How to determine the number of plagioclase?
a) Plagioclases albite,Na[AlSi3О8], and anorthite, Ca[A12Si208], are components of solid solutions. Accurate determination of plagioclase and its composition is an important diagnostic feature for identifying a rock group. All plagioclases are divided into the following types based on the content of albite and anorthite components (Table 6.1).
| №№ PP | Titles | Molecular content, ab % an | Composition |
| Albite | 90 – 100 | 0- 10 | acid plagioclase |
| Oligoclase | 70-90 | 10- 30 | acid plagioclase |
| Andesine | 50-70 | 30-50 | medium plagioclase |
| Labrador | 30 – 50 | 50-70 | basic plagioclase |
| Bytovnit | 10- 30 | 70-90 | basic plagioclase |
| Anorthitis | 0-10 | 90 – 100 | basic plagioclase |
The content of the anorthite molecule in plagioclase shows its number. The refractive indices of plagioclase closely match the composition of plagioclase (Figure 6.29). Therefore, using the immersion method, it is possible to accurately determine the composition of plagioclase. In terms of refractive indices, albite belongs to group II (according to Lodochnikov), oligoclase belongs to group III, and medium and basic plagioclases belong to group IV. Accordingly, the relief, shagreen surface, and Becke stripe can be different in different plagioclases. 
Figure 6.28 – Optical properties of plagioclases In general, plagioclases in thin sections are colorless, have low interference colors, and twins are observed. Interference colors do not occur above white even in different sections. Polysynthetic twins in crossed nicols are observed as parallel individuals. Neighboring individuals, the directions of the indicatrix axes of which have different names, also have different extinction angles. In igneous rocks, plagioclases form regular prisms or tabular shapes. In hypabyssal magmatites, plagioclases have long prismatic forms, the so-called laths. In effusive rocks, plagioclase occurs in the bulk in the form of small prismatic crystals, which are called microlites. In acidic plagioclases, polysynthetic twins are thinner, while in basic ones they are wider. In medium plagioclases, their zonal structure is often observed, which indicates a change in the composition of plagioclase, corresponding to the zones in which changes in both optical properties and the orientation of the optical indicatrix are observed. Under a microscope, zoning of plagioclase can be observed; width, angles and colors of interference of these zones. In zoned plagioclases, the content of anorthite molecules decreases from the center to the outside of the crystals. However, in some magmatites, especially in volcanic and hybrid rocks, it can, on the contrary, increase outward. The extinction angle changes with changes in the composition of plagioclase. This property is used to determine the name of plagioclase. Oligoclase has an almost right extinction angle; from albite to anorthite, the extinction angle increases. To more accurately determine the plagioclase number, it is necessary to use a section perpendicular to (010), (001). In this section, the twins exhibit symmetric extinction. The following is the procedure for determining the name of plagioclase: 1) we find a plagioclase twin, in which, when the twin seam is combined with the vertical thread of the eyepiece, all individuals should have the same interference color; 2) we measure the extinction angles for both systems of individuals relative to the twin seam. If the difference between these angles is no more than 5°, then we take the arithmetic mean. We carry out such measurements 5-6 times, and select the larger one from all the results obtained. 3) using the diagram (Figure 6.30), taking into account the genetic type, we find the composition and name of plagioclase corresponding to a certain extinction angle. At the same time, for plagioclases with an extinction angle of less than 20°, we also take into account the group relative to the refractive indices: for IV, we take the reading with a positive sign, for II, with a negative sign. 
