GOUDEYITE: ANOTHER RARE COPPER ARSENATE SPHERE

This has been one of those weeks where the pandemic almost seems it will never end.  I am right at a hundred days of trying to make the best of a somewhat unpleasant and confining situation.  However, my mind always pops up and notes that I am well and safe and should be thankful for my home as a location to stay well.  So, off I go to read or write or exercise—attitude is everything.  Life is good.

You cannot tailor-make the situations in life, but you can tailor-make the attitudes to fit those situations.       Zig Ziglar

Goudeyite is another one of those colorful hydrated copper arsenates also containing a hydroxyl: AlCu₆(AsO₄)(OH)₆-3H₂O [sometimes REEs substitute for some of the aluminum].  It is not overly abundant for collectors and usually is considered as rare.  My specimen came from the Majuba Hill Mine in Nevada [the Type Locality], the home to a variety of arsenate minerals found in the oxidized zone of the sulfide ore body.  All of these secondary copper arsenates seem to have been derived from the primary arsenic-bearing mineral, arsenopyrite (FeAsS).  My question has been, why do all of these different secondary copper arsenates form from the same primary mineral?  Well, I found an interesting article by Magalhães, Pedrosa de Jesus, and Williams (1988) noting that solubility products and formation-free energy seem to control the formation of different minerals.  These authors have produced a number of different stability field diagrams (equilibrium models) “illustrating the chemical conditions under which the various species may crystallize from aqueous solution.”  I had to dig into the deep recesses of my brain to think about some of my chemistry classes and the use of stability diagrams!  They also noted that some of the earliest-formed copper arsenates minerals may be replaced during later chemical changes where new minerals form.  Interesting stuff.

Above,length of goudyite crust ~4.75 mm.

 

Photomicrographs at different scales showing botroydial crust of green goudeyite.  Note the nice sphere in the lower photo whose diameter is somewhat less than half a millimeter.

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). 

Goudeyite often forms as acicular crystals that forms tuffs or masses of hair like fibers or spherical masses of radiating hexagonal crystals.  In other instances, the crystals are virtually impossible to observe but form a crust (such as my specimen). Goudeyite is green to yellow-green to blue-green, usually has a vitreous luster and a hardness of ~3.5 (Mohs). As a member of the Mixite Group it is tough to identify and may be confused with other members.

REFERENCES CITED

 Magalhães, M.C.F., J.D. Pedrosa de Jesus and P.A. Williams, 1988, The Chemistry of formation of some secondary arsenate minerals of Cu(II), Zn(II) and Pb(II): Mineralogical Magazine v. 52, no. 368.

Trites, A.F., Jr., and R.H. Thurston, 1958, Geology of Majuba Hill, Pershing County, Nevada: U.S. Geological Survey Bulletin 1046-I.