Tuesday, October 21, 2014


The July-August (v.54) issue of the American Mineralogist  listed several names for new minerals recently approved by the International Mineralogical Association.  One of these was a rare copper arsenate named strashimirite discovered in Bulgaria and named for a Professor at Sofia University (Strashimir Dimitrov).  Well, I am a sucker for arsenate minerals since I am fascinated with the common interchangeability of the phosphate (PO4), arsenate (AsO4), vanadium (VO4) radicals.  Jones (2011) noted that solid solution series commonly exist between these radicals with both end members and intermediate members between the arsenate and vanadate radicals and the phosphate and arsenate radicals.  There are no intermediate members between the vanadate and phosphate end members. 

Many of the arsenates are quite colorful (for example see Blogs 5/10/14; 5/18/14/;  6/8/14) and with this in mind I could not resist picking up a specimen with beautiful azurite crystals and tiny spherules of strashimirite.  I mean, when you don’t have the slightest idea what a mineral “is” (strashimirite), and the price is right (cheap), pick it up for the collection!
Specimen from Majuba Hill, Nevada. Azurite crystals are obvious; however, there are a number of minerals in the specimen quite difficult for me to identify.  width ~5.5 cm.
Azurite crystals (photomicrograph).  Unknown red globules.  Field of view ~1.1 cm.
This copper arsenate [Cu8(AsO4)4(OH4)-5(H2O)] usually occurs as a pale green to white crust of radiating spherulitic aggregates although the crystals (monoclinic) are usually tabular or elongate (mighty small in the spherules).  It has sort of a nondescript luster described as greasy or pearly.  I would describe the spherules as “dull” and quite soft (~2.5 Mohs).

Photomicrograph. Spray of strashimirite acicular crystals---although it could be a similar copper arsenate, parnauite. Width of spray less than 1 mm.
Photomicrograph showing globules of strashimirite. Field of view ~1.0 cm.

Photomicrograph. Spray of strashimirite acicular crystals, although it could be parnauite. Width of spray less than 1 mm.
Since its discovery in Europe, strashimirite has been located in a few localities in the U.S. (see MinDat.org): Tintic and Gold Hill Districts in Utah, four mines scattered across Nevada, and one in Montana. My specimen was collected Majuba Hill Mine (Copper Stope), Antelope District, Pershing County, Nevada (western). It appears that Majuba Hill has produced the largest number of specimens on the collector’s market.

The Majuba Hill Mine is a copper-tin-arsenic deposit that Trites and Thurston (1958) described as a complex plug of rhyolitic rocks intruding Triassic sedimentary rocks.  Copper (27,000 tons of copper ore shipped between 1916 and 1949) and tin (350 tons of shipped ore) were the major commodities with small amounts of gold, lead, arsenic and silver.  Uranium is also known from the mine (area) but has not been mined (I think).  The copper and tin were mined in the supergene area that was enriched by percolating solutions along faults and fractures (maximum depth average ~200 feet).  Strashimirite (and azurite) is a mineral of the enriched oxidation zone (average depth ~60 feet) that would be located above the supergene (see previous discussions on atacamite and chlorargyrite). 

Chalcopyrite, pyrite and arsenopyrite are the major hypogene minerals with chalcocite being the enriched copper ore mineral of the supergene. As for the tin, cassiterite is found in the primary hypogene ore, the supergene enrichment area and the zone of oxidation. 

Majuba Hill, Tintic and Gold Hill are all areas known for their specimens of colorful arsenate minerals.  My question---what is the source of the arsenic at Majuba Hill?  I have not found a reference that explicitly states XXX mineral is the source.  However, my best guess is the arsenic leached from the arsenopyrite (FeAsS about 46% by weight arsenic).  Trites and Thurston (1958) noted that chalcopyrite, pyrite and arsenopyrite were primary hypogene ores and that “arsenopyrite is notably abundant in the copper-and tin-bearing vein in the copper stope.” Arsenic is easily oxidized from arsenopyrite and in fact, arsenic is a common minor element of most copper ore.  The “loose” arsenic then is able to combine with metallic cations like copper and produce the copper arsenate minerals.  At other time the arsenic is released into the mine drainage and helps to produce some toxic and nasty water.  Arsenic is also known to transfer from a solid state to a gaseous state and fly out of smelter smokestacks into the atmosphere and ultimately to the ground as fine particles.

Arsenic is not nice stuff but it can produce some very attractive minerals! 

Jones, B., 2011, The Frugal Collector, v. 1:  Ventura, CA., Miller Magazines.  
Trites, A.F., Jr., and R.H. Thurston, 1958, Geology of Majuba Hill, Pershing County, Nevada: U.S. Geological Survey Bulletin 1046-I.

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