{{Infobox mineral | name = Pearceite | category = | boxwidth = 24 | boxbgcolor = | image = Pearceite-4jg57a.jpg | caption = Pearceite from Butte, Montana, US | formula = {{chem2|Cu(Ag,Cu)6Ag9As2S11}}<ref name=IMA>{{Cite web|url=http://rruff.info/ima/|title = IMA Mineral List with Database of Mineral Properties}}</ref> | IMAsymbol = Pea<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = 2,096.80 g/mol | strunz = 2.GB.15 (10 ed) <br /><small>2/E.05-20 (8 ed)</small> | dana = 3.1.8.1 | system = Monoclinic or trigonal | symmetry = ''P{{overline|3}}m1'' (no. 164) | color = Black | habit = Pseudohexagonal prisms | twinning = | cleavage = {001} Poor<ref name=Webmin>{{Cite web|last1=Barthelmy|first1=David|year=2014|url=http://www.webmineral.com/data/Pearceite.shtml|access-date=7 August 2022|title = Pearceite Mineral Data|website=Webmineral.com}}</ref> | fracture = Conchoidal to irregular | tenacity = Brittle | mohs = 3 | luster = Metallic | refractive = 2.7 | opticalprop = Biaxial | birefringence = 2.7 | pleochroism = RL Pleochroism (in reflected plane polarised light): Very weak in air, fair in oil<ref name=Webmin/><ref name=HOM>{{cite web |last1=Anthony |first1=John W. |last2=Bideaux |first2=Richard A. |last3=Bladh |first3=Kenneth W. |last4=Nichols |first4=Monte C. |title=Pearceite |url=http://www.handbookofmineralogy.org/pdfs/pearceite.pdf |website=Handbook of Mineralogy |publisher=Mineral Data Publishing |access-date=7 August 2022 |date=2005}}</ref> | streak = Black | gravity = 6.15 | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Opaque | other = Non-fluorescent, nonmagnetic, not radioactive | references = <ref name=Webmin/><ref name=HOM/><ref name=Dana>Gaines et al (1997) Dana’s New Mineralogy, Eighth Edition. Wiley</ref><ref name=Mindat>{{mindat|id=3138|title=Pearceite|access-date=7 August 2022}}</ref>}} '''Pearceite''' is one of the four so-called "ruby silvers", pearceite {{chem2|Cu(Ag,Cu)6Ag9As2S11}}, pyrargyrite {{chem2|Ag3SbS3}}, proustite {{chem2|Ag3AsS3}} and miargyrite {{chem2|AgSbS2}}.<ref name=Mindat/> It was discovered in 1896 and named after Dr Richard Pearce (1837–1927), a Cornish–American chemist and metallurgist from Denver, Colorado.<ref name=HOM/>

== Nomenclature == Pearceite and polybasite are closely related minerals that form the pearceite-polybasite series. Originally pearceite was thought to be an arsenic analogue of polybasite {{chem2|Cu(Ag,Cu)6Ag9Sb2S11}}, and was called arsenpolybasite, and one polytype of polybasite was called antimonpearceite. Arsenpolybasite was found to represent two different polytypes, arsenpolybasite-221 and arsenpolybasite-222.<ref name=AM48>{{cite journal | author=C. Frondel | date= 1963 | title= Isodimorphism of the polybasite and pearceite series | url= https://pubs.geoscienceworld.org/msa/ammin/article-abstract/48/5-6/565/542084/Isodimorphism-of-the-polybasite-and-pearceite | journal= American Mineralogist | volume= 48 | issue= 5–6 | pages= 565–572 | access-date= 2024-07-01 }}</ref> In modern usage the old name pearceite is replaced by the polytype name pearceite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c and antimonpearcite by polybasite-Tac.<ref name=AM92>{{cite journal |author1= L. Bindi | author2= M. Evain | author3= P.G. Spry |author4= S. Menchetti | year=2007 | title=The pearceite-polybasite group of minerals: Crystal chemistry and new nomenclature rules |journal= American Mineralogist |volume= 92 | issue= 5–6 |pages= 918–925 | doi= 10.2138/am.2007.2440 | bibcode= 2007AmMin..92..918B | url= https://pubs.geoscienceworld.org/msa/ammin/article-abstract/92/5-6/918/134492/The-pearceite-polybasite-group-of-minerals-Crystal | access-date= 2024-07-01| url-access= subscription }}</ref> Pearceite-Tac forms a series with polybasite-Tac.

