Marrite
The two arrows point to the marrite microcrystals (one or two at each arrow, dull), the rest is galena (bright)
General
Category	Sulfosalt mineral
Formula	PbAgAsS3
IMA symbol	Mrr[1]
Strunz classification	2.JB.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Unit cell	a = 7.29 Å, b = 12.68 Å, c = 5.99 Å; β = 91.22°; V = 553.57 Å3; Z = 4
Identification
Formula mass	486.19 g/mol
Color	Lead gray, steel gray
Crystal habit	Tabular or striated crystals
Twinning	Partly bent twin lamellae observable in polished section.
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3
Luster	Metallic
Streak	Black with brownish tint
Diaphaneity	Opaque
Specific gravity	5.82
Optical properties	Biaxial
Refractive index	Anistrophic
Pleochroism	White, with red internal reflections.
Other characteristics	Of hydrothermal origin along with other sulfosalts in dolomite.
References	[2][3][4][5]

Marrite (mar'-ite) is a mineral with the chemical formula PbAgAsS₃. It is the arsenic equivalent of freieslebenite (PbAgSbS₃), but also displays close polyhedral characteristics with sicherite and diaphorite.^[6] Marrite was first described in 1905,^[7] and was named in honor of geologist John Edward Marr (1857–1933) of Cambridge, England.^[5]

Marrite is part of the monoclinic crystal class, and ²⁄_m point group.^[4] The symmetry reveals that this mineral is composed of 3 axes of unequal lengths. Two of the axes are perpendicular at 90 degrees, while one axis intersects at an angle less than 90 degrees. Crystal habit includes striated, meaning it forms parallel lines along crystal faces; or tabular, meaning that structure dimensions are thin in 1 direction.^[4]

Under a microscope, marrite has a distinct anisotropic refractive index, meaning the velocity of light varies depending on the direction being traveled through the mineral. It is typical of anisotropic minerals to display colorful, optical properties when viewed under cross polarized light. This differs from its vague gray color when viewed in plane polarized light, which is the reason optical mineralogy is essential for correctly identifying similar minerals.^[3] Marrite belongs to the biaxial optical class, which signifies that it has two optic axes. An optic axis is the direction of light that travels at 0 birefringence, causing the mineral to appear isotropic. When viewing the indicatrix of biaxial minerals, both optic axes are always perpendicular to one of the two circular sections.

The only known occurrence of Marrite is the type locality of the Lengenbach quarry in Binntal, Valais, Switzerland. It is primarily formed because of hydrothermal activity, which involves water and high temperatures. Marrite is predominantly found in dolomite along with a variety of other sulfosalts.^[5]