ALGODONITE
Algodonite is a rare arsenide that integrates complex ores of Cu and Ag with As.
It may contain Ag, and there is a synthetic variety with Ag.
In older literature, there is a differentiation between α-algodonite and β-algodonite. Similarly, domeykite, which occurs in association, was “subdivided” into α-domeykite, β-domeykite, and γ-domeykite. Some references even mention δ-forms, various varieties, and mixtures. Care must be taken with the characteristics described.
Some of the characteristics initially described for algodonite refer to native copper rich in As (up to 11.5% As), called “whitneyite,” which occurs closely intergrown with algodonite.
1. Characteristics
Crystal system: Hexagonal, dihexagonal bipyramidal.
Color: From steel gray to silver white, tarnishes to matte.
Habit: Massive, granular. Incrustations formed by very distorted, minute crystals.
Cleavage: No.
Tenacity: Malleable.
Twinning: No.
Fracture: Subconchoidal.
Mohs Hardness: 4
Parting: No.
Streak: No information available.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 8,38
2. Geology and Deposits
Algodonite is uncommon, typical of hydrothermal deposits containing Cu arsenides. It generally occurs in very small quantities and in a fairly dispersed manner. Only in some locations does it occur in larger volumes.Algodonite can form through the migration of groundwater in desert environments.
3. Mineral Associations
It is associated with typical gangue minerals such as quartz and calcite.
It occurs with other arsenides such as domeykite, koutekite, and Ag arsenides.
With Cu minerals (native copper, chalcocite, covellite, cuprite, tenorite), native silver, nichelite, and prehnite.
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 algodonite 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 algonite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Algodonite takes on a very good polish. Its hardness upon polishing is medium, greater than that of chalcocite, similar to that of native copper with As (“whitneyite”) and domeykite, and less than the hardness of breithauptite. Microscopic analysis must be performed quickly, as algodonite tarnishes rapidly.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light creamy white, very similar to the reflection colors of domeykite and native copper with As (“whitneyite”). It is a very luminous, very metallic color. It tarnishes quickly in the air.
Pleochroism: Very faint, clear at intergranular boundaries.
Reflectivity: 65.06% (very high!)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak anisotropy.
Internal reflections: No.
May be confused with: The identification of algodonite 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”) and domeykite have very similar reflection colors, but are isotropic, as shown in the table below.
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.
General Characteristics:
Grain shape: Algodonite generally occurs massively, in aggregates of rounded grains. It never shows well-developed crystalline faces. It can occur as cement for sandstones and breccias.
Polishing grooves are usually still present.
Twinning it does not exhibit.
Pseudomorphs of chalcocite and covellite on algodonite can occur.
Intergrowths of algodonite with niqueline are possible.
Replacement of algodonite by domeykite occurs.
