WHITEMOREITE: BLACK HILLS PHOSPHATE

 

JAY GATSBY

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My quest to become a life-long learner has led me in many different directions from trying to relearn some basic chemistry to reviewing some of my high school/college lit class assignments (yes I remember). The other day I was thumbing through the Cliff Notes copy of The Great Gatsby by F. Scott Fitzgerald, one of St. Paul, Minnesota’s favorite sons.  I really didn’t like the novel when I was 18 or 20, and it seems my “likes” have not changed in half a century. What I did remember was a quote that stuck in my mind and reappeared every decade or so: Let us learn to show our friendship for a man when he is alive and not after he is dead. Today I have sort of paraphrased and rewritten that sentence to: Let us learn to honor our friends when they are alive and not after they are dead!  I am certain some of my Blog readers are now thinking that Mike is at it again, way off his rocker.  I am thinking that the editor of this Blog is an ole softee and will put his red pencil away (that red pencil phrase dates me) and allow me to zig-zag round.  So, I will; please stick around!

The last few days I have been wandering through MinDat looking at data and photos from the many mines in the Black Hills of South Dakota, places with fond memories.  I came across information about the Big Chief Mine located not far from the tourist town of Keystone. Active in the late 1800s, the Big Chief was never a large producer of primary feldspar and mica and secondary beryl hacked out of a small cut and a short drift in a couple of Proterozoic (Precambrian) zoned pegmatites. But, this small mine has yielded 73 different minerals and varieties including the phosphates olmsteadite, perloffite, and metavivianite that call the Big Chief their Type Locality (MinDat.org). Recently a beryllophosphate [Ba[Be₂P₂O₈]-H₂O] was described and the Mine became the Type Locality of mineral #4 (Yang and others, 2022).  This phosphate was named after Tom Loomis, the longtime owner and operator of the on-line mineral store Dakota Matrix.com out of Rapid City, South Dakota. I refer to Tom as Mr. Phosphate since I am convinced that he is about the only person who can visually identify the myriad of tiny phosphate minerals that are found in the pegmatites of the Black Hills.  He has helped me with mineral identification a number of times and for this I am thankful.  All of this ties in with the paraphrased F. Scott Fitzgerald quote noted above.  It is nice to honor, by naming a new mineral,  a very alive and kicking mineralogist rather than waiting for the inevitable. Congratulations Tom.

Big Chief Mine exposing Precambrian pegmatites. Photo courtesy of Dakota Matrix Minerals.  

 

According to Loomis, writing on the website at Dakota Matrix Minerals, the Big Chief  could very well had a fifth Type Locality mineral with the discovery of the secondary iron phosphate whitmoreite  [Fe²⁺⁺Fe³⁺⁺⁺₂(PO₄)₂(OH)₂-4H₂O]. Bill Roberts, the famous collector of South Dakota minerals, first found a new mineral (later whitmoreite) in 1971 but somehow was edged out of the naming rights by specimens found in New Hampshire (also in 1971) and in Maine (1973).  The Maine mine, Palermo No. 1, was designated as the Type Locality in 1974 (Moore and others).

I was able to acquire a specimen of whitmoreite collected from the Big Chief and mounted by Art Smith in 1979 after he purchased it from Black Hills Minerals.  Loomis noted that the crystals from the Big Chief are amber to greenish brown in color with chisel-shaped terminations. At times the crystals are found as isolated individuals or scattered groups, but the most recognizable crystals resemble “floating naval mines,” a term he picked up from Moore (Moore and others, 1974). Like many secondary phosphate minerals the minute crystals are measured in millimeters or less.

A "floating naval mine" of minute whitmoreite crystals. Width of crystal mass is less than 1 mm, probably about .5 mm, and is the best my camera could produce. The darker "out-of-focus" crystals are probably rockbridgeite.  

Whitmoreite is a member of the Arthurite Group (monoclinic arsenate and phosphates), named for arthurite, a hydrous copper/iron arsenate: CuFe⁺⁺⁺(AsO₄,)₂(OH)₂-4H₂O--the copper/iron cations combine with the arsenate radical and water.  In other members of the Group the copper cation is replaced by cobalt, iron++, zinc, or manganese. Notice that in whitmoreite the copper is replaced by the divalent cation ferrous iron (Fe++). In addition, the phosphate radical (PO₄) may replace the arsenate radical in some minerals (as in whitmoreite).  So, there are a variety of minerals, or possible arthurite-like minerals, forming with changes/substitutions in the cations and/or the radicals!  

Arthurite is emerald to dark apple green in color with a hardness of ~4 (Mohs).  Crystals are acicular to prismatic and quite vitreous.  Like the other arsenates, arthurite is found in the secondary oxidized zone and is derived from arsenopyrite or enargite (Cu₃AsS₄).  

Crystals of arthurite from Majuba Hill, Nevada. Some crystal sprays are very dark green in color and quite vitreous and my camera refuses to pick up the individuals Width FOV ~ 7 mm.

REFERENCES CITED

Moore, P.B., Kampf, A.R. and Irving, A.J.,1974, Whitmoreite, Fe²⁺Fe³⁺₂(OH)₂(H₂O)₄[PO₄]₂, a new species: Its description and atomic arrangement. American Mineralogist: 59.

Yang, H., Gu, X., Gibbs, R. B., and Scott, M. M., 2022, Loomisite, IMA 2022-003, in: CNMNC Newsletter 67. European Journal of Mineralogy: 34.