It is also the main rock-forming mineral in the crust. The timely silica composition is close to 100%, and it can contain trace amounts (about 0. 1× 10-6) of iron, magnesium, aluminum, calcium, lithium, sodium and potassium, and often contains gas, liquid and solid inclusions. There is a frame structure, Ng∑c, with two variants. High-temperature β -strain is a hexagonal system with a hexagonal bipyramid with a short cylindrical surface, which is often corroded; Low-temperature α -strain is a triangular crystal system with columnar development. At atmospheric pressure, their phase transition temperature is 573℃, and all natural time is α-time. The original β -time has been transformed into α-time, but the illusion of β -time still exists. In the flake, it is colorless and transparent, and in the aggregate, it is mostly irregular granular, without cleavage, with positive and low protrusions and rough surface; The interference color is grade I yellow and white, and there is often wave extinction (note: wave extinction refers to the phenomenon that the lattice is twisted at a small angle (< 5)). Almost no damage to particles can be seen under single polarization; But under the orthogonal mirror, you can see the shadow with unclear interface. When rotating the stage, the shadow (extinction light) constantly scratches the crystal grain), and the uniaxial crystal is positive. No weathering products.
Second, the general characteristics of calcite
Calcite is widely distributed in nature and is the main mineral component of carbonate rocks, carbonate rocks and marble. Calcite is a carbonate compound of metal cation Ca2+, which can contain a small amount of Mn2+, Fe2+, Mg2+, Pb2+, Sr2+, Ba2+, Re, Co, etc. Calcite belongs to cubic crystal system, Np∑c, with irregular equiaxed grains, or rhombic crystals, or a hexagonal column, complex triangular polyhedron and rhombic polyhedron. Calcite flake is colorless and transparent, with rhombic cleavage, and the flicker process is negative to low; Advanced white interference color; Often with flaky twins (plate ⅵ-1); Negative optical properties of uniaxial crystals.
Third, the general characteristics of nepheline
Nepheline minerals are also important rock-forming minerals, especially in alkaline rocks. Nepheline minerals mainly include nepheline, potassium nepheline and cancrinite. Nepheline is widely exposed.
Comparing the chemical formula M [ALSIO4] of nepheline with the chemical formula M [Al (Al, Si) SiO8] of feldspar, we can see that it is characterized by poor SiO2. Therefore, nepheline is mainly produced in alkaline rocks rich in sodium and poor in silicon, and is considered as a sign mineral of poor silicon in rocks.
Nepheline belongs to hexagonal system, Np∑c, which is a skeleton aluminosilicate. Automorphic nepheline is hexagonal column, usually granular. In the thin slice, its distinguishing characteristics are: colorless and transparent, plate-like or granular, negative and low positive and low protrusion; Grade I gray interference color, parallel extinction, negative ductility; Negative optical properties of uniaxial crystals.
Four. Comparison table of plagioclase, potash feldspar and chronological optical characteristics
Crystal optics and rock-forming minerals
5. Optical orientation diagrams of Yingshi, nepheline and calcite (Figures 7-33, 7-34 and 7-35).
Figure 7-33 Time-dependent (Q) Optical Orientation
Figure 7-34 Optical Orientation of Nepheline (ne)
Figure 7-35 Optical Orientation (Cc) of Calcite
(1) crystal system _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
(2) Axial _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
(3) the optical character _ _ _ _ _ _ _ _ _ _ refers to the _ _ _ _ _ _ _ symbol in the optical character.
(4) Optical axis position: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.
(5) Extensible symbol _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
(6) Extinction type and extinction angle _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
(7) Shape and cleavage _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Six, thinking about the problem
1. What are the isomorphic characteristics of potash feldspar minerals and their main differences?
2. What's the difference between the optical properties of K-feldspar and K-feldspar?
3. What is the difference in optical characteristics between Shishi and nepheline?