Glauberite

Glauberite is a monoclinic sodium calcium sulfate mineral with the formula Na₂Ca(SO₄)₂.

Glauberite
Glauberite crystal group from the Bertram Siding Sulfate deposit, Imperial County, California
General
Category	Sulfate minerals, anhydrous sulfate subgroup
Formula (repeating unit)	Na2Ca(SO4)2
Strunz classification	7.AD.25
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 10.129, b = 8.306 c = 8.533 [Å]; β = 112.19°; Z = 4
Identification
Color	Gray or pale yellow, colorless
Crystal habit	Tabular prismatic crystals
Cleavage	Perfect on {001}, imperfect on {110}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	2.5 - 3
Luster	Vitreous to waxy, pearly on cleavages
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.75–2.85
Optical properties	Biaxial (-)
Refractive index	nα = 1.507 - 1.515 nβ = 1.527 - 1.535 nγ = 1.529 - 1.536
Birefringence	δ = 0.022
2V angle	24° to 34°
Dispersion	strong r > v
Solubility	HCl and H2O (water) soluble
Alters to	readily alters to gypsum
Other characteristics	often a pseudomorph
References	[1][2][3]

It was first described in 1808 for material from the El Castellar Mine, Villarrubia de Santiago, Toledo, Castile-La Mancha, Spain. It was named for the extracted Glauber's salts after the German alchemist Johann Rudolf Glauber (1604–1668).[1]

Glauberite often forms in continental and marine evaporite deposits, but may also form from hydrothermal deposits, as mineral sublimates deposited near fumaroles, in amygdules in basalt, and in nitrate deposits in arid climates. It occurs associated with halite, polyhalite, anhydrite, gypsum, thenardite, mirabilite, sassolite and blodite.[3]

Because of its solubility, glauberite is often dissolved away from the crystal matrix leaving a distinctly shaped hollow cast. Its mineral composition is readily altered into other minerals as pseudomorphs. Gypsum pseudomorphs are common due to increased humidity.

Glauberite, its cast impressions, and its pseudomorphed crystals are often easily recognizable due to its common crystal twinning, and crystal habit displayed by uniquely shaped flattened, often seeming rhombohedral, large individual 'floater crystals'.