Hand Specimen identification: Sanidine may be difficult to tell from other feldspars, but its restricted occurrence in felsic volcanic rocks and associations are helpful diagnostic tools. Certain identification requires X-ray analysis or thin sections. Plagioclase and microcline have different kinds of twins than sanidine, and sanidine never shows twin lamellae.

Chemical Composition : (K,Na)AlSi3O8

Hardness : 6

Specific Gravity2.56

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

Luster/Transparency : vitreous/transparent to


Color : white, variable

Streak : white

Optical Properties : Sanidine is similar to orthoclase but with greater 2V. Carlsbad twins may divide crystals into halves. Manebach and Baveno twins may also be present. Biaxial 1- 2, a = 1.521, b = 1.525, g = 1.528, d = 0.007, 2V varies depending on structure.

Crystallography : Monoclinic, a = 8.56, b = 13.03, c = 7.17, b = 116.58°, Z = 4; space group C 2/m; point group 2/m.

Habit : Crystals are prismatic, may be tabular or elongate, and often have a square cross section. Carlsbad twins are common.

Structure and composition : Similar to orthoclase, the structure of sanidine con-sists of a three-dimensional framework of SiO4 and AlO4 tetrahedra. In sanidine the two kinds of tetra-hedra are randomly distributed in the structure, while in orthoclase they are partially ordered. K+ ions occupy available holes between the tetrahedra. Na may replace K; complete solid solution between sanidine and albite 1NaAlSi3O82 is possible at high temperature.

Occurrence and associations : Sanidine occurs in silicic igneous rocks but is re-stricted to rocks that have cooled quickly. If cooling is slow, orthoclase will be present instead. Typical occurrences are as phenocrysts in rocks such as tra-chyte or rhyolite.

Varieties :

Related Minerals : Related minerals include the other KAlSi3O8 poly-morphs, orthoclase and microcline, and the plagio-clase feldspar series.

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Hand Specimen identification: The luster, hardness, and color of orthoclase are similar to other feldspars, …

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