Gypsum
Gypsum has been known since Antiquity and the name is derived from the Greek
'gypsos' meaning plaster - plaster as in plaster of Paris - and Gypsum is indeed the raw
material for plaster of Paris, filler, spackle, etc. Gypsum is hydrated calcium sulfate,
CaSO4¥2H2O, and when heated to around 120 °C it
looses most of the water (H2O) - this is, however, a reversible loss which
is used for the plaster of Paris. When the partly dehydrated Gypsum is mixed with
water, it swiftly incorporates water and forms Gypsum (well, a synthetic analogue of
Gypsum at least) - this can be used for casting, filling cracks, etc., and has been so since
antiquity. Modern products are often modified to allow slower setting, giving you more
time to manipulate the material.
Most Gypsum, and certainly all commercially used deposits, are formed as evaporitic
sediments - enclosed areas of sea water evaporating, precipitating Gypsum and other
salts. Gypsum is moderately soluble in water - around 2.2-2.4 g per liter - but is also
one of the first minerals to precipitate from evaporating sea water. Gypsum in
collections comes from various sources, and is often formed by the oxidation of sulfides
producing sulfuric acid, subsequently acting on Calcite and finally precipitating Gypsum
crystals in cavities. Remarkable Gypsum crystals are formed by the oxidation of Pyrite
in German lignite deposits, carrying large, transparent, idiomorphic crystals.
Most Gypsum in collections is in the form of 'desert roses', intergrown aggregates of
bladed Gypsum crystals, often incorporating sand, and not having any distinct faces and
sharp angles. Massive Gypsum is called Alabaster and often used for carving vases and
other crafts. Masses of fibrous Gypsum is called 'satin spar' and indeed produces a
strong silky sheen when carved and polished. Transparent Gypsum crystals are often
called Selenite, but please note neither Selenite nor Gypsum contains any selenium.
Gypsum itself is colourless or white, but it may incorporate grains of foreign minerals
giving sometimes quite vivid colours and patterns.
Gypsum is very soft, is easily scratched by a nail, and should be handled with great care.
It has a distinct cleavage running parallel to layers of water molecules (H2O) in the
crystal lattice.
Our records indicate that Gypsum from a range of deposits occurs on specimens that
also carry one or more of the following minerals: Ammoniojarosite, Anhydrite,
Antigorite, Apatite, Aragonite, Arsenopyrite, Atacamite, Basaluminite, Bianchite,
Bindheimite, Biotite, Blatonite, Bonattite, Boracite, Botryogen, Brianyoungite, Calcite,
Chalcanthite, Chalcopyrite, Chenevixite, Chrysocolla, Cinnabarite, Copper, Creedite,
Cuprite, Cuspidine, Devilline, Ferrimolybdite, Fluorite, Galena, Gaspeite, Gaudefroyite,
Glauconite, Goethite, Halite, Halotrichite, Hawleyite, Hematite, Hšrnesite,
Hydrohonessite, Hydrozincite, Jarosite, Kankite, Ktenasite, Livingstonite,
Magnesiochromite, Malachite, Marshite, Melanterite, Muscovite, Namuwite, Nantokite,
Natrojarosite, Ordonezite, Pyrite, Pyrolusite, Quartz, Rapidcreekite, Rhšnite,
Rockbridgeite, Ršsslerite, Schršckingerite, Schulenbergite, Siderite, Slavikite,
Sphalerite, Stibnite, Sulfur, Tamarugite, Tyrolite, Uranopilite, Wurtzite, and Zykaite.
Specimen Handling
Gypsum is for all practical purposes stable in a normal household environment. It is not
harmed by light, changes in temperature in the normal comfort range, or known to
decompose. Gypsum specimens can be brittle and should be handled with care like any
other mineral specimen. Gypsum is very soft and susceptible to mechanical damage from
dings or abrasion, and should not be wiped with brushes or cloth. Gypsum is slightly
soluble in water, and should not be washed.
Bibliography
Agricola, Georgius. 1556 (and subsequent editions, several languages). De Re
Metallica [translated by Herbert Clark Hoover & Lou Henry Hoover, 1912, 1950,
1986]
Anthony, John Williams, Sidney A. Williams, Richard A. Bideaux & Raymond
W. Grant. 1995. Mineralogy of Arizona, 3rd ed.
Blackburn, William H. & William H. Dennen. 1997. Encyclopedia of mineral
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Deer, W.A., R.A. Howie & J. Zussman. 1963. Rock forming Minerals.
Gaines, Richard W., H. Catherine W. Skinner, Eugene E. Foord, Brian Mason,
Abraham Rosenzweig & Vandall T. King. 1997. Dana's new mineralogy: the
system of mineralogy of James Dwight Dana and Edward Salisbury Dana, 8th
ed.
Hintze, Carl (ed.) 1929-1930. Handbuch der Mineralogie, vol. 1, section 3, part
2.
Hintze, Carl (ed.) 1931-1933. Handbuch der Mineralogie, vol. 1, section 4, part
2.
Noe-Nygaard, Arne. 1966. Mineralogi, 3rd ed.
Palache, Charles, Harry Berman & Clifford Frondel. 1951. The system of
mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University
1837-1892, 7th ed. Vol. 2.
Panczner, William D. 1987. Minerals of Mexico.
Ramdohr, Paul & Hugo Strunz. 1980. Klockmann's Lehrbuch der Mineralogie,
16th ed.
Roberts, Willard Lincoln, Thomas J. Campbell & George Robert Rapp jr. 1990.
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This page is authored by Claus Hedegaard.
