Marrite

Marrite
General
Category	Sulfosalt mineral
Chemical formula	PbAgAsS3
Strunz classification	02.JB.15
Crystal symmetry	Monoclinic 2/m - Prismatic
Unit cell	a = 7.29 Å, b = 12.68 Å, c = 5.99 Å; β = 91.22°; V 553.57 ų; Z = 4
Identification
Molar mass	486.19 g
Color	Lead gray, steel gray
Crystal habit	Tabular or striated crystals
Crystal system	Monoclinic
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	[1][2][3]

Marrite (mar'-ite) is depicted by the chemical formula PbAgAsS₃. It is the arsenic equivalent of mineral freieslebenite (PbAgSbS₃), but also displays close polyhedral characteristics with sicherite and diaphorite.^[4] Marrite was named in honor of geologist John Edward Marr (1857–1933) of Cambridge, England.^[3]

Crystal habit

Marrite is part of the monoclinic crystal class, and ²⁄_m point group.^[2] 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.^[2]

Optical mineralogy

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.^[1] 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.

Occurrence

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 predominately found in dolomite along with a variety of other sulfosalts.^[3]

References

1. ^ ^{a b} "Marrite Mineral Information and Data." http://www.mindat.org/min-2579.html. Accessed 08 September 2010.
2. ^ ^{a b c} Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1990) Marrite. Handbook of Mineralogy. 313 p. Mineral Data Publication. Tucson, Arizona.
3. ^ ^{a b c} "Marrite Mineral Data." http://webmineral.com/data/Marrite.shtml. Accessed 08 September 2010.
4. ^ Berlepsch, P., Makovicky E., and Armbruster, T. (2002) Structural relationships between sicherite, marrite, freieslebenite, and diaphorite: Analysis based on anionic nets and polyhedral characteristics(178th edition) 75-91 p. E. Schweizerbart'sche Verlagsbuchhandlung. Stuttgart, Germany.