The Mineral METAVAUXITE

• Chemistry: FeAl2(PO4)2(OH)2 - 8H2O, Hydrated Iron Aluminum Phosphate Hydroxide.
• Class: Phosphates
• Uses: Only as mineral specimens.
• Specimens

Metavauxite is a rare phosphate mineral with two closely named and closely related mineral cousins. Metavauxite is closely related to its basic namesake vauxite. A look at the two formulae shows that metavauxite is different from vauxite only in the number of water molecules in the structure. Metavauxite has eight water molecules while vauxite has only six in its formula. The two minerals have different structures since metavauxite is monoclinic and vauxite is triclinic in symmetry. The only different properties is metavauxite's green to colorless color and prismatic crystal form verses vauxite's blue color and tabular or fibrous habit.

Paravauxite is the other closely named and closely related mineral to metavauxite. It is dimorphous with metavauxite. Dimorphous means that two minerals have the same exact chemistry, but different structures (di = two; morph= shape). In this case, the structure of paravauxite is triclinic while the structure of metavauxite is monoclinic in symmetry. Unfortunately there is little to tell metavauxite from paravauxite without X-ray laboratory tests. The rarer metavauxite lacks evident cleavage and forms prismatic crystals compared to paravauxite's perfect cleavage and typical tabular habit.

Although one might think so, vauxite and metavauxite are not dimorphs since they share neither the same structure nor the same chemistry (thanks to those water molecules). Vauxite, paravauxite and metavauxite are all found at the famous tin oxide deposits at Llallagua, Potosi, Bolivia. All are associated with the primary tin ore, cassiterite. They form as a result of precipitation from hydrothermal solutions.

PHYSICAL CHARACTERISTICS:

• Color is colorless, white or pale green.
• Luster is vitreous to silky.
• Transparency: Specimens are translucent to transparent.
• Crystal System is monoclinic.
• Crystal Habits include prismatic crystals and radiating clusters.
• Cleavage is not evident.
• Fracture is conchoidal.
• Hardness is 3
• Specific Gravity is approximately 2.3 (average).
• Streak is white.
• Associated Minerals include cassiterite, paravauxite, vauxite, wavellite, quartz and limonite.
• Notable Occurrences are limited to the famous tin deposits at Llallagua, Potosi, Bolivia and a few minor localities around the world.
• Best Field Indicators are color, locality, crystal habit and associations.