MAJUBA HILL: CLINOCLASE & ARTHURITE: COPPER ARSENATES

Clinoclase crystals, with some other copper minerals, fo Majuba Hills Mine.  Width FOV ~4.0 cm.

Clinoclase is another one of those colorful hydrous copper arsenates: Cu₃(AsO₄)(OH)₃.  It is more common than the previously mentioned (see Blog) strashimirite [Cu₈(AsO₄)₄(OH)₄-5H₂O] but still is not overly abundant for collectors and usually is considered as rare.  My specimen came from the Majuba Hill Mine in Nevada (as did the strashimirite), 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!  I don’t even pretend to understand all of the basic chemistry behind the diagrams but certainly can observe that clinoclase comes out of solution with a higher pH.  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.  Mass of dark green arthurite crystals. Width FOV ~1.9 cm.

Clinoclase crystals,photomicrograph.  Width FOV ~1.3 cm.

Clinoclase (monoclinic) commonly forms nice crystals that generally are acicular in form, and have a nice vitreous luster.  The color varies but generally crystals are some sort of a blue-green although a dark blue is not uncommon.  It is rather soft at ~3 (Mohs) or less.

Mass of dark green arthurite crystals. Width FOV ~1.9 cm.

The mineral at Majuba Hill that seems able to take advantage of whatever anions are present in the aqueous solutions is arthurite, a hydrous copper/iron arsenate: CuFe+++(AsO₄,)₂(OH)₂-4H₂O.  The copper/iron usually combines with the arsenate radical (the originally described arthurite) but can also attach some phosphate or sulfate.  In addition, arthurite has a number of related minerals (Arthurite Group), seemingly in solid solution, where the copper cation is replaced by cobalt, iron++, zinc, or manganese.  So, there are a great variety of minerals, or possible arthurite-like minerals, with changes in the cations and/or the radicals!  I suppose that one needs sophisticated instrumentation to determine exact chemical composition.  I call my specimen arthurite since MinDat agrees with that mineral from Majuba Hill.

Arthurite crystals and sprays, photomicrograph.  Width FOV ~1.1 cm.

Arthurite (monoclinic) is “greener” than strashimirite or clinoclase and is an more of an emerald to dark apple green.  It is somewhat harder at ~4 (Mohs).  Crystals are acicular to prismatic and quite vitreous.  Like the other arsenates, arthurite is found in the secondary oxidized zone and is derived from arsenopyrite or enargite (Cu₃AsS₄).  However, I believe enargite is a minor component at Majuba Hill so the arsenic must come from the arsenopyrite.

Try to learn something about everything and everything about something.  Thomas Huxley

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.