Terlinguaite

Terlinguaite is the naturally occurring mineral with formula Hg4+3Hg²⁺Cl₂O₂. It is formed by the weathering of other mercury-containing minerals. It was discovered in 1900 in the Terlingua District of Brewster County, Texas, for which it is named.^[5] Its color is yellow, greenish yellow, brown, or olive green.

Terlinguaite
Terlinguaite, collected from Mariposa Mine, Terlingua District, Brewster County, Texas, United States
General
Category	Halide mineral
Formula	Hg4+3Hg2+Cl2O2
IMA symbol	Tlg[1]
Strunz classification	3.DD.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 19.51 Å, b = 5.91 Å c = 9.47 Å; β = 143.81°; Z = 4
Identification
Color	Sulfur-yellow, greenish yellow, brown
Crystal habit	Aggregates of equant to elongated crystals, powdery, massive
Cleavage	Perfect on [101]
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Brilliant adamantine
Streak	Lemon-yellow, turning olive-green
Diaphaneity	Transparent to translucent
Specific gravity	9.22
Optical properties	Biaxial (-)
Refractive index	nα = 2.350 nβ = 2.640 nγ = 2.660
Birefringence	δ = 0.310
Pleochroism	Weak, green and yellow
2V angle	Measured: 20°
Alters to	turns olive-green on exposure to light
References	[2][3][4]