THE MINERAL MIXITE

Chemistry: BiCu6(AsO4)3(OH)6 - 3H2O, Hydrated Bismuth Copper Arsenate Hydroxide
Class: Phosphate Class
Subclass: Arsenates
Group: Mixite
Uses: Only as mineral specimens
Specimens

Mixite is one of just a handfull of bismuth minerals and one of even fewer bismuth arsenates. Other bismuth arsenates include walpurgite and atelestite, to name a couple. Mixite is probably the best known of these, but is still a rare mineral by most considerations. It forms in the oxidation zone of metal ores that probably contained primary bismuth sulfides such as emplectite.

As a mineral collection specimen, mixite is truly a wonderful addition. It forms nice radial clusters called spherules, made up of fine acicular crystals. The color is typically a brilliant green of one shade or another coupled with an attractive silky luster.

Mixite lends its name to a small group of rather obscure minerals called the Mixite Group. The Mixite Group is a group of hexagonal, hydrated copper arsenate and phosphate hydroxides with a general formula of ACu6(XO4)3(OH)6 - 3H2O The A in the formula can be either bismuth, aluminum, calcium, cerium, lanthanum, yttrium, neodymium or thorium. The X can be either arsenic or phosphorous.

These are the members of the Mixite Group:

Agardite (Hydrated Lanthanum Yttrium Calcium Cerium Neodymium Copper Arsenate Hydroxide)
Goudeyite (Hydrated Aluminum Yttrium Copper Arsenate Hydroxide)
Mixite (Hydrated Bismuth Copper Arsenate Hydroxide)
Petersite (Hydrated Yttrium Calcium Copper Phosphate Hydroxide)

Agardite actually represents at least a couple of minerals and is sometimes considered a mineral group itself. Agardite and mixite are difficult to differentiate. But if the specimen is associated with other bismuth minerals, than this can prove diagnostic in mixite's favor.

PHYSICAL CHARACTERISTICS:

Color is green, emerald-green, blue-green, yellow-green or off-white.
Luster is vitreous or silky to dull.
Transparency: Crystals are transparent to translucent.
Crystal System is hexagonal.
Crystal Habits include radiating clusters, tufts or spherules of acicular or fibrous crystals, also massive, capillary and earthy.
Cleavage not noticeable.
Fracture is fibrous.
Hardness is 3 - 4.
Specific Gravity is approximately 3.8 - 4.0 (above average for translucent minerals).
Streak is light green.
Associated Minerals are pharmacosiderite, zeunerite, emplectite, limonite, malachite, wulfenite, mimetite and barite.
Notable Occurrences include Germany; El Carmen Mine, Durango, Mexico; France; Jachymov of the former Czechoslovakia; Inyo County, California and Gila and Pinal Counties in Arizona, USA.
Best Field Indicators are color, crystal habit, density and associations with bismuth ores.

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