DOMEYKITE

Domeykite or Domeykite-α – Cu3As – is a rare arsenide. It participates in complex Cu and Ag ores with As.

It is dimorphic with β-domeykite, which is a Cu-deficient domeykite, macroscopically visually indistinguishable from domeykite and which transitions to α-domeykite at approximately 225ºC. Domeykite is isotypic with dienerite and may contain Sb and S.  There is an artificial variety containing Ag, “argentodomeykite”.

In older literature, there is a differentiation between α-domeykite (cubic), β-domeykite (hexagonal) and γ-domeykite. Similarly, algodonite, which occurs associated, was “subdivided” into α-algodonite and β-algodonite. Some references even mention δ-forms, various varieties and mixtures. Care must be taken with the characteristics described for the minerals that make up this paragenesis.

1. Characteristics

Crystal system: Cubic, hexatetrahedral.          

Color: White as tin to gray as steel. Fades to yellowish, then brown, then iridescent.     

Habit: Massive, reniform, botryoidal.       

Cleavage: No.      

Tenacity: Malleable.        

Twinning:  No.      

Fracture: Irregular.       

Mohs Hardness: 3 – 3.5

Parting: No.         

Streak: No information available.         

Lustre:  Metallic.         

Diaphaneity: Opaque.           

Density (g/cm³):  7.2 – 7.9

 

2. Geology and Deposits

Domeykite is uncommon, typical of hydrothermal deposits containing Cu arsenides.

 

3. Mineral Associations

It is associated with typical gangue minerals such as quartz, calcite, and prehnite.

It occurs with Cu minerals such as native copper, chalcocite, covellite, cuprite, chrysocolla, olivenite, tenorite, malachite, algodonite (Cu1-xAsx), and koutekite (Cu5As2).

Also with native silver and Ni minerals such as nickeline, rammelsbergite, and Ag arsenides.

It is also associated with mohawkite, which is a mixture of algodonite, domeykite, As-rich native copper, and perhaps other minerals, originally described from the Mohawk mine (Michigan, USA).

Older literature also associates domeykite with “whitneyite,” which is not a mineral but a variety of As-rich native copper (up to 6.6%).

 

4. Transmitted Light Microscopy

This does not apply, as domeykita is completely opaque.

5. Reflected Light Microscopy

Sample preparation:  Domeykite polishes very well. Its hardness upon polishing is medium, greater than that of chalcocite and similar to that of algodonite. Some polishing grooves may be persistent. It tarnishes quickly, especially if other ore minerals are present.      

PLANE POLARIZED LIGHT – PPL

Reflection color:  White yellowish to very light gray.

Domeykite-β is somewhat bluish-gray in contrast, less yellowish than domeykite-α, but very similar. This difference is only visible in freshly polished sections!

Compared to the color of algodonite, the color of domeykite is darker and more yellowish-gray.

Compared to the color of “whitneyite”, the color of domeykite is darker and more yellowish-gray.      

Pleochroism: No.

Domeykita-β has weak to distinct pleochroism.      

Reflectivity: Very high, at 65.06%,        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic.

Domeykite-β exhibits quite distinct anisotropy in orange hues. Colors may vary between samples from different locations.

Internal reflections: No.      

May be confused with: The recognition of domeykite and its associated minerals is difficult. Knowing the paragenesis reduces the possibility of mistakes, but there are many cases of complicated and unexpected mixtures, so observations must be careful.

Native copper with As (“whitneyite”) has very similar reflection colors, but it is isotropic. It probably has many polishing grooves, like native copper poor in As.

The minerals can occur in isolation, but generally domeykite contains algodonite, algodonite contains whitneyite, and whitneyite contains algodonite. The whitneyite + domeykite association is very rare. 

Here are the most common minerals in this paragenesis. None of them show birreflectance nor internal reflections. All four have very high reflectivity, between 45 and 65%.

Algodonite:  In PPL, light creamy white, tarnishes like domeykita and very weak pleochroism. In XPL, weak anisotropy.

Cu+As – “Whitneyite”: In PPL, creamy white, tarnishes very quickly (more than native Cu), no pleochroism. In XPL, isotropic.

Domeykite-α: In PPL, steel gray. tarnishes to yellowish, iridescent, and brownish crust, no pleochroism. In XPL, isotropic.

Domeykite-β: In PPL, slightly more blue-gray in contrast, with distinct pleochroism. In XPL, distinct anisotropy in orange hues.

Koutekite: In PPL, bluish gray like cuprite, with weak pleochroism in bluish hues with gray. In XPL, strong anisotropy.

 

Copper ore from Houghton Co., Michigan, USA, in PPL. According to the literature and known minerals from the site, the dark gray mineral, with excellent polish, is domeykite. Around it, in slightly lower relief, is a complex mixture of algodonite and other Cu+As minerals, possibly mohawkite (see paragenesis above). Cuprite also occurs in the sample, but there is no native copper. In black, holes. In XPL, all minerals are isotropic (like domeykite) or show very weak anisotropy (the others); all are very dark.
From the same polished section as the previous image, the distribution of domeykite (darker gray) in aggregates with rounded outlines. PPL.
In this image, from the same polished section, the rounded shapes of the domeykite aggregates (darker gray) are more visible. Holes are shown in black.