Tapering crystal of syngenite (size: 4.4 x 1.3 x 0.6 cm)
Category Sulfate mineral
(repeating unit)
Strunz classification 7.CD.35
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group P21/m
Unit cell a = 9.77 Å, b = 7.14 Å
c = 6.25 Å; β = 104.01°; Z = 2
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 [1][2][3]

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

Discovery and occurrence

It was first described in 1872 for an occurrence as druse on halite in the Kalusa Salt deposit, Ivanovo-Frankovsk Oblast', Ukraine.[2] The name is from Greek 'συγγενής' (related) due to its chemical similarity to polyhalite.[3][2]

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.[1]


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