Tuesday, October 28, 2014

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,PO4,SO4)2(O,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.

Tuesday, October 21, 2014

STRASHIMIRITE: RARE COPPER ARSENATE



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!Azurite crystals (photomicrograph).  Unknown red globules.  Field of view ~1.1 cm.
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 meteoric 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 explicably 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! 
 
REFERENCES CITED

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.

Sunday, October 19, 2014

MORE CHLORIDES: SYLVITE & CHLORARGYRITE



As noted in the previous blog, the halide minerals include those in which the halogen anions (chlorine, bromine, fluorine and iodine), with a negative charge, combine with metal cations (positive charge).  Many halide minerals seem to have low specific gravities, are essentially non-conductors of electricity, some good cleavage, are transparent to translucent (mostly depending upon impurities), and are soft (2-3+ Mohs).

Atacamite, described previously, is a chloride as are sylvite, halite, and chlorargyrite.  The initial two minerals are known as evaporites since their sedimentary depositional environment is usually a restricted circulation and drying basin.  Chlorargyrite, a silver chloride (AgCl), is much different and is a secondary mineral found in rocks that produce silver (similar to atacamite). 

In the United States the best known marine evaporites, described in my well-used Glossary of Geology (distributed by the American Geological Institute) as “water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution” are from the great Permian Basin.  The Basin was part of  a broad and shallow cratonic (inland) sea that extended from Mexico to southern central Canada; Permian rocks are well-exposed and much studied from west Texas north through the Plains states and Rocky Mountains.  As the Permian continued (that is going from older Permian time to younger) the world’s continents begin to coalesce into the vast end-of-Paleozoic supercontinent termed Pangaea. This event had the effect, in the later Permian, of driving off the earlier Permian shallow seas and creating very restricted circulation basins where water evaporated, the seas became more “saline,” and evaporitic minerals began to form.  In my native Kansas, where Permian rocks are well-exposed in the eastern one-half (and southwestern quadrant) of the state, observant rockhounds can literally see the rocks move from fossiliferous limestones and shales to inhospitable (for life) beds of red shale, halite (subsurface only), gypsum, and anhydrite.  However, the thickest and best known of the Permian rocks are located in the Permian Basin, a Permian subsistence basin occupying west Texas and parts of southeastern New Mexico. 

The Delaware Basin is a subsection of the greater Permian Basin and includes the section around Carlsbad, New Mexico, where redbeds and evaporites are common and gypsum crops out over a wide area.  Halite, anhydrite and potash (potassium salts) are widespread in the subsurface.  In addition, the Delaware Basin is a major producer of hydrocarbons.

The Carlsbad Potash District produces rock salt from dry mines, brine fields, and solar-salt operations at 18 locations; gypsum is mined at 13 sites; potash is produced from five underground mines; and sulfur is produced by the Frasch process at one site (Johnson, 1997).
 
Permian Basin.  Public Domain map.
Klein (2002) noted that in marine evaporitic basins the minerals precipitate out in a select order, and in the reverse order of their solubility.  The first to come out, and therefore the most common minerals in the rock column, are the carbonates: calcite [CaCO3], and dolomite [CaMg(CO3)2] when evaporation reduces the original sea water by ~50%; gypsum [CaSO4-2H2O] and/or anhydrite [CaSO4] when ~20% of the original volume is left (anhydrite, rather than gypsum, with higher salt concentration and higher temperatures); halite [NaCl] when ~10% is left; and finally the much rarer magnesium and potassium sulfates langbeinite [K2Mg2(SO4)3], polyhalite [K2Ca2Mg(SO4)4-2H2O], kieserite [MgSO4-H2O], and chlorides sylvite [KCl] and carnallite [KMgCl3-6H2O].  Although rare in most deposits, sylvite can form thick deposits and is mined extensively (for potassium) in the Carlsbad Potash District. 
 
Sylvite from the Carlsbad Potash District.  Width ~5.2 cm.
I have a mineral from the District that is somewhat difficult to identify—sylvite or halite!  Both minerals are found in similar environments and actually may be found together in the same specimen. Both have very similar physical characteristics: isometric with a cubic habit, usually found in massive granular masses, varied color from colorless to others due to impurities, soft (~2.5 Mohs), white streak.  In other words, they look alike. 

