The Mineral PARAVAUXITE

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

Paravauxite is closely related to its basic namesake vauxite. Paravauxite appears to be different from vauxite only in the number of water molecules in the structure. Paravauxite has eight water molecules while vauxite has only six in its formula. It is the presence of these water molecules that alters the structure to the point that paravauxite has perfect cleavage while in sharp contrast, vauxite has no cleavage at all. Cleavage is one property that is undeniably tied to the structure of a mineral. The only other strikingly different property is paravauxite's green to colorless color verses vauxite's blue color.

Metavauxite is another closely named and closely related mineral to paravauxite. It is a dimorph of paravauxite. A dimorph is a mineral that has the same chemistry, but a different structure. In this case, the structure of paravauxite is triclinic while the structure of metavauxite is monoclinic in symmetry. Although one might think so, vauxite and paravauxite are not dimorphs since they share neither the same structure (although both are triclinic) 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.

Paravauxite is the name of a somewhat rare phosphate mineral and an obscure mineral group. The Paravauxite Group is composed of other triclinic phosphates that have the following general formula:

XY2(PO4)2(OH)2 - 8H2O

or XY2(PO4)2(OH)3 - 7H2O

The X ion can be either iron(+2), magnesium, manganese or iron(+3) while the Y ion can be either aluminum, iron(+3) or to a lesser extent, chromium. The second formula is used when the X ion is the iron(+3) in stead of the iron(+2) and the additional positive charge requires the additional hydroxide and one less water molecule.

These are the members of the Paravauxite Group with their respective formula:

Gordonite MgAl2(PO4)2(OH)2 - 8H2O
Laueite MnAl2(PO4)2(OH)2 - 8H2O
Paravauxite FeAl2(PO4)2(OH)2 - 8H2O
Sigloite FeAl2(PO4)2(OH)3 - 7H2O
Ushkovite MgFe2(PO4)2(OH)2 - 8H2O

THE PHYSICAL CHARACTERISTICS OF PARAVAUXITE:

Color is colorless, white or pale green.
Luster is vitreous to pearly.
Transparency: Specimens are translucent to transparent.
Crystal System is triclinic.
Crystal Habits include tabular crystals and radiating fibrous masses.
Cleavage is perfect.
Fracture is conchoidal.
Hardness is 3
Specific Gravity is approximately 2.4 (average).
Streak is white.
Associated Minerals include cassiterite, metavauxite, 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, associations and perfect cleavage.