== Crystallography and Structure == Two structural varieties, trigonal and monoclinic, are known.<ref name=Mindat/> The trigonal variety crystallizes in the hexagonal scalenohedral class {{overline|3}}m ({{overline|3}} 2/m), space group P{{overline|3}}m1 (P{{overline|3}} 2/m 1).<ref name=Webmin/><ref name=Mindat/> The monoclinic variety crystallises in the prismatic 2/m class, space group C2/m.<ref name=HOM/><ref name=Dana/><ref name=AM48/>

Unit cell parameters

*Monoclinic variety: There are two formula units per unit cell (Z = 2), the lengths of the sides of the unit cell are a = 12.64 Å, b = 7.29 Å, c = 11.90 Å and the angle between the c and a directions is β = 90.0°.<ref name=HOM/><ref name=Dana/> *Trigonal variety: There is one formula unit per unit cell (Z = 1), two of the sides are of equal length a = 7.3876&nbsp;Å and the third side, parallel to the threefold axis, is c = 11.8882&nbsp;Å.<ref name=Webmin/><ref name=Mindat/>

The crystal structure consists of sheets stacked along the c axis. The arsenic atoms form isolated {{chem2|(As,Sb)S3}} pyramids, copper cations link two sulfur atoms and the silver cations are found in various sites with low coordination numbers, 2,3 and 4, as is usually the case with silver.<ref name=AC>{{cite journal | author1= L. Bindi | author2= M. Evain | author3=S. Menchetti | journal= Acta Crystallographica B | volume= 62 | date= 2006 | pages= 212–219 | title= Temperature dependence of the silver distribution in the crystal structure of natural pearcite, (Ag,Cu)<sub>16</sub>(As,Sb)<sub>2</sub>S<sub>11</sub> | issue= Pt 2 | doi= 10.1107/S010876810600108X | pmid= 16552154 }}</ref>

== Properties == Pearceite is often granular and massive;<ref name=Webmin/> crystals are short, tabular pseudohexagonal prisms with bevelled edges, showing triangular striations on faces parallel to the plane containing the a and b axes, and rosettes of such crystals, to 3&nbsp;cm across.<ref name=HOM/> The mineral is black, and in polished section it is white with very dark red internal reflections.<ref name=HOM/> It has a black to reddish black streak and a metallic luster, generally opaque, but translucent in very thin fragments.<ref name=HOM/>

It is biaxial with a very high refractive index of 2.7<ref name=Webmin/><ref name=Mindat/> and maximum birefringence δ also 2.7.<ref name=Mindat/> Dispersion of the optic axes is relatively strong.<ref name=Mindat/>

Reflected light anisotropism is the property of appearing to change color when viewed under crossed polarised light in a reflected light microscope. Pearceite exhibits moderate anisotropism, often dark violet.<ref name=Webmin/><ref name=HOM/><ref name=Dana/> The color in reflected plane polarised light is white, with very dark red internal reflections<ref name=Webmin/> and very weak pleochroism in air, fair in oil.<ref name=Webmin/><ref name=HOM/> Reflectivity in air at 540&nbsp;nm is about 30%. It is not fluorescent.<ref name=Webmin/>

Pearceite is a brittle mineral that breaks with a conchoidal to irregular fracture. It is soft, with hardness only 3, the same as calcite. The silver content gives it a high specific gravity of 6.15, the highest of the ruby silvers. Cleavage is either absent or poor. The mineral is neither magnetic nor radioactive.<ref name=Webmin/><ref name=HOM/><ref name=Dana/><ref name=Mindat/>

== Occurrence and associations == The type locality is the Mollie Gibson Mine, Aspen, Aspen District (Roaring Fork District), Pitkin County, Colorado,<ref name=Mindat/> where the mineral occurs in hydrothermal deposits formed at low to medium temperatures, associated with acanthite, tetrahedrite, native silver, proustite, quartz, baryte and calcite. Type material is lodged at Yale University, New Haven, Connecticut, references 3.4270, 3.4292, 3.4293, and at The Natural History Museum, London, England, reference 84843.<ref name=HOM/>

==References== {{reflist}}

==External links== * [http://www.microscopyu.com/articles/polarized/polarizedintro.html Nikon: Introduction to Polarized Light Microscopy] * [http://www.olympusmicro.com/primer/techniques/polarized/polarizedhome.html Olympus: Polarized Light Microscopy] {{Webarchive|url=https://web.archive.org/web/20090221172601/http://www.olympusmicro.com/primer/techniques/polarized/polarizedhome.html |date=2009-02-21 }} * [http://earth2geologists.net/Microscopes/index.htm Geological Microscopes] * [http://gerdbreitenbach.de/crystal/crystal.html A virtual Polarizing Microscope] {{Commons category}}

Category:Sulfide minerals Category:Silver minerals Category:Copper(I) minerals Category:Arsenic minerals Category:Aluminium minerals Category:Trigonal minerals Category:Minerals in space group 164 Category:Monoclinic minerals Category:Minerals described in 1896