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.
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.
Beautiful Piece and Awesome Information :)Have a Wonderful day. I Will keep checking in :)
ReplyDelete