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Gypsum - CaSO₄*2H₂O

Gypsum is the most abundant sulphate mineral; it often occurs in extensive masses of great thickness, in association with Iimestones and shales and in evaporite deposits. Among the various forms in which gypsum occurs are: selenite, a clear crystalline variety well known for its use in a testing instrument with the polarizing microscope (gypsum plate); alabaster, a fine-grained massive variety used as an ornamental stone; and satin-spar, a fibrous variety.

First known mention of Gypsum is by Theophrastus about 300-325. The name gypsum is from the Greek gypsos, which was probably applied to the mineral itself or to the product of calcination. Selenite is from the Greek selenites, moon-stone; alabaster, from alabastros, is said to have been named after a town in Egypt.

In the structure of gypsum, determined by Wooster (1936), two sheets of SO4 ions parallel to (010) are bonded together by Ca ions, and these double sheets are separated by layers of water molecules; each Ca ion is coordinated by eight oxygens, six belonging to SO4 tetrahedra and two to water molecules. The water molecules lie near the centres of triangles formed by a calcium ion and two oxygens (of sulphate groups) to which they are hydrogen bonded.

Paragenesis

The principal modes of formation of gypsum are direct deposition by the evaporation of brine, by hydration of anhydrite, by oxidation of sulphides and by the action of acid sulphate solutions on calcium-bearing rocks.

Gypsum-anhydrite deposits exhibit a range of different textures including lamellar, nodular, pegmatitic (lensoid selenite crystals) and massive. Sea-water contains about 3,5 Wt% of dissolved materials, about 80% of which is sodium chloride and about 4% calcium sulphate. The usual sequence of deposition of salts from sea-water has been shown experimentally to be: calcium carbonate; calcium sulphate; sodium chloride; sulphates and chlorides of magnesium-sodium bromide and potassium chloride.

If all the salt of a 1000 m column of water were precipitated it would make only 15 m of salt deposits, of which about 0.4 m would be calcium sulphate, 11.6 m halite and the remainder potassium and magnesium-bearing salts. It is not essential, however, to have deep basin conditions for the formation of calcium sulphate; it can occur in a lagoonal environment, and in salt flat or sabkha conditions like those of the present day in areas such as the Trucial Coast and the Gulf of California.

Evaporating column of seawater.

In evaporites, the calcium sulphate sometimes occurs as gypsum, sometimes as anhydrite, and very often both minerals together. Different opinions have been expressed with reference to particular localities as to whether the gypsum or the anhydrite was the first to precipitate from solution. Some geological evidence indicates that the original material of most gypsum beds was anhydrite. This view is apparently supported where gypsum beds are observed to grade into anhydrite at depth, and by textural relationships which indicate that gypsum is formed later than anhydrite; in both cases, however, the anhydrite itself may have resulted from the dehydration of gypsum. Later opinions have allowed for the possibility that either gypsum or anhydrite may occur as the primary deposit.

Gypsum sometimes forms in sandy areas, and crystals may trap sand inside when forming, causing a specimen to become brown or gray and opaque. These sand inclusions sometimes form hourglass formations in a crystal. They are also present in the well-known Desert Rose, which is rosette shaped Gypsum with sand inclusions. (The term Desert Rose also applies to rosette shaped Barite with sand inclusions, and the two should not be confused).

Giant gypsum crystals in Naica Mine. chihuaha, Mexico.

Giant gypsum crystals in Naica Mine. chihuaha, Mexico.

Gypsum, selenite variety.

Veins of satin-spar gypsum.

Gypsum, alabaster variety.

Desert rose gypsum variety.

Optical characteristics

• Colour: Colourless.
• Form: platy to coarse prismatic, or acicular, crystals.
• Cleavage: Perfect on (001), good on (010) and (100).
• Twinning: swallow-tail twins on (100) are common.
• Birifrangence: Low refraction and birefringence (grey to white interference colours of the 1st order, similar to K-feldspar).

Bibliography

• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc

Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)
Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)
Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 1x (Field of view = 9mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 1x (Field of view = 9mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless) and unidentified beige material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gray) and unidentified isotropic material. Pagherola Mine (Oltressenda Alta), Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. PPresolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum and calcite crystals in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)	Prismatic gypsum and calcite crystals in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (colourless and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. PPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 2x (Field of view = 7mm)
Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 10x (Field of view = 2mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 10x (Field of view = 2mm)	Prismatic gypsum crystals (I order gary and rich of carbonate inclusions) in a carbonatic rock. Presolana Mine, Bergamo, Italy. XPL image, 10x (Field of view = 2mm)
Anhydrite (high relief) and gypsum crystals. Roccastrada (Grosseto), Tuscany, Italy. PPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high relief) and gypsum crystals. Roccastrada (Grosseto), Tuscany, Italy. PPL image, 2x (Field of view = 7mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high relief) and gypsum crystals. Roccastrada (Grosseto), Tuscany, Italy. PPL image, 2x (Field of view = 7mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 2x (Field of view = 7mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high relief) and gypsum crystals. Roccastrada (Grosseto), Tuscany, Italy. PPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Anhydrite (high interference colours) and gypsum (I order gray) crystals. Roccastrada (Grosseto), Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). PPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). PPL image, 2x (Field of view = 2mm)
Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)
Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)	Prismatic gypsum crystals (I order gray). "Molino Mastricarro" mine (La Fiumarella river, Catanzaro, Calabria region, Southern Italy). XPL image, 2x (Field of view = 2mm)
Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)	Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL and compensator plate. Image, 2x (Field of view = 2mm)	Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)
Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)	Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)	Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)
Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)	Radial gypsum crystals (I order gray). Ortighera, Lombardia region (Northern Italy). XPL image, 2x (Field of view = 2mm)