The major differences seem to be that sylvite has a “salty” taste but is more bitter than halite, and does not fluoresce under UV light (halite is commonly reddish orange under short wave and reddish to green-orange under long wave UV).  Sylvite from the Carlsbad Potash District.  Width ~5.2 cm.

OK, it is not wise to do a “taste test” but---I scraped a small amount of powder and unwisely put it on my tongue immediately rinsing with copious amounts of water.  I found it to be very bitter.  In addition, the specimen does not fluoresce. So, I pronounced it sylvite, a very late forming potassium chloride.
A nondescript sample of chlorargyite with arrows pointing at blackish "balls" of the minerals.  Other dark-colored areas may also be chlorargyrite.  The light-colored (blueish) vugs are some sort of a clay mineral.  

A photomicrograph of chlorargyrite with the arrow pointing at blackish "balls" of the minerals.  Width of view ~5 mm.
Chlorargyrite is also a chloride mineral (AgCl) but forms in a much different environment than the evaporitic chlorides.  Chlorargyrite is a secondary mineral and is found in the oxidized zone of sulfide deposits.  At a “typical” sulfide ore body meteoric water dissolves and leaches out several minerals as it percolate downwards.  This action has an oxidation effect on rocks above the water table but oxidation stops at the top of the table.  As this percolating solution reaches the water table mineral sulfides (secondary) begin to precipitate and a zone of mineral enrichment develops (the supergene). However, at times the water table fluctuates up and down, the primary surface (gossan) minerals (lots of quartz, iron oxides) dissolve, and  the percolating solutions drop new (secondary) metals in the zone of oxidation.  For example, chlorargyrite is the silver mineral found in the oxidation zone whereas acanthite [Ag2S] is the silver mineral of the supergene deposits.  Azurite [Cu3(CO3)2(OH)2], malachite [Cu2(CO3)(OH)2] and chrysocolla [(CuAl)2H2Si2O5(OH)4-nH2O)] are copper oxidation minerals while chalcocite [Cu2S] and bornite [(Cu5FeS4] are supergene minerals.  Both the supergene and oxidized zones are greatly enriched with minerals (chlorargyrite can be 75% silver), are usually fairly close to the surface, and therefore easy to extract. Generally they are/were the first to be mined. 

Chlorargyrite is a very soft mineral (1-2 Mohs), crystallizes in the isometric (hexoctahedral) system but rarely is found as yellowish crystals.  Mostly it is massive, sometimes columnar, and is usually dark brown to dark purple to almost black in color.  The specimen in my collection has patches of tiny, almost black, melted together, “balls.”  It matches photos shown on MinDat.  At times bromine or iodine ions partially substitute for the chlorine with resulting bromian chlorargyrite (also known as embolite) or idoargyrite.

My small purchased specimen was collected from the “Turquoise District—Courtland-Gleeson District” Cochise County, Arizona.  Mineralization at Courtland-Gleeson-Pearce is of several types: (1) copper carbonates and oxides in irregular blanket deposits where the Cambrian quartzite is thrust over Mississippian limestone creating a fault breccia (broken rock) close to a contact with an igneous intrusion; (2) lead and zinc carbonates, lead sulfates and zinc silicates with silver chloride, manganese and minor copper and gold in irregular ore bodies in Pennsylvanian-Permian limestones along fractures and faults; (3) turquoise in near-surface stringers and lenses in altered granite and quartzite—solution in fracture zones; (4) manganese oxides in irregular masses along fractures in limestone; and (5) spotty base metal ores with gold and silver values in veins located in intrusive rocks (MinDat, 2011).  What all this means is that faulting in the area created fracture zones that allowed heated (from the igneous intrusions) and mineralized solutions to travel through and deposit the metallic ores.

So, that the story.  Attach a chloride anion to something like a potassium cation and a salt is produced.  Sylvite is an important for use in fertilizers (the K) in the formula.  Attach the chloride to a silver cation and a very rich silver ore results.  I have not noticed a town by the name of Sylvite; however, I have visited Chloride, Arizona, an old silver mining town!

REFERENCES CITED 


Johnson, K.S., 1997, Permian evaporites in the Permian basin of southwestern United States: Prace - Panstwowego Instytutu Geologicznego, Issue 157, pt. 2.

Klein, C., 2002, the 22nd edition of the Manual of Mineral Science: John Wiley and Sons, New York.