3 Photos
Gaylussite
Chemical Formula	Na2Ca(CO3)2·5H2O
Species	Carbonates
Crystal System	Monoclinic
Mohs Scale	2
Specific Gravity	1.93-1.99
Color	Colorless, white, yellow, and grey
Streak	white
Luster	Vitreous
Refractive Index	n = 1.444 n = 1.516 n = 1.523
Diaphaneity	Transparent, Translucent
Cleavage	PerfectOn perfect; rather difficult.
Fracture	Conchoidal
Crystal Habit：Tabular prismatic crystals also granular
Geological Setting：Soda lakes.

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.

It is formed as an evaporite from alkali lacustrine waters. It also occurs rarely as veinlets in alkalic igneous rocks.

It was first described in 1826 for an occurrence in Lagunilla, Mérida, Venezuela. It was named for French chemist Joseph Louis Gay-Lussac (1778-1850).