Albite

Hand Specimen identification: Cleavage, hardness, luster, association, and fine polysynthetic twinning help identify albite and other plagioclase feldspars. If not twinned, it may be diffi-cult to tell from K-feldspar. Distinguishing albite from other plagioclase feldspars cannot be done precisely without detailed X-ray or optical data. Plate 6.8 shows rare euhedral albite.

Chemical Composition : NaAlSi3O8

Hardness : 6

Specific Gravity2.62

Cleavage/Fracture : perfect (001), good (010), poor

{110}/uneven

Luster/Transparency : vitreous/translucent

Color : white, gray, green

Streak : white

Optical Properties : In thin section, plagioclase shows no color, has low relief, and exhibits gray interference colors. It is sim-ilar to K-feldspar and superficially similar to quartz. However, cleavage, biaxial character, and “zebra stripes” caused by polysynthetic twinning usually serve to identify it. Biaxial 1+ 2, a = 1.527, b = 1.531, g = 1.538, d = 0.011, 2V = 77°. Plates 5.7 and 5.8 show plagioclase in thin section.

Crystallography : Triclinic, a = 8.14, b = 12.79, c = 7.16, a = 93.17°, b = 115.85°, g = 87.65°, Z = 4; space group P1; point group 1.

Habit : Masses or subhedral grains are common. Rare euhedral crystals are prismatic, tabular, or bladed. Most crystals are twinned according to thepericline law, and some are twinned by the albite law. Albite twins give plagioclase the characteristic polysynthetic twinning that is often visible as fine striations in hand specimen and as stripes in thin section

Structure and composition : Albite is an end member of both the plagioclase feldspar and the alkali felspar series. As with K-feldspar, ordering of AlO4 and SiO4 tetrahedra decreases with increasing temperature, leading to minor changes in structure. Low albite’s structure is similar to that of low microcline; high albite’s struc-ture is more disordered. At very high temperature a completely disordered albite, called monalbite be-cause it is monoclinic, is stable. At all but the lowest temperatures, complete solid solution exists between albite and the other plagioclase end member, anor-thite, CaAl2Si2O8. Albite, and other plagioclase feldspars, also form limited solid solutions with or-thoclase, KAlSi3O8.

Occurrence and associations : The most abundant mineral of the Earth’s crust, plagioclase feldspars are found in a wide variety of igneous, metamorphic, and, less commonly, sedi-mentary rocks. Most are intermediate between albite and anorthite, but compositions approaching end members are known. Albite, defined as plagio-clase with greater than 90% NaAlSi3O8, is found in silicic igneous rocks such as granite, syenite, trachyte, or rhyolite, where it associates with quartz and orthoclase.

Varieties : Clevelandite is a form of albite, typified by curved plates, found in pegmatites. Opalescent varieties of albite or other plagioclase feldspars are called moonstone.

Related Minerals : Albite is closely related to the other, more calcic pla-gioclase feldspars, and to the other alkali feldspars (orthoclase, sanidine, and microcline).

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Anorthite

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