Mineral phase
IMA name
Gypsum
Secondary names
Selenite
Class
non-silicate mineral
Compound type
sulfate
Comments
commonly used non-standard space groups: A2/a, A1, I12, I2/a
Formula
$Ca(SO_4) \bullet 2(H_2O)$
Chemical formula
CaSO4.2(H2O)
Elemental formula
Ca S O6 H4
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Ca$ Calcium element 1 constituent element main
$S$ Sulfur element 1 constituent element main
$O$ Oxygen element 6 constituent element main
$H$ Hydrogen element 4 constituent element main
Composition comments
Slightly soluble in water. Poorly deliquescent > 99% RH at 20°C
Chemical functions
  Function Number
$(SO_4)^{2-}$ 1
Chemical bonds
  Bond Number
$\dddot - \dddot - \dddot - S \dddot - O^{-1/2}$ 4
Oxides
Formula $CaO$ $SO_{3}$ $H_{2}O$
Mass fraction (%) 32.57 46.5 20.93

Strunz

Class
sulfates - chromates - molybdates - tungstates (07)
Division
sulfates (selenates, etc.) without additional anions, with H2O (07.C)
Family
with only large cations (07.CD)
Code
07.CD.40

Dana

Major class
sulfates, chromates, selenates (VI)
Class
hydrated acid and sulfates (29)
Type
where AXO4.x(H2O) (29.06)
Group
-Gypsum- group (Dihydrates, monoclinic) (29.06.03)
Code
29.06.03.01
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/c
Crystal sites
Label Nb Type Wp Specie Specie coordinence Specie occupancy Specie oxidation state
Ca 1  $Ca$ 1.0
S 1  $S$ 1.0
O1 2  $O$ 1.0
O2 2  $O$ 1.0
Ow 2  $O$ 1.0
H1 2  $H$ 1.0
H2 2  $H$ 1.0
Composition order
ordered
Molar mass
172.17 $g/mol$
Density
2.31 $g/cm^3$
State NTP
solid
Refringence type
biaxial
Refringence sign
positive
Birefringence
0.0090 - 0.0100
Refraction index $n_{\alpha}$
1.519 - 1.521
Refraction index $n_{\beta}$
1.522 - 1.523
Refraction index $n_{\gamma}$
1.529 - 1.530
Dispersion V
strong
Pure color
colorless
Color
colorless, white, yellowish white, greenish white, brown
Diaphaneity
transparent to translucent
Luster
pearly