Syngenite

Syngenite
Tapering crystal of syngenite (size: 4.4 x 1.3 x 0.6 cm)
General
Category	Sulfate mineral
Formula (repeating unit)	K2Ca(SO4)2·H2O
Strunz classification	7.CD.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Unit cell	a = 9.77 Å, b = 7.14 Å c = 6.25 Å; β = 104.01°; Z = 2
Identification
Color	Colorless, milky white to faintly yellow due to inclusions
Crystal habit	Tabular to prismatic crystals, lamellar aggregates and crystalline crusts
Twinning	Common on {101} contact twins
Cleavage	Perfect on {110} and {100}, distinct on {010}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.579–2.603
Optical properties	Biaxial (-)
Refractive index	nα = 1.501 nβ = 1.517 nγ = 1.518
Birefringence	δ = 0.017
2V angle	Measured: 28°
References

Syngenite is an uncommon potassium calcium sulfate mineral with formula K₂Ca(SO₄)₂·H₂O. It forms as prismatic monoclinic crystals and as encrustations.

It was first described in 1872 for an occurrence as druse on halite in the Kalusa Salt deposit, Ivanovo-Frankovsk Oblast', Ukraine. The name is from Greek 'συγγενής' (related) due to its chemical similarity to polyhalite.

It occurs in marine evaporite deposits as a diagenetic phase. It also forms as a volcanic sublimate, as vein fillings in geothermal fields and in caves where it is derived from bat guano. It occurs in association with halite and arcanite in salt deposits; and with biphosphammite, aphthitalite, monetite, whitlockite, uricite, brushite and gypsum in cave environments.