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.