Submillimeter yellow "scales" of volborthite partially covering matrix. Width of specimen ~3.6 cm. |
I have a couple of specimens purchase several years
ago before Ackley’s Minerals, a small shop here in Colorado Springs, closed as
the proprietors “retired” to the farm north of town. Volborthite is a rare mineral that usually is
observed as small, yellow to yellow-green “scales” encrusting part of a rock’s
matrix. The luster ranges from vitreous
to opaque but is usually classifies as pearly.
The “scales” often may be seen as individuals but many times they are
stacked on one another, and without magnification, appear as massive
encrustations. On some specimens, but not mine, the small plates of volborthite
form clusters of rosettes. The hardness is ~3.5 (Mohs) and I was able to get a
light to pale green streak. Volborthite
does not fluoresce under a UV light.
Photomicrograph of volborthite "scales." Width of photo ~6 mm. |
Above two photomicrographs show submillimeter "scales" of volborthite. |
Volborthite is usually found in the highly-oxidized
zone where hydrothermal solutions (coming off hot bodies of intruded magma)
have deposited other vanadium-bearing minerals.
In the Colorado Plateau Type deposits, a sandstone-hosted mineral
deposit, vanadium minerals often occur with uranium minerals; therefore, rockhounds
originally believed minerals like volborthite were radioactive (just the
radioactivity coming off the uranium minerals).
But, vanadium is a critical component of some uranium minerals such as
carnotite: K2(UO2)2(VO4)2--3H2O.
Karpenkoite [Co3V2O7(OH)2—2H2O]
is the cobalt analogue of volborthite since cobalt substitutes for the copper. Martyite is the zinc analogue when zinc
replaces the copper [Zn3V2O7(OH)2—2H2O].
Engelhauptite has the water (H2O)
molecules replaced by potassium and chlorine molecules [KCu3(V2O7)(OH)2Cl].
My two specimens came from the Copper Hill Mine,
Picuris District, Taos County, New Mexico, and according to the rock shop proprietors,
were collected decades ago. The Picuris
District is part of the Sangre de Cristo Mountains and “is best known for the
Harding pegmatite, a Proterozoic [late Precambrian] complex-zoned pegmatite,
which has produced substantial amounts of beryl, lepidolite, spodumene, and
tantalum-niobium (microlite) minerals” (McLemore and Mullen, 2004). However, the Copper Hill Mine is a “strata-bound”
copper-silver-antimony deposit” (Williams and Bauer, 1995) that produced the volborthite specimens.
I have been unable to locate much information about
volborthite from the Copper Hill Mine.
In fact, MinDat does not list a primary vanadium mineral from Copper
Hill; therefore, I “do not know” (no surprise here) where the vanadium “came
from”?
REFERENCES
CITED
McLemore,
V.T., and K.E. Mullen, 2004, Mineral Resources in Taos County, New Mexico in New Mexico Geological Society
Guidebook. 55th Field Conference, Geology of the Taos Region.
Williams, M.L., and P.W. Bauer, 1995, The Copper Hill
Cu-Ag-Sb Deposit, Picuris Range, New Mexico: retrograde mineralization in a
brittle-duct trap: Economic Geology, v. 90.
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