Analcime

Analcime or analcite (from the Greek analkimos - "weak") is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi₂O₆·H₂O. Minor amounts of potassium and calcium substitute for sodium. A silver-bearing synthetic variety also exists (Ag-analcite).

Analcime
Reddish crystals of analcime up to 1.8 cm in size on matrix
General
Category	Zeolite
Formula (repeating unit)	NaAlSi2O6·H2O
Strunz classification	9.GB.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Ibca
Identification
Color	White, colorless, gray, pink, greenish, yellowish
Crystal habit	Typically in crystals, usually trapezohedrons, also massive to granular.
Twinning	Polysynthetic on [001], [110]
Cleavage	Very poor [100]
Fracture	Uneven to subconchoidal
Mohs scale hardness	5 - 5.5
Luster	Vitreous
Streak	White
Specific gravity	2.24 - 2.29
Optical properties	Isotropic; anomalously biaxial (-)
Refractive index	n = 1.479 - 1.493
Fusibility	3.5
Other characteristics	Weakly piezoelectric; weakly electrostatic when rubbed or heated.
References	[1]

Analcime is usually classified as a zeolite mineral, but structurally and chemically it is more similar to the feldspathoids. Analcime occurs as a primary mineral in analcime basalt and other alkaline igneous rocks. It also occurs as cavity and vesicle fillings associated with prehnite, calcite, and zeolites.