Gypsum

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Gypsum
Mineralogical name	Gypsum
Chemical name	Calcium sulfate dihydrate
Trivial name	plaster of paris, Alabaster, Satin Spar, Selenite
Chemical formula	Ca[SO4]•2H2O
Other forms	Anhydrite (CaSO4) Hemihydrate (CaSO4•0.5H2O)
Crystal system	monoclinic
Crystal structure
Deliquescence humidity 20°C
Solubility (g/l) at 20°C	2.14 g/l
Density (g/cm³)	2.2-2.4 g/cm³
Molar volume	74.69 cm3/mol
Molar weight	172.17g /mol
Transparency	transparent to opaque
Cleavage	perfect, clearly visible formation of fibre
Crystal habit	flat, prismatic, needle- like crystal, granular, massive aggregate
Twinning	very common
Phase transition
Chemical behavior	hardly soluble in water
Comments
Crystal Optics
Refractive Indices	α = 1.519-1.521 β = 1.522-1.523 γ = 1.529-1.530
Birefringence	Δ = 0.010
Optical Orientation	biaxial positive
Pleochroism	colorless
Dispersion	58°
Used Literature
{{{Literature}}}

Authors: Hans-Jürgen Schwarz , Nils Mainusch, Tim Müller
English Translation by Sandra Leithäuser
back to Sulfate

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Microscopy

Laboratory examination: Gypsum is slightly water soluble, therefore gypsum-containing sample material only dissolves slightly, when mixed with distilled Water. In solution, gypsum- containing sample material recrystallizes by carefully concentrating the solvent. At first, single needles form, then increasingly needle- like gypsum aggregate in proximity of the seam of the solvent emerges. Alternatively, sample material can be dissolved in hydrochloric acid, which also leads to the formation of crystal needles. Compared to other salts that can recrystallize in needle-like shapes, e.g. sodium carbonate, gypsum needles are clearly shorter.

Refraction indices:    n_x = 1.521; n_y =1.523; n_z =1.530
birefringence:      Δ = 0.009
crystal class:            monoclinic

Polarized light microscopy examination:
Apart from the typical acicular habit of gypsum crystals, (especially in recrystallized material) different morphological characteristics appear. These can be useful for identifying gypsum. Gypsum particles (in raw material samples) display shapes of rounded fragments and plate- like rhombohedra, clearly showing the inner cleavage planes. Furthermore, the occurrence of twinning shapes is typical for gypsum crystals, whether they are lath- shaped, tabular or lamellar. The assignment of refractive indices is carried out in accordance with the immersion method using media with indices nD=1,518 und nD=1,53. Due to the often small- scale particles the examination using the Schoeder van der Kolk method is more significant and reliable than the Becke- Line test. Gypsum crystals belong to the class of monoclinic crystals. Thus, they show, depending on the orientation of the single particle under the microscope, a parallel or respectively a symmetrical extinction, but mainly exhibit a characteristicly oblique axis position in the extinction position. On well developed crystal rhombi the oblique extinction can clearly be measured. Of all calcium sulfate crystals, gypsum has the lowest birefringence. Under crossed polarizers, gypsum has very low interference colors, lying within the gray to yellowish white range of the first order, (of course depending on the thickness of the particles).

Possibility for mistakes:
The Analysis methods mentioned above clearly identify gypsum, provided the following evaluation criteria are explicitly clarified.

• low water solubility
• characteristic needle- like morphology of the recrystallized particles
• all observable indices have a n_D –value from 1,518 and 1,530
• gypsum crystal show low interference colors
• gypsum crystals have an oblique extinction

Table 1: Salt phases with a gypsum- like chemical and optical properties

salt phase	differentiating features
Syngenite K2Ca(SO4) • 2H2O	all observable indices; 1,518
Tachyhydrite CaMg2Cl6 • 12H2O	mostly observable index < 1,518 / only parallel and symmetrical extinction
Hydromagnesite Mg5[OH(CO3)2]2 • 4H2O	mostly one index > 1,53

Literatur

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A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral