Marrite
{{Use dmy dates|date=July 2021}}
{{Infobox mineral
| name = Marrite
| category = Sulfosalt mineral
| image = Galena-Marrite-rar09-mf04b.jpg
| caption = The two arrows point to the marrite microcrystals (one or two at each arrow, dull), the rest is galena (bright)
| formula = PbAgAsS3
| molweight = 486.19 g/mol
| strunz = 2.JB.15
| system = Monoclinic
| class = Prismatic (2/m)
(same H-M symbol)
| symmetry = 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
| color = Lead gray, steel gray
| habit = Tabular or striated crystals
| twinning = Partly bent twin lamellae observable in polished section.
| cleavage = None
| fracture = Conchoidal
| tenacity = Brittle
| mohs = 3
| luster = Metallic
| refractive = Anistrophic
| opticalprop = Biaxial
| pleochroism = White, with red internal reflections.
| streak = Black with brownish tint
| gravity = 5.82
| density =
| diagnostic =
| diaphaneity = Opaque
| other = Of hydrothermal origin along with other sulfosalts in dolomite.
| references = [https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Marrite Mineralienatlas]
}}
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.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. Marrite was first described in 1905,{{cite journal|last=Solly|first=RH|year=1905|title=Some new minerals from the Binnenthal, Switzerland|journal=Mineralogical Magazine|volume=14|pages=72-82|url=https://rruff.geo.arizona.edu/doclib/MinMag/Volume_14/14-64-72.pdf|doi=10.1180/minmag.1905.014.64.03}} and was named in honor of geologist John Edward Marr (1857–1933) of Cambridge, England."Marrite Mineral Data." http://webmineral.com/data/Marrite.shtml. Retrieved 8 September 2010.
Crystal habit
Marrite is part of the monoclinic crystal class, and 2⁄m point group.[http://rruff.geo.arizona.edu/doclib/hom/marrite.pdf 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.] 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.
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."Marrite Mineral Information and Data." http://www.mindat.org/min-2579.html. Retrieved 8 September 2010. 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 predominantly found in dolomite along with a variety of other sulfosalts.