Plagioclases (from ancient Greek πλάγιος - oblique and κλάσις - breaking, splitting) - a group of minerals of the isbitic series of albite - anortite . They have rock-forming significance, belong to the group of frame aluminosilicates - feldspar . By chemical composition, they are a continuous isomorphic series of Na - Ca aluminosilicates — albite and anorthite — with unlimited mixture . In the form of impurities, they sometimes contain an isomorphic impurity K (up to a few percent, with the amount of K usually being inversely correlated with the content of the anortite minal ), Ba, Sr, impurities FeO, Fe 2 O 3 , etc.
| Plagioclase | |
|---|---|
 | |
| Formula | NaAlSi3O8 - CaAl2Si2O8 |
| Color features | |
| Hardness | 6 |
| Density | 2.62 (albite) - 2.76 (anortite) g / cm³ |
| Syngony | Triclinic |
At the suggestion of E. S. Fedorov, the composition of plagioclases is designated by numbers, which express the percentage of anortite component in the plagioclase. For example, plagioclase No. 72 is an isomorphous mixture containing 72% anorthite and 28% albite. Within the group, separate mineral species are distinguished according to the content of the anortite component (or relative to 100 * Ca / (Ca + Na) at.%). With an increase in the anorthite component in plagioclase, the silica content decreases. In this connection, acidic (no. 0-30), medium (no. 30-50) and basic (no. 50-100) plagioclases are isolated.
Content
Structure and morphology of plagioclase
Plagioclases have a triclinic syngony , often exhibiting polysynthetic twinning .
Physical Properties
Depending on the composition and degree of ordering of Al-Si in the structure, the properties of a plagioclase vary naturally over a wide range; from pure albite to pure anortite, the density increases: 2620–2760 kg / m3, mineralogical hardness 6–6.5; Melting point 1100–1550 ° C.
| Title | % An | 2V Ng | Np |
|---|---|---|---|
| albite | 0-10 | 80-85 | 1.527-1.532 |
| oligoclase | 10-30 | 85-96 | 1.532-1.543 |
| andesine | 30-50 | 96-80 | 1.543-1.553 |
| Labrador | 50-70 | 80-86 | 1.553-1.564 |
| bitovnite | 70-90 | 86-98 | 1.564-1.572 |
| anortitis | 90-100 | 98-105 | 1.572-1.577 |
Optical properties
The refractive indices in the series of albite-anortite vary from 1.53 to 1.58. By studying the refractive indices, the angle of the optical axes, the position of the optical indicatrix, the laws of twinning and other optical properties using a polarizing microscope using the Fedorov table and using special diagrams of the dependence of the properties of plagioclase on their composition, one can determine the number of plagioclase, that is, its composition and degree of order . To quickly determine the number of plagioclase using the method of Michel Levy .
Spread in nature
The main mass of plagioclase is formed during the crystallization of magma ; they are part of igneous rocks as the most important rock-forming minerals. They are also found in contact-metamorphic formations (for example, in hornfelses, etc.), in hydrothermal veins (albite), in sedimentary rocks, where they fall during weathering of other rocks.
Space Spread
The content of plagioclase in the soil sample delivered by the Hayabus apparatus from the Itokawa asteroid is 13%. It is the second most abundant mineral in the sample, after olivine (39%). [one]
Secondary changes
When weathered, plagioclases easily pass into hydromica, epidote group minerals, and clay minerals — kaolinite , montmorillonite , etc.
Application
Oligoclase ( moonstone ) and labrador , and plagioclases with regular outgrowths of small scales of mica or hematite ("solar stone"), iridescent with bluish, blue and golden color, are used as ornamental stones . Kaolin formed during the weathering of plagioclase is a valuable raw material for the ceramic, paper and pharmacological industry.
Literature
- Dir W.-A., Howie R.-A., Zusman J., Rock-forming minerals, trans. from English, vol. 4, M., 1966
- Marfunin A. S. , Feldspar - phase relationships, optical properties, geological distribution, M., 1962