WHAT IS THAT GREEN STUFF? "MEDMONTITE"

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 20^th; 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 

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.