Tuesday, April 2, 2013

ARROJADITE: A RARE PHOSPHATE--BLACK HILLS



The state of South Dakota has always occupied a place in my heart since graduating from the “U” in the late 1960’s.  As a native Kansan I was excited about traveling north to a different university and seeing new and exciting localities.  After an early fall field trip to the Black Hills, I was hooked on the state---forever.  I have returned to South Dakota many times since those halcyon days of my youth and always had my eyes to the ground looking for new specimens.  In addition, while attending rock/mineral shows and/or visiting rock shops I am on the lookout for specimens labeled as being collected from South Dakota.

At a recent spring rock show in Apache Junction, Arizona, I came across a dealer with a number of specimens acquired at a liquidation sale of some sort.  Much to my surprise, and excitement, one specimen was labeled “Arrojadite, Nickel Plate Mine, Pennington  Co., SD.  Collector: Bud Ehrle, 1960’s”, and priced for $3.  I snatched it up although not certain if I could differentiate between arrojadite, Arrowsmith, or Arrowhead Stadium! 
 

ARROW IS POINTING AT A SINGLE GREEN CRYSTAL OF ARROJADITE ALTHOUGH MANY OTHER GREEN PARTIAL CRYSTALS ARE VISIBLE.  LENGTH OF SPECIMEN: 2.3 CM.
After returning to my library and computer to search the references and try to find out if arrojadite was even a valid mineral name, I begin to wonder what sort of a strange specimen I had acquired.  Turns out that arrojadite is an exceeding complex anhydrous phosphate with a “long” chemical formula: KNa4Ca(Mn,Fe)14Al(PO4)12(OH)2, or at least something close.  That caveat is here because the latest work on the arrojadite, at least that I could locate (Camara and others, 2006), indicates the mineral is in a solid solution series with three end members: Arrojadite (KNa) [potassium-sodium aluminum phosphate], Arrojadite (SrFe) [strontium-iron aluminum phosphate], and Dickinsonite (KMnNa) [potassium-manganese-sodium aluminum phosphate].  However, www.MinDat.org noted there are many other “in-between” series members with various combinations of barium, lead, potassium, sodium, iron, strontium, and manganese. I certainly agree with Huminicki and Hawthorne (2002) when they stated arrojadites are infernally complex structures…and Camara and others (2006) who noted there is no doubt that the structure and crystal chemistry of arrojadites, [are] perhaps among the most complex found so far.  The good news is that mineralogists do understand that the South Dakota Nickel Plate Mine specimens are the arrojadite (KFe) variety with a green to green-yellow color, a hardness of ~5 (Mohs), a conchoidal fracture, and a rather vitreous luster. 


GLASSY GREEN STREAKS OF ARROJADITE.  WIDTH FOV ~ 9 MM.

Any member of the arrojadite group is rare in the mineral record.  In western North America there are minor occurrences in Canada (Yukon), Arizona, and South Dakota.  In the Black Hills www.MinDat.org noted arrojadite has been found in the Victory Mine (Custer County), Big Chief Mine, Etta Mine, White Cap Mine, and Nickel Plate Mine (Pennington County).  The latter mine, a one-time tin (cassiterite)- producing project, is the “type locality” for arrojadite (KFe) from a pegmatite near (east) of the Keystone cemetery. Dickinsonite (arrojadite group) and a variety of other minerals are also known from the mine.  I am hoping on my next visit to the Hills that my friends in South Dakota will guide me to the Nickel Plate Mine claim.  In the meantime, I have acquired a nice (but small), cheap ($3) specimen of a rare phosphate mineral. 
INDIVIDUAL CRYSTALS OF ARROJADITE LESS THAN 1 MM IN LENGTH.


REFERENCES CITED
Cámara, F., R. Oberti, C. Chopin, and O. Medenbach, 2006, The arrojadite enigma: I. A new formula and a new model for the arrojadite structure. American Mineralogist, 91, 1249-1259.

Huminicki, D.M.C. and F.C. Hawthorne, 2002, The crystal chemistry of the phosphate minerals, in M.L. Kohn, J. Rakovan, and J.M. Hughes, Eds., Phosphates: geochemical, geobiological, and materials importance: Reviews in Mineralogy and Geochemistry, Mineralogical Society of America, Chantilly, Virginia.

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