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Category  Arsenide mineral
Strunz classification  2.AC.10a
Space group  P3 or P3
Formula (repeating unit)  Pd8As3
Crystal system  Trigonal
Crystal class  Pyramidal (3) or rhombohedral (3) H-M symbol: (3 or 3)

The palladium arsenide mineral Stillwaterite has a general formula of Pd8As3. Stillwaterite was first discovered in the Banded and Upper zones of the Stillwater igneous complex in Montana, US, and has been reported in the Lac-des-Iles area of Ontario, Canada. Outside of North America, this rare mineral has been found in northern Finland.



Stillwaterite has hexagonal 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 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 crystals because they have only one direction, along the optic axis, in which light is not reoriented.


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.


Stillwaterite Wikipedia

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