxene is the most important mineral in ultrabasic rocks and basic rocks, and porphyry often appears in andesite . Most pyroxene is greenish black and a small amount of pyroxene is grayish green (such as diopside ). Pyroxene produced in intrusive rocks is generally equiaxed in granular form. Pyroxene has two sets of nearly vertical cleavage, often forming an irregular stepped fracture. The most common secondary changes in pyroxene are serpentine and fibrin. Serpentine accommodating pyroxene often forms a "silver" with silky luster.

6. Peridot olivine is a common mineral in ultrabasic rocks and basic rocks, and its content is an important basis for the classification and identification of these two types of rocks. Fresh olivine is a sugar-like crystal, which is olive green or yellow-green. It is generally lustrous and oily. It has a shell-like fracture and is not cleavable, so it is easier to distinguish it from pyroxene. Intrusions olivine often altered to serpentine and talc, olivine-serpentine alteration often has a dark green, black, with oil sheen, and is often visible from the fine iron ore magnetic pulse consisting of fine mesh. The olivine phenocrysts in the eruptive rock (basalt) are often eclipsed into a maroon idyl stone with an olivine illusion. Peridot is a mineral with weak resistance to weathering, and it is difficult to see fresh olivine on the surface.

7. The biotite biotite mainly appears in acidic rocks. The fresh biotite is black or dark brown, faded after weathering, often golden yellow, and the cleavage is extremely complete, often in the form of flakes, which are often seen in hand specimens. A flat reflective surface of uniform crystal size with pearl luster and a hardness less than that of a knife. According to the above features, it is not difficult to distinguish it from ordinary hornblende and pyroxene.

8. Nepheline is only found in SiO ₂ unsaturated alkaline rock. The nepheline in the intrusive rock is often granular in shape due to crystallization. The color is flesh red or grayish white, the cleavage is incomplete, and it often has grease luster and is easily confused with quartz. The difference with quartz is that quartz is generally smoky gray, which is convex on the weathered surface, while nepheline is generally red in meat, weak in weathering resistance, and often has weathering products. In addition, quartz is generally symbiotic with potassium-rich alkaline feldspar, acid plagioclase, and biotite, and nepheline is often associated with sodium-rich alkaline feldspar (such as celsian) and basic pyroxene. According to the cleavage is not developed, the grease shines, can be distinguished from feldspar.

Second, the mineral content

Determining the mineral content is important for determining the name of the magma. The following are two methods for estimating mineral content in rocks in the wild.

1. The visual estimation method is to estimate the content of minerals in rocks by field, and its accuracy is related to experience, and the error is large. When assessing the mineral content, since the dark minerals are more conspicuous, the estimated content tends to be high, which should be avoided.

2. The straight line method is a representative outcrop selected in the field. The length of a mineral on a plurality of parallel straight segments is measured with a small steel ruler and recorded. Then, the volume percentage of the mineral to be tested is determined according to the following formula. content.

The volume percentage of a mineral in the rock = the total length measured by the mineral / the total length of the line * 100% In principle, the longer the straight line segment, the more accurate the measured result. The total length of the general line should not be less than 100 times the particle size of the mineral particles, and the line spacing should not be less than the average particle size of the mineral.

Third, the magma rock field identification table

In the field identification of magmatic rocks, in addition to the observation of the minerals and contents of the rocks, the structure, structure and occurrence of the rocks should be systematically studied to determine the location of the lava condensation, so that the magmatic rocks can be Correct identification. According to the location of the lava condensation, the magmatic rocks can be divided into plutonic rocks, epizoic rocks, subvolcanic rocks and volcanic rocks. The characteristics of these four types of magmatic rocks are shown in Table 1.

Table 1 Lithological characteristics of various magmatic rocks with different condensation depths

1. Sedimentary field identification table

In the field identification of deep rock, the color ratio of rock, the presence and content of quartz, olivine, feldspar, the presence and content of potassium feldspar and plagioclase, and the types of dark minerals should be considered. Refer to Table 2 for identification.

Table 2 The most common deep rock identification field

Note: A is potassium feldspar and P is plagioclase

2. Field identification of epithelial rocks (including subvolcanic rocks)

The field identification of epithelial rocks (including subvolcanic rocks) can be roughly similar to that of plutonic rocks. Due to the fine crystal structure and large structural changes of the diagenetic rocks, special attention should be paid to the mineral composition and structure of the phenocrysts. For the identification of superficial rocks in the field, refer to Table 3.

Table 3 Field identification table of superficial diagenesis (including subvolcanic rocks)

3. Field identification table of volcanic rocks

The volcanic rocks are poorly crystallized, and the matrix is ​​often cryptocrystalline and vitreous. The identification of the field is mainly based on color, structure, phenocryst composition and secondary changes.

The color of volcanic rocks is generally darker than the intrusive rocks of the corresponding constituents. The phenocryst minerals of volcanic rocks are generally more "basic" than the minerals in the matrix. For example, pyroxene phenocrysts are common in andesite, and olivine phenocrysts are common in basalt. Refer to Table 4 for the identification of volcanic rocks in the field.

Table 4 Main characteristics of various volcanic rocks in the field identification

Fourth, the field description

The field description of magmatic rocks is an important task in the geological record of magmatic rocks. It is also important for indoor identification. The description should strive to be comprehensive, accurate, and refined so that the reader can see the description and have a more comprehensive and clear image of the rock. This requires careful observation, description of language specifications, accurate use of geological terms, and adherence to the order of description.

The main contents and order of description of magmatic rocks in the wild are: color, structure, structure, types and contents of major minerals and secondary minerals, secondary changes, and occurrence.

When describing colors, distinguish between primary colors and secondary colors, and describe them separately. Note the change in rock color caused by groundwater leaching or sedimentation of groundwater and assimilation of contamination in the vicinity of the joint or contact zone.

When describing the structure of the rock, care should be taken to separate the phenocryst structure from the matrix structure and to estimate the phenocrytic content and the content of different minerals in the phenocryst.

In the description of minerals, in addition to the description of the main characteristics and content of minerals, the secondary changes of minerals should be given full attention.

Example of magma rock field description

1. Huiluo Xiuyan: The fresh surface is gray-black, the weathered surface is gray-green, and the plaque-like structure. The matrix is ​​medium-fine grain merging structure, massive structure, and the main minerals are pyroxene and plagioclase. A small amount (5%) of plagioclase constitutes phenocrysts. On the weathered surface, the plagioclase is a self-shaped elongated crystal. The pyroxene is filled in the pores of the plagioclase crystal. The plagioclase and the pyroxene account for about half of the rock and a small amount of pyrite. At the surface, it has been weathered into limonite, and feldspar has been weathered into kaolinite.

2. Granite: the fresh surface is grayish yellow, the weathered surface is often yellowish brown; the medium and coarse grain structure, block structure; the main minerals are potassium feldspar (60%) and quartz (30%); the minor minerals are amphibole and Black mica (10%); potassium feldspar twin crystal is not obvious, the color is grayish yellow, it is granular; quartz is smoky gray, oily luster, it is granular; hornblende is black, granular; biotite is golden yellow after weathering, Pearl luster, flakes, a set of extremely complete cleavage.

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