Gaylussite
General
Category	Carbonate mineral
Formula (repeating unit)	Na2Ca(CO3)2·5H2O
IMA symbol	Gyl[1]
Strunz classification	5.CB.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	I2/a
Identification
Color	Colorless, white, yellow, and grey
Crystal habit	Tabular prismatic crystals also granular
Cleavage	perfect [110]
Fracture	Conchoidal, brittle
Mohs scale hardness	2.5
Luster	vitreous
Streak	white
Specific gravity	1.93 – 1.99
Optical properties	Biaxial (−)
Refractive index	nα = 1.444 nβ = 1.516 nγ = 1.523
Birefringence	δ = 0.079
Solubility	decomposes in water
Other characteristics	efflorescent
References	[2][3][4]

Gaylussite is a carbonate mineral, a hydrated sodium calcium carbonate, formula Na₂Ca(CO₃)₂·5H₂O. It occurs as translucent, vitreous white to grey to yellow monoclinic prismatic crystals. It is an unstable mineral which dehydrates in dry air and decomposes in water.^[2]

Discovery and occurrence

It is formed as an evaporite from alkali lacustrine waters. It also occurs rarely as veinlets in alkalic igneous rocks.^[2] It was first described in 1826 for an occurrence in Lagunillas, Mérida, Venezuela. It was named for French chemist Joseph Louis Gay-Lussac (1778–1850).^[3]

The mineral has been recently (2014) reported from drill core in Lonar lake in Buldhana district, Maharashtra, India. Lonar lake was created by a meteor impact during the Pleistocene Epoch^[5] and it is one of only four known hyper-velocity impact craters in basaltic rock anywhere on Earth.^[6][7]