Getchellite

Getchellite
Getchellite from Khaidarkan, Fergana Valley, Osh Oblast, Kyrgyzstan. Specimen size 3.6 cm.
General
Category	Sulfide mineral
Formula (repeating unit)	AsSbS3
Strunz classification	2.FA.35
Dana classification	2.11.1.2
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) H-M symbol: (2/m)
Space group	Monoclinic Space group: P21/a
Identification
Formula mass	293.81 g/mol
Color	Dark red, sometimes with a purple to green iridescent tarnish
Crystal habit	Subhedral crystals and massive with a platy texture
Twinning	Simple and polysynthetic twins with the twin plane and composition plane parallel to {001}
Cleavage	Perfect on {001}
Fracture	Splintery
Tenacity	Sectile; cleavage flakes are flexible and inelastic
Mohs scale hardness	1.5 to 2
Luster	Vitreous to pearly on cleavage surfaces, otherwise resinous
Streak	Orange red
Diaphaneity	Transparent
Specific gravity	3.92 (observed) 4.0 (calculated)
Optical properties	Biaxial (+), 2V=46°
Refractive index	n = 2.720. Dispersion r > v strong
Melting point	340 °C to 355 °C
Other characteristics	Not radioactive
References

Red Getchellite and yellow Orpiment from the Getchell Mine, the type locality.

Getchellite is a rare sulfide of arsenic and antimony, AsSbS₃, that was discovered by B. G. Weissberg of the New Zealand Department of Scientific and Industrial Research in 1963, and approved as a new species by the International Mineralogical Association in 1965. Many metal sulfides are grey to black, but a few are brightly colored. Orpiment is yellow to brownish gold, cinnabar is deep red and getchellite is a bright orange red.

Thermal properties

Getchellite turns darker red when heated, becoming black by the time it reaches its melting point. Close to this temperature it sublimes (changes directly from a solid to a vapor) and recrystallizes on cooler surfaces as minute acicular black crystals.
Melting point: 340 °C to 355 °C. Boiling point: near 470 °C.

Structure

Each of the semimetal atoms, arsenic and antimony, is bonded to three sulfur atoms to form a trigonal pyramid characteristic of elements in group V of the periodic table. These (As,Sb)S₃ pyramids form 8-membered (As,Sb)₈S₈ rings which in turn combine to form sheets parallel to (001), with each sulfur atom bonded to two semimetal atoms. The occupancy of the metal sites is disordered, and within the sheets the 8-membered rings are orientated normal to the plane of the sheet. The sheet structure is responsible for the cleavage and twin planes of getchellite Unit cell a = 11.949 Å; b = 9.028 Å; c = 10.130 Å; β= 116.15°; V = 980.9 Å³; Z = 8. Space group P2₁/a.

Powder diffraction

X-Ray Powder Diffraction:
d spacing	2.89	4.44	3.63	2.54	2.33
relative intensity	10	8	7	6	6

Discovery

In August 1962 Weissberg visited the Getchell mine at Adam Peak, about 32 km northeast of Golconda, in Humboldt County, Nevada, US. The purpose of his visit was to collect samples for a study of the relationship between various fairly common sulfides, not to look for new minerals. Getchellite was in some of the samples, but it was not discovered until the rocks were re-examined, a year later. The new mineral was named after the Getchell Mine, which in turn became the type locality. Samples from the mine are preserved as type material (a reference material for the mineral), at the Smithsonian National Museum of Natural History, Washington, D.C., US.

Environment

At the type locality getchellite is found in an epithermal (formed at low temperature) arsenical gold deposit in a narrow, steeply dipping fault zone cutting across interbedded shales, argillites (lithified muds and oozes) and limestones, near an intrusion of granodiorite.
Associated minerals are orpiment, realgar, stibnite, cinnabar and quartz, as well as galkhaite, laffittite, chabournéite, christite, lorandite, marcasite, barite, fluorite and calcite.

Distribution

Getchellite is found at the type locality in Nevada, United States, and also in Azerbaijan, China, Iran, Japan, Kyrghystan and Russia.

Synthesis

Getchellite has been synthesized from As₃SbS₆ glass in sodium sulfide solutions at 2600 C. and 1,000 bars pressure.