ARKANSAS PHOSPHATE: KIDWELLITE

The phosphate minerals, those containing the PO4 radical, have always been interesting to me.  This radical, with a negative three charge, is about the about the same size as the arsenate (AsO4) and vanadate (VO4) radicals and some minerals are involved in a solid solution relationship (see numerous previous posts). Members of the group also seem to have an affinity for the hydroxide ion, OH. Most of the phosphate minerals, with the exception of the apatite group, are rare or at least uncommon.  The phosphate radical also commonly attaches to metallic ions and produces, for example, wavellite and variscite (aluminum), turquoise (copper and aluminum), strengite (iron),  rockbridgeite (iron and manganese), pseudomalachite (copper), cacoxinite (iron and aluminum), cornetite (copper), and a host of others.  However, I often remain confused about their identification!

 

Kidwellite (K) from Polk County, Arkansas.  ? represents an unknown iron phosphate, perhaps beraunite, while G points to masses of tiny pisolitic goethite. Width FOV ~4.4 cm.

I have a nice specimen of kidwellite, an uncommon hydrated sodium iron phosphate hydroxide: NaFe⁺⁺⁺₉(PO₄)₆(OH)₁₁-3H₂O.  My specimen was collected from Polk County, Arkansas, an area well-known for producing nice phosphate minerals.  Kidwellite comes in a variety of colors including light green, greenish-blue, greenish-yellow, greenish-white, gray-green, and yellow; however, the streak is yellow.  It is fairly soft at 3 (Mohs) and often occurs as a crust of botryoids displaying a matte surface.  When fractured the botryoids display radiating fibers.  It has a very dull luster grading to waxy or resinous.  Kidwellite is formed by replacing rockbridgeite [Fe⁺⁺Fe₄⁺⁺⁺(PO₄)₃(OH)₅] or beraunite [Fe⁺⁺Fe₅⁺⁺⁺(PO₄)₄(OH)₅-6H₂O] and may occur with other phosphatic botryoids---hence the tough identification.

Kidwellite is one of those many minerals first described from Arkansas and seems to be restricted to fracture fillings in the Arkansas Novaculite (Devonian in age and a type of chert) and found in Polk and Montgomery counties and is mostly associated with manganese mines and prospects. Kidwellite owes its origins to circulating ground water and the presence of phosphate pellets and nodules in the sedimentary rocks (Howard, 1987; Howard 2014).

Photomicrograph of unknown iron phosphate.  Note radiating fibers.  Perhaps beraunite?  Width FOV ~ 1.7 cm.

Now, in my specimen there is a band of radiating fibers with a few botryoids that are much larger than the “normal” kidwellite botryoids---a mystery mineral.  It seems to be an iron phosphate and my guess, and that is about all that I can do, is call it either beraunite, a scarce hydrated iron phosphate hydroxide [Fe⁺⁺Fe⁺⁺⁺₅(PO₄)₄(OH)₅-6H₂O], or a different example of kidwellite!  The radiating fibers seem a different shade of green/brown than fibers exposed within the kidwellite botryoids.  Since beraunite does occur with kidwellite perhaps that is the answer.  On the other hand, the identification of phosphates is tough for an ole paleontologist like me and I often wish for just a small bit of Tom's (www.dakotamatrix.com)  knowledge of phosphate minerals. But, I am still learning:

There is no end to education.  It is not that you read a book, pass an exam, and finish with an education.  The whole of life, from the moment you are born to the moment you die, is a process of learning.

                   Jiddu Krishnamurti

 

Photomicrograph of pisolitic goethite (showing iridescence) with larger spherules of kidwellite.  Note broken spherules of kidwellite showing internal radiating fibers.  Width FOV ~ 7 mm.

Photomicrograph of “beaded” goethite along with kidwellite.  Note broken sphere at upper left.  Width FOV ~1 cm.

Finally, a third mineral present on the specimen is goethite, an iron oxyhydroxide: α-FeO(OH).  The alpha symbol α refers to goethite being one of four polymorphs of iron oxide-hydroxide.  Goethite occurs in many physical forms; however, the Arkansas specimen has a pisolitic form that reminds me of “fish eggs”!  Some masses show a slight iridescence.  Goethite is a secondary mineral and oxidizes where iron, and usually manganese, is present. 

REFERENCES CITED

Howard, J. M., 1987, Mineral Species of Arkansas - a digest: Arkansas Geological Commission Bulletin 23.

Howard, J. M., 2014, Rockhounding Arkansas: www.rockhoundingar.com.