Origin of Name
From the Greek word sanis (tablet) and idios (appearance), referring to this mineral’s typical habit.
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.
Physical Properties
| Hardness | 6 |
| Specific Gravity | 2.56 |
| luster/transparency | vitreous/transparent to translucent |
| Color | White, Variable |
| streak | white |
| Cleavage | perfect (001), Good (010) |
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 (—), Alfa = 1.521, Beta = 1.525, Gama = 1.528, Dealta = 0.007, 2V varies depending on structure.
Crystallography
Monoclinic, a = 8.56, b = 13.03, c = 7.17, Beta = 116.58°, Z = 4, space group C2/m; point group 2/m.
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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 consists of a three-dimensional framework of SiO4 and AlO4 tetrahedra. In sanidine the two kinds of tetrahedra 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 (NaAISi3O8) is possible at high temperature.
Occurrence and Associations
Sanidine occurs in silicic igneous rocks but is restricted to rocks that have cooled quickly. If cooling is slow, orthoclase will be present instead. Typical occurrences are as phenocrysts in rocks such as trachyte or rhyolite.
Related Minerals
Related minerals include the other KAISi3O8 polymorphs, orthoclase and microcline, and the plagioclase feldspar series.
3D Structure
Size: 7.5 x 6.4 x 1.8 cm
Formula: K(AlSi3O8)
Type: Igneous extrusive (volcanic) rock
Description: The extrusive, aphanitic, often porphyritic, equivalent of syenite, composed mainly of alkali feldspar and minor mafic minerals (biotite, hornblende, or pyroxene). As there are two principal groups of syenite, there are likewise two groups of trachyte: alkali trachyte and calc-alkali trachyte. (Gk. trachys, rough; from its rough appearance).
In this specimen, the tabular crystal of Sanidine is embedded in Trachyte matrix showing the typical striations.
Essential minerals: alkali feldspar, biotite, pyroxene, amphibole, (± plagioclase less than 10% of the total feldspar).
Accessory minerals: zircon, titanite, apatite, etc.