Arkansas rocks yield several collectable phosphate
minerals that are green (commonly) in color a due to vanadium chromophores. Color in minerals seems a very complex event
and often is beyond my comprehension!
Bob Jones has written a series of recent articles in Rock & Gem that go into great detail
explaining the color of minerals and I would point interested readers in that
direction. For this article I would
simply state that some minerals (idiochromatic coloration) exhibit certain
colors due to the chemical composition of the minerals—cuprous copper, Cu+,
gives a green color to malachite (Cu2CO3(OH)2. Other minerals only have trace amounts of
impurities that impart the color (allochromatic coloration)—titanium, Ti, imparts
a blue color to sapphire (Al203). Vanadium is a common impurity and an
important allochromatic chromophore.
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Wavellite with broken spherules, “Cat’s Eye.” Width individual spherules ~.50-.75 cm.
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Wavellite. Width specimen ~12 cm. Color due to vanadium +3 chromophore.
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The mineral that may be the second most popular
species to collect in Arkansas (after quartz) is wavellite. This fascinating
mineral is a hydrated aluminum phosphate [Al3(PO4)2(OH,F)3·5H2O] that commonly appears
in botryoidal and radial aggregates, a trait that makes specimens quite
interesting. Fairly soft at ~3.5-5.0
(Mohs), the mineral has a vitreous to silky luster and is translucent. Although the green color is most impressive
to me, it may occur as colorless to yellow and light blue. On the piece in my collection several green
spherules were broken and therefore the radiating fibers are clearly visible
(cat’s eye to collectors). Wavellite is
most often a secondary and low temperature mineral found in vugs and fissures
of the associated host rock—in this case, at Mauldin Mountain, the Bigfoot Chert
of Ordovician age. The green color (most
common) of wavellite is mostly due to the presence of vanadium. Vanadium has five different oxidation states
(2-5 are accessible in aqueous solution) with three of them imparting some
color to wavellite: +3 giving it a green color, +4 imparts a blue color while
+5 furnishes a yellow color. The absence
of vanadium will leave wavellite white or colorless (Foster and Schaller, 1966:
Holt, 1972).
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Broken wavellite spherules without color (due to
absence of vanadium chromophore). Width
~3.2 cm.
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My specimen of wavellite was collected at the
Mauldin Mountain Quarries, Montgomery
County, Arkansas, a fairly "famous" collecting locality near
Mt Ida.
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Variscite, width ~5.5 cm.
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I have second specimen of a green phosphate mineral
collected at or near Dug Hill (Garland County, previously part of Montgomery
County) that was labeled as variscite, another hydrated aluminum phosphate
[AlPO2-2H2O] that commonly is found in the same
localities as wavellite (Smith, date unknown).
It does not look anything like the variscite that I have collected in
Utah so I looked at other possibilities.
Another similar-looking hydrated aluminum phosphate is planerite [Al6(PO4)2(HPO4)2(OH)8-4H2O)],
a member of the turquoise group. Like
variscite and wavellite, planerite is found in botryoidal masses. However, in planerite the individual balls
are quite tiny, a millimeter or so. In
addition, variscite occurs as more of a bright-green crust than the pale-green
masses of planerite. It is a tough call
but will stay with variscite (partially influenced by the photograph of
variscite on www.mifind.com
that seems identical to my specimen). I
love phosphate minerals; however, they often confuse me---something that is not
really hard to do!
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Cacoxinite, yellow-orange “streaks’ labeled C. maximum width specimen ~2. cm.
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Photomicrograph cacoxinite crystal bundles. Width of view 1.2 cm.
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Another phosphate mineral that is collectable and
found in Arkansas is known as cacoxenite [Fe24Al(PO4)17O6(OH)12-17H2O],
where the hydrated aluminum phosphate has added some iron. It seems to be rarer than many other
phosphates, and has a different morphology than wavellite and variscite. In most instances cacoxinite displays
crystals that are acicular and found as radial bundles or radial aggregates and
are yellow to brownish yellow to orange in color. Cacoxinite is found in phosphorous-rich
igneous rocks, in novaculite, and in a few other phosphatic rocks. My specimen came from the Coon Creek Mine
(York Mine), Polk County, Arkansas, a former open pit manganese mine with
mineralization in the Arkansas Novaculite.
This rock unit is Devonian in age and is a form of chert that was
subjected to some low grade metamorphism.
The cacoxinite owes its origin to circulating ground water containing
iron and phosphate (Howard, 2011) and seems to be one of the last phosphates to
form (Smith, unknown date).
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Rockbridgeite spherules. Width view ~1.7 cm.
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The Coon Creek Mine has also produced another
interesting phosphate mineral named rockbridgeite, a hydrated iron phosphate
[(Fe++Fe+++4(PO4)3(OH)5]. Rockbridgeite then drops the aluminum but
picks up the iron---both ferrous and ferric.
As with the other phosphates described here, rockbridgeite is a
secondary mineral resulting from oxidation of other phosphate minerals and
iron. The mineral has a botryoidal habit
like other phosphates, and is similar to botryoidal limonite. Rockbridgeite in fresh exposures displays the
“green” tones of other hydrated phosphates but also is found in brown and black
colors. In fact, due to the iron, the
mineral commonly oxidizes to a reddish-brown to black color and surficially
resembles limonite and/or goethite.
EPILOGE
Wavellite is not a common mineral in Colorado but is
randomly found in some of the turquoise mines and a few igneous centers. I had never seen Colorado wavellite or
variscite until purchasing a specimen at an auction last spring. It was simply labeled “Wavellite, Colorado”. I don’t have the slightest idea where the
specimen was collected.
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Wavellite collected “Colorado” width 3.5 cm.
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The element vanadium, the chromophore for wavellite
and others, is found in 60-70 different minerals. It is probably best known as a major
component of the mineral vanadinite [Pb5(VO4)3Cl],
that of the beautiful orange-red hexagonal crystals. Natural vanadium occurs in
the human body but its biological use is unknown. However, vanadium is an important element in
certain tunicates (Phylum Chordata) called sea squirts. These marine filter feeders concentrate
vanadium in their bodies to a level one million times higher than the
concentration of vanadium in seawater (www.wou.edu)! Why? I
just don’t know as I have been unable to communicate with the
little fellas.
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Halocynthia
sp., a sea squirt. Public domain photo.
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REFERENCES
CITED
Foster, M. D., and W. T. Schaller, 1966, Cause of
Colors in Wavellite from Dug Hill, Arkansas: The American Mineralogist, v. 51.
Howard, J. M., 2011, Rockhounding Arkansas: www.rockhoundingar.com
Smith, A. E., date unknown, Arkansas Aluminum
Phosphates: Wavellite with variscite and
Planerite: www.acrystalmine.com