Mirabilite

Mirabilite, also known as Glauber's salt, is a hydrous sodium sulfate mineral with the chemical formula Na₂SO₄·10H₂O. It is a vitreous, colorless to white monoclinic mineral that forms as an evaporite from sodium sulfate-bearing brines. It is found around saline springs and along saline playa lakes. Associated minerals include gypsum, halite, thenardite, trona, glauberite, and epsomite.

Mirabilite
General
Category	Sulfate minerals
Formula	Na2SO4·10H2O
IMA symbol	Mrb[1]
Strunz classification	7.CD.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Identification
Formula mass	322.9 g/mol
Color	Colorless, white, yellowish-white, greenish-white
Crystal habit	Granular or well-formed coarse crystals
Twinning	Interpenetration twinning on {001}; also on {100}
Cleavage	{100} perfect, {001} poor, {010} poor
Fracture	Conchoidal
Mohs scale hardness	1.5–2
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent to opaque
Specific gravity	1.49
Optical properties	Biaxial (–), 2V=75.93°
Refractive index	nα = 1.396, nβ = 1.4103, nγ = 1.419
Birefringence	δ = 0.023
Pleochroism	none
Other characteristics	Not radioactive, non-fluorescent
References	[2][3][4]

Mirabilite is unstable and quickly dehydrates in dry air, the prismatic crystals turning into a white powder, thenardite (Na₂SO₄). In turn, thenardite can also absorb water and converts to mirabilite.

Mirabilite is used as a purgative and anti-inflammatory remedy in the Traditional Chinese medicine; in Mandarin, it is called máng xiāo 硭硝. The name 'mirabilite' is based on the phrase "Sal mirabilis" (Latin for "wonderful salt") used by Johann Rudolph Glauber when he inadvertently synthesized mirabilite.^[4][5]

Mirabilite is found in several areas within the Mammoth Cave System, where it appears to have been mined by Late Archaic and Early Woodland peoples, probably for use as a laxative. ^[6]

Four mirabilite mounds were documented on the south shore of the Great Salt Lake, Utah, United States, in January 2020. These developed where springs surfaced along the beach, which had been exposed due to lower lake elevations, and cold air helped preserve the salt precipitate. This was documented by the Utah Geological Survey^[7] as well as reported in the press.^[8]

Crystal structure of mirabilite