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

Evaporite

Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...

 from alkali lacustrine

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 waters. It also occurs rarely as veinlets in alkalic igneous rock

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

s.

It was first described in 1826 for an occurrence in Lagunilla

Lagunilla

Lagunilla is a municipality in the Province of Salamanca, Spain. It is 95 km from the provincial capital of Salamanca.It is bordered by Aldeacipreste, Colmenar de Montemayor, Valdelageve, Sotoserrano and Zarza de Granadilla....

, Mérida

Mérida, Mérida

Santiago de los Caballeros de Mérida, Venezuela, is the capital of the municipality of Libertador and the state of Mérida, and is one of the principal cities of the Venezuelan Andes...

, Venezuela

Venezuela

Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...

. It was named for French chemist Joseph Louis Gay-Lussac

Joseph Louis Gay-Lussac

- External links :* from the American Chemical Society* from the Encyclopædia Britannica, 10th Edition * , Paris...

(1778-1850).