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Tapering crystal of syngenite (size: 4.4 x 1.3 x 0.6 cm)
CategorySulfate mineral
(repeating unit)
Strunz classification7.CD.35
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Unit cella = 9.77 Å, b = 7.14 Å
c = 6.25 Å; β = 104.01°; Z = 2
ColorColorless, milky white to faintly yellow due to inclusions
Crystal habitTabular to prismatic crystals, lamellar aggregates and crystalline crusts
TwinningCommon on {101} contact twins
CleavagePerfect on {110} and {100}, distinct on {010}
Mohs scale hardness2.5
DiaphaneityTransparent to translucent
Specific gravity2.579–2.603
Optical propertiesBiaxial (-)
Refractive indexnα = 1.501 nβ = 1.517 nγ = 1.518
Birefringenceδ = 0.017
2V angleMeasured: 28°

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]


Syngenite can be artificially produced by the action of a potassium sulfate solution on gypsum.[4]


  1. ^ a b Handbook of Mineralogy
  2. ^ a b c Syngenite on
  3. ^ a b Syngenite data on Webmineral
  4. ^ Ennaciri, Yassine; Alaoui-Belghiti, Hanan El; Bettach, Mohammed (May 2019). "Comparative study of K2SO4 production by wet conversion from phosphogypsum and synthetic gypsum". Journal of Materials Research and Technology. doi:10.1016/j.jmrt.2019.02.013. open access