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.
Wavellite with broken spherules, “Cat’s Eye.” Width individual spherules ~.50-.75 cm.
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).
My specimen of wavellite was collected at the Mauldin Mountain Quarries, Montgomery County, Arkansas, a fairly "famous" collecting locality near Mt Ida.
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!
Cacoxinite, yellow-orange “streaks’ labeled C. maximum width specimen ~2. cm.
Photomicrograph cacoxinite crystal bundles. Width of view 1.2 cm.
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).
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.
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.
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.
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