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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)
(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
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[edit]

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]