The

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself nam ...

arsenide mineral An arsenide mineral is a mineral that contains arsenide as its main anion. Arsenides are grouped with the sulfides Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containi ...

Stillwaterite has a general formula of Pd₈As₃. Stillwaterite was first discovered in the Banded and Upper zones of the

Stillwater igneous complex The Stillwater igneous complex is a large layered mafic intrusion (LMI) located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles (48 km) of the north flank of the Beartooth Mount ...

Montana Montana () is a U.S. state, state in the Mountain states, Mountain West List of regions of the United States#Census Bureau-designated regions and divisions, division of the Western United States. It is bordered by Idaho to the west, North ...

, United States, and has been reported in the Lac-des-Iles area of

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

, Canada. Outside of North America, this rare mineral has been found in northern

Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bo ...

Symmetry

Stillwaterite has

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A ''regular hexagon'' has ...

symmetry, meaning that it has four crystallographic axes: three are horizontal axes of equal length, and the fourth is a vertical axis of different length.

Optical properties

Because it is opaque, stillwaterite is most commonly viewed under reflected light, appearing light creamy gray in color. It is weakly

anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's phys ...

in air, displaying dark gray to brownish gray color. In oil immersion, it shows distinct anisotropy with brownish black color and a blue to yellow-brown tinge. Hexagonal minerals such as stillwaterite are referred to as

uniaxial crystal Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringe ...

s because they have only one direction, along the optic axis, in which light is not reoriented.

Significance

Since its discovery in the mid-1970s, Stillwaterite has been studied to better understand the characteristics of complex mineral assemblages of gold, silicates, and other palladium arsenides with which it is associated. Minerals containing palladium can have economic significance, and are mined for use in industrial and commercial applications.

References

{{reflist Palladium minerals Arsenide minerals Trigonal minerals Minerals in space group 143 Minerals in space group 147