Origin of Name
From the Latin word albus, meaning “white.”
Chemical Formula - NaAlSi₃O₈
Hand Specimen Identification
Cleavage, hardness, luster, association, and fine polysynthetic twinning help identity albite and other plagioclase feldspars. If not twinned, it may be diffcult to tell from K-feldspar. Distinguishing albite from other plagioclase feldspars cannot be done precisely without detailed X-ray or optical data.
Physical Properties
| Hardness | 6 |
| Specific Gravity | 2.62 |
| luster/transparency | pearly, vitreous/translucent |
| Color | White, Gray, Green |
| streak | white |
| Fracture | Uneven |
| Cleavage | Perfect (001), Good (010), Poor (110) |
Optical Properties
In thin section, plagioclase shows no color, has low relief, and exhibits gray interference colors. It is similar 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 (+), alfa = 1.527, Beta = 1.531, gama = 1.538, delta = 0.011, 2V = 77°.
Crystallography
Triclinic, a = 8.14, b = 12.79, c = 7.16, alfa = 93.17%, Beta = 115.85°, gama = 87.65°, Z = 4; space group P1bar; point group 1.
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Habit
Masses or subhedral grains are common. Rare euhedral crystals are prismatic, tabular, or bladed. Most crystals are twinned according to the pericline law, and some are twinned by the albite law. Albite twins give plagioclase the characteristic polyaynthetic twinning that is often visible as fine atrlations 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 structure is more disordered. At very high temperature a completely disordered alhite, called monalbite because it is monoclinic, is stable. At all but the lowest temperatures, complete solid solution exists between albite and the other plagioclase end member, anorthite, CaAl2Si3O8. Albite, and other plagioclase feldspars, also form limited solid solutions with orthoclase, 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, sedimentary rocks. Most are intermediate between albite and anorthite, but compositions approaching end members are known. Albite, defined as plagioclase with greater than 90% NaAlISi3O8 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 plagioclase feldspars, and to the other alkali feldspars (orthoclase, sanidine, and microcline).
3D Structure
Quartz-albite-garnet-beryl (emerald) pegmatite in the WVU Petrology collection. Sampling location unknown. The garnet and beryl are intergrown. The garnet is rimed with biotite where it is in contact with the albite feldspar. Pegmatites often include unusual minerals that contain volatile elements like Be (beryl), Li (spodumene, lepidolite), and F (topaz); this is the result of them crystallizing very late from a magma saturated in volatiles, and probably at low temperatures.