Plagioclase (Plagioclase) is a subfamily of class quality of albite and anorthite end parts of two members of the group consisting of the same stone as the series, i.e., _{NaAlSi 3 O 8 -CaAl 2 Si 2} O 8, at room temperature, but in some sections They are not miscible with each other, forming a microscopic continuous body of two-phase feldspar, but it is still not correctly regarded as a complete isomorphic series. The sub-Asian people are divided into six types:

Basic plagioclase changes due to their chemical composition, structural characteristics, physical properties, etc., so they are combined.

[Chemical composition] Or is often present in the composition of plagioclase and may contain a very small amount of Cn molecules. Generally speaking, the higher the An plagioclase, the less the Or molecule contains less than 5%, but the one with less An is slightly more. After analysis, it was also found that plagioclase contains a small amount of Ti, Fe ³⁺ , Fe ²⁺ , Mn, Mg, Sr and the like. Ti and Fe ³⁺ should replace Al in the structure, while other ions, if not mixed, should replace Ca in the structure.

[Crystal structure] triclinic system; sodium feldspar: ; a ₀ = 0.814 nm, b ₀ = 1.279 nm, c ₀ = 0.715 nm; α = 94 ° 13', β = 116 ° 31 ', γ = 87 ° 42; Z = 4. Anorthite: C1i-P1 and I1; a ₀ = 0.818 nm, b ₀ = 1.288 nm, c ₀ = 1417 nm; α = 93 ° 10', β = 115 ° 51 ', γ = 91 ° 13'; Z = 8. It can be seen from the above that the space groups of anorthite and albite are P1 and C1, respectively, and the c ₀ value of anorthite is twice that of albite, indicating that the difference in the structure of the two members is large, resulting in some The immiscibility of the interval.

[Form] Single crystal is parallel {010} extended, plate-like, sometimes extending along the a-axis, but rarely extending along the c-axis (Fig. G-52). There is a leaf-shaped albite, called cleavelandite, whose leaves are also parallel (010) and formed under high temperature conditions. For example, extending along the b-axis, called pericline, is formed under low temperature conditions.

Figure G-52 plagioclase crystal

Figure G-53 Polycrystalline twin crystals of plagioclase ((010) is a double crystal plane

(quoted from Cornelius S. Hurlbat, JR., 1977)

(a) Polycrystalline twin crystal model, (b) plagioclase specimen with polycrystalline twin crystal

The plagioclase has a variety of twin crystals, the most common being the albite law and the sorghum feldspar law. Except for a few albite formed by spontaneous action, it is extremely rare that plagioclase does not appear in the agglomerate. This polycrystalline twin crystal, each monomer is very thin, generally in micrometers, and when developed well, it can be seen on the {001} cleavage surface (Fig. G-53). No, it is usually done by a polarizing microscope. Kasbah is also quite common, with fewer Bavenor and Mani Baba. It is worth noting that the feldspar often has a sodium feldspar Casbah composite twin, and the case of this composite twin is described above. Unlike the lattice twin crystals, the two twin crystal laws coexist and produce a new twin crystal law. The joint surfaces of these three twin crystal laws are {010}.

[Physical properties] White or grayish white, if other colors appear, it is often caused by impurities; glass luster. {001} and {010} are completely cleaved. Hardness 6 to 6.5. The relative density is 2.61 to 2.76. Many physical properties of plagioclase such as relative density and refractive index change with the regular change of composition. For example, the relative density of those with high Ab is small, and the more molecules containing An, the greater the relative density. In addition, there are several variations depending on the structure and physical properties:

Labradorite: A rainbow effect occurs due to the interference of light by the polycrystalline twin structure.

Sun stone (sunstone):] Since the uniformly distributed, fine inclusions of orientation (hematite, goethite, green, mica, etc.) to produce a flash.

[genesis and occurrence] plagioclase is a widely distributed rock-forming mineral. High-temperature plagioclase is produced in some volcanic rocks and superficial diagenes. Low-temperature plagioclase is produced in deep diagenetic and regional metamorphic rocks. The plagioclase varies with the type of igneous rock. The plagioclase is usually divided into three categories: acidic, neutral and basic, with the boundary between the two points being An ₃₀ and An ₅₀ . Less than 30 are acidic plagioclase, more than 50 are basic plagioclase, and the middle is plagioclase. Acidic plagioclase is produced in acidic and alkaline rocks. Neutral plagioclase is produced in neutral rocks, and basic plagioclase is produced in basic and ultrabasic rocks. Only albite or aragonite is found in the pegmatite. Only a few basic pegmatites have been found to have coarse-grained medium-basic plagioclase. The An-content of plagioclase formed during the process of regional metamorphism will increase with the deepening of metamorphism. The situation is similar to those formed under contact with metamorphic conditions.

The so-called sodium long petrochemical action in the hydrothermal alteration process is the process of forming albite or aragonite. There may be albite in the sedimentary rocks as authigenic minerals. There may also be plagioclase in the clastic rock, but it is far less common than alkaline feldspar.

[Identification characteristics] The various differences in plagioclase can be roughly distinguished from acidic, neutral and basic plagioclase according to the type and type of rock, but accurate and reliable identification generally depends on the optical rotation and X-ray test. data of.

[Main use] See the overview of the Changshi people.