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| CHRYSOCOLLA (FORMALLY KNOWN AS MEDMONTITE) FROM THE OK MINE. WIDTH IS 3.75 CM. |
I love green minerals (but also red, blue, purple,
etc.)! Perhaps the copper content in
many green minerals catches my eye? Perhaps
my fondness is due to various color deficiencies in my sight---I can “see”
bright green minerals but have serious problems noticing more subdued green objects—like
the green stoplights mixed in with various city lights. I try to avoid driving in a city during
Christmas celebrations, or at least have a stoplight spotter in the passenger
seat! So, I am always combing the hills
and deserts for bright-colored minerals.
In the early 1990’s I was working on an Environmental
Impact Statement (EIS) for a new petroleum pipeline that would run from
southwestern Wyoming diagonally across Utah to near St. George and then on to
southern California. This was a great
project and I hoofed many a mile looking for occurrences of important
fossils. The line crossed the Mineral Mountains
near the Utah town of Milford (southwestern Utah) so I was able to take a
closer look at several of the old mining areas to the west of town.
The Beaver Lake Mountains are situated to the
northwest of Milford and contain several small copper prospects and mines. Some are skarn-type copper deposits (altered Paleozoic
limestone) while others (i.e. the OK Mine) are copper porphyry deposits hosted
in quartz monzonite; many produced a few thousand tons of copper in the early-to-mid
1900’s (Wilson, 1995). In rummaging through
mine dumps along the road near the OK Mine I picked up a small piece of greenish-colored
something or other, called it malachite, and threw it in my bag. We ran into a local mining geologist who said
that I had a piece of medmontite, a name with which I was completely unfamiliar
(and essentially am today)!
In getting ready for my CSMS presentation in
September (the 20th; Rockin’
Thru Utah), I was trying to gather together my Utah specimens and came
across this long forgotten piece of medmontite.
So, the first thing that I did was to search the internet, and my books
on Utah---not much luck.
Mineral
Data (www.mindat.org) noted medmontite was
named in 1950 (Chukhrev and Anosov) and described as “a copper bearing mineral
of the montmorillonite group”---it seemed to be a copper infused clay (and a
synonym of cupromontmorillonite). The presence
of copper would explain the green color.
But, MinDat further noted that medmontite was discredited as a distinct
species and was “simply” a mixture of chrysocolla and montmorillonite.
So, my question was: what do you call the
specimen? And, I still don’t know the
answer but I guess chrysocolla. MinDat
is mum on the subject and virtually no internet resources describing the discredited
mineral could be located. The closest I
came was to note a crystal store in Oxfordshire, UK, selling medmontite
specimens (£4.99 plus shipping but including VAT) from the OK Mine that are “dead ringers” for my small piece. They describe medmontite as a mixture of chrysocolla
and mica that “is a stone of harmony”!
Wilson (1995) listed the following minerals from the
OK Mine: malachite,
azurite, bornite, chalcopyrite, pyrite, cuprite, brochantite, molybdenite, and chrysocolla. Therefore, I guess I will go with chrysocolla,
a hydrated copper silicate-- (Cu,Al)2H2Si2O5(OH)4·nH2O (but it still looks like malachite
to my untrained mineralogical eye)!
mike
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| PHOTOMICROGRAPH OF ABOVE SPECIMEN (2.40 MM WIDTH). |
REFERENCES CITED
Chukhrev, F.V. and F. Y. Anosov, 1950, Medmontite, a
Copper bearing Mineral of the Montmorillonite Group: Vses. Mineralog. Obshch.
Zapiski, ser. 2, 79(1), 23-27.
Wilson, J. R., 1995, A Collector’s Guide to Rock,
Mineral & Fossil Localities of Utah: Utah Geological Survey Misc. Pub.
95-4, 148 p.