Figure 6.30 – Determination of plagioclase number by extinction angle Secondary minerals develop from plagioclases, these include the processes of pelitization, sericitization, and saussuritization. Pelitization is the process of development of very fine clay minerals, which leads to a change in the color of the surface of plagioclases, the surface becomes covered with a grayish or brownish coating. The entire crystal undergoes pelitization, but more often develops in spots. In crossed nicols, the pelitized mineral appears isotropic. Pelitization is characteristic mainly of albite, but also occurs in oligoclase. Sericitization is the process of development of fine-flaky muscovite, the so-called sericite aggregate. Sericitization is characteristic of oligoclase; in ansesine, it develops slightly. An aggregate of sericite with zoisite develops along andesine. Sericite in parallel nicols is poorly visible, in crossed ones it has a yellow interference color, sometimes red and blue colors are noted. The main plagioclases undergo a process of saussuritization; this is an aggregate of sericite, zoisite, epidote, and calcite. With one nicol, saussuritization is clearly visible in a clear relief, and with crossed nicols, it is clearly visible in the bright colors of the interference (Figure 6.31). In addition to the above-mentioned processes of secondary alteration in plagioclases, the development of other minerals is also possible: zeolites, scapolites, prehnite, chlorites, actinolite. In intrusive rocks, plagioclases can form special formations, the so-called myrmekites, which occur at the boundary of plagioclases and potassium feldspars. Myrmekites are fine-crystalline quartz aggregates that crystallized due to excess silicic acid during the formation of plagioclases from K-feldspars. Did you like the article? Bookmark it (CTRL+D) and don’t forget to share it with your friends: Plagioclases (from the Greek plágios – oblique and klásis – breaking, splitting) – a group of minerals of the NaAlSi albite series3O8 – anorthite CaAl2Si2O8 Common rock-forming minerals belonging to the group of framework aluminosilicates are feldspars. In terms of chemical composition, they represent a continuous isomorphic series of sodium-calcium aluminosilicates – albite and anorthite with unlimited miscibility. Sometimes they contain K2O (up to several percent), BaO, SrO, FeO, Fe2O3, etc. as impurities. According to the proposal of E. S. Fedorov, the composition of polymer is designated by numbers that express the percentage of the anorthite component in the polymer. For example, P. No. 72 is an isomorphic mixture containing 72% anorthite and 28% albite. Within the group, individual mineral species are distinguished according to the content of the anorthite component (or according to the ratio 100*Ca/(Ca+Na) at.%):
| Name | %An | 2VNg | 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 |
| bitovnit | 70-90 | 86-98 | 1.564-1.572 |
| anorthitis | 90-100 | 98-105 | 1.572-1.577 |
With an increase in the anorthite component in plagioclases, the silica content decreases, and therefore P. from No. 0 to No. 30 are called acidic, No. 30-50 – medium and No. 50-100 – basic. Structure and morphology of plagioclases Plagioclases have a triclinic system and often have polysynthetic twins. 
Series of plagioclases at different temperatures Plagioclases are triclinic minerals. Albite, anorthite and some intermediate varieties each have several structural varieties that are close to each other, but still differ from each other in the metric and symmetry of the lattice (primitive, centered, transitional elementary cell, single and doubled have been identified). Therefore, isomorphism in plagiocalases appears differently at different temperatures.
Homogeneous plagioclases of the continuous albite-anorthite series are possible only at high temperatures. As the temperature decreases, structural transformations occur in plagioclases with the formation of several structural varieties. Plagioclase of bulk composition No. 5-25 consists of two feldspars (albite 0 and oligoclase 20-35). Onir form the finest lamellar accretion with each other. Such structures are called peristerites. Their presence is externally manifested in the blue internal glow of plagioclase – iridescence. Plagioclase of labradorite composition has a two-phase structure. The finest intergrowths of polagioclase 45 and plagioclase 50-60 were identified here. They are called accretion Böggilda. Plagioclase of this structure and composition shimmers brightly with peacock colors from the inside. accretion Huttenloscher composed of the thinnest plates of plagioclase 65 and one of the structural types of anorthite. physical properties Depending on the composition and degree of ordering of Al-Si in the structure, the properties of plagioclase change naturally over a wide range; from pure albite to pure anorthite they increase: density 2620-2760 k/m3, hardness on the mineralogical scale 6-6,5; Melting point 1100-1550 °C. Optical properties The refractive indices in the albite-anorthite series 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 a Fedorov stage, and using special diagrams of the dependence of the properties of the photons on their composition, it is possible to determine the number of the photons, i.e., its composition and degree of order. To quickly determine the plagioclase number, the Michel-Levy method is used. Occurrence in nature The main mass of magma 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 hornfels, etc.), as well as in hydrothermal veins (albite). Secondary changes During weathering, P. easily transforms into hydromicas, minerals of the epidote group, and clay minerals – kaolinite, montmorillonite, etc. Iridescent in bluish, blue and golden colors, oligoclase (moonstone) and labradorite are used as ornamental stones.
- Deere W.A., Howie R.A., Zusman J., Rock-forming minerals trans. from English, vol. 4, M., 1966
- ZLOBIN V.L., SONYUSHKIN V.E., TISLOV Y.S. Manifestation of iridescent plagioclase on the Anabar shield. Northern Siberia. – World of Stones, 1996, No. 11, pp. 42-43 (54).
- Marfunin A. S., Feldspars – phase relationships, optical properties, geological distribution, M., 1962
