Gypsum

Gypsum
Gypsum
General
Category	Sulfate minerals
Formula; (repeating unit)	CaSO4·2H2O
IMA symbol	Gp
Strunz classification	7.CD.40
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) ; H-M symbol: (2/m)
Space group	Monoclinic ; Space group: I2/a
Unit cell	a = 5.679(5), b = 15.202(14) ; c = 6.522(6) Å; β = 118.43°; Z = 4
Identification
Color	Colorless (in transmitted light) to white; often tinged other hues due to impurities; may be yellow, tan, blue, pink, dark brown, reddish brown or gray
Crystal habit	Massive, flat. Elongated and generally prismatic crystals
Twinning	Very common on {110}
Cleavage	Perfect on {010}, distinct on {100}
Fracture	Conchoidal on {100}, splintery parallel to [001]
Tenacity	Flexible, inelastic
Mohs scale hardness	1.5–2 (defining mineral for 2)
Luster	Vitreous to silky, pearly, or waxy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.31–2.33
Optical properties	Biaxial (+)
Refractive index	nα = 1.519–1.521; nβ = 1.522–1.523; nγ = 1.529–1.530
Birefringence	δ = 0.010
Pleochroism	None
2V angle	58°
Fusibility	5
Solubility	Hot, dilute HCl
References
Major varieties
Satin spar	Pearly, fibrous masses
Selenite	Transparent and bladed crystals
Alabaster	Fine-grained, slightly colored

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO₄·2H₂O.^[4] It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk.^[5]^[6]^[7]^[8] Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.

Fine-grained white or lightly tinted forms of gypsum known as alabaster have been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England.

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2003). "Gypsum" (PDF). Handbook of Mineralogy. Vol. V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America. ISBN 978-0962209703. Archived (PDF) from the original on 6 February 2006.
^ Gypsum. Mindat
^ ^a ^b Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1985), Manual of Mineralogy (20th ed.), John Wiley, pp. 352–353, ISBN 978-0-471-80580-9
^ Institute, Canadian Conservation (14 September 2017). "Care of Objects Made of Plaster of Paris – Canadian Conservation Institute (CCI) Notes 12/2". www.canada.ca. Retrieved 20 January 2023.
^ Make your own sidewalk chalk. (1998, July 21). Christian Science Monitor. 13.
^ "Plaster | Definition, Uses, Types, & Facts | Britannica". www.britannica.com. Retrieved 20 January 2023.
^ "Definition of DRYWALL". www.merriam-webster.com. Retrieved 20 January 2023.