Roscoelite

The chemical formula is K(V³⁺,Al,Mg)₂AlSi₃O₁₀(OH)₂^[1]. Crystals of roscoelite take on the monoclinic form, and are from the 2/m point group. The appearance is semi transparent to translucent coloured olive brown to green brown. The luster is pearly. The mineral shows pleochroism with X showing green-brown, and Y and Z axes showing olive-green colour. The mineral was named after Henry Enfield Roscoe who first produced vanadium metal.

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Chemical properties

Roscoelite is a muscovite with aluminium substituted with vanadium. Vandium can also be substituted by magnesium, iron, or manganese.

Physical properties

It is soft and the density is 2.93±0.01. The unit cell has dimensions a = 5.26 ångströms (1 ångström = 0.1 nanometres) b = 9.09 ångströms c = 10.25 ångströms, angle between axes β=101.0°. The tetrahedral cation-oxygen atom distance is 1.641 ångströms, The distance from the cation to the oxygen in the octahedral plan is 2.02 ångströms^[2].

Formation

Two kinds of mineral deposits contain roscoelite, either gold-silver-tellurium low temperature epithermal deposits where it occurs along with quartz, fluorite, pyrite and carbonates, or oxidized low temperature uranium-vanadium ores in sedimentary rocks, where it occurs with corvusite, hewettite, carnotite and tyuyamunite. Roecoelite is considered a gangue mineral of no value when found with gold^[3]. However it has also been used as a vanadium ore.

In the Mt. Kare mine in New Guinea the mineral occurs with gold and is an important maker of gold deposits. The temperature of the geothermal fluid that depositied the roscoelite was from 127 to 167 °C. The fluid contained a high level of salt and also contained carbon dioxide, methane, carbonyl sulfide and other minor amounts of rock forming elements.^[4]

Occurrence

The mineral has been found in numerous places in USA, Australia, Japan, Gabon, Fiji, New Guinea^[5]and Czech Republic. In the United States, it was the principal vanadium ore mineral at the mines at Placerville, Colorado. In Australia roscoelite has been found at Kalgoorlie, Radium Hill and the Kintore Open Cut at Broken Hill.

References

1. ^ Roscoelite information
2. ^ Xray analysis
3. ^ Karen Kelley, Theodore Armbrustmacher, and Douglas Klein Au-Ag-Te Vein Deposits section of ore and Gangue mineralogy
4. ^ E. Ronacher, and J. P. Richards, Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea
5. ^ Buffalo Gold at Mt Kare