BLUE BOLEITE & DIABOLEITE: CSMS SHOW

As noted in the previous Post, the CSMS annual Show was a huge success with a large number of vendors, great display cases, and numerous satisfied visitors.  After visiting Tucson in February, and locating several blue-colored minerals (Show theme was Shades of Blue), I was now on the lookout at the CSMS show for additional blue minerals that were out of my price range in Arizona.  Much to my surprise I was able to pick up small specimens of boleite and diaboleite; both minerals are complex halides rather uncommon in the rock record.

Boleite [KPb₂₆Ag₉Cu₂₄(OH)₄₈Cl₆₂] has been known since the early 1890s but was not well understood until the late 20th century with the advent of sophisticated instrumentation.  The discovery locality, the Boleo Copper District located in Baja California Sur, Mexico, near the town of Santa Rosalia, has been the site of major sulfide mining (open pit until the 1980s) and at one time (at least in the 1950s) was the second largest producer of copper in Mexico (Wilson and Rocha, 1955). Underground mining started in 2012 with the first production in 2014 and I presume mining is still active.   As best I can decipher, the copper deposits are in an uplifted belt of Neogene (Miocene or Pliocene) rocks within the El Boleo Formation (deltaic and near-shore marine claystone-siltstone-sandstone beds).  The major sulfide ore minerals are chalcocite (Cu₂S) accompanied by chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), covellite (CuS), and native copper (Cu).  The oxidized zone (above the sulfide deposits) has a large variety of copper oxides, copper carbonates, copper silicates, manganese oxides, and rare halide minerals such as boleite, pseudoboleite, and cumengite.  The Boleo deposits are the type locality of the latter three halide minerals. The productive ore contains rather large amounts of copper, manganese, zinc, cobalt, lead, and silver; however, few minerals except copper were produced in commercial quantities.  Evidently the hydrothermal solutions traveling through the underlying Comondu volcanics (Miocene?) from even older (?Cretaceous) intrusive igneous rocks ( a quartz monzonite) carried metallic elements upwards in fractures and faults before depositing the sulfide ores in sedimentary rocks and sediments of the El Boleo Formation.  (above geological information from Wilson and Rocha, 1955).  The mineral deposits at Boleo are termed epigenetic since they formed after deposition of the host rocks.  Syngenetic would indicate deposition of mineral deposits simultaneously with deposition of the host rocks

The Boleo ores are interesting in that they are a Manto ore deposit.  These deposits are usually polymetallic in nature and form sheet-like bodies along bedding planes of sedimentary rocks.  The hydrothermal, metal-bearing solutions circulating off nearby igneous rocks (usually intrusive) supply the ore minerals. The types of primary sulfide minerals vary with the composition of the hydrothermal solutions.  Evidently the hydrothermal solutions at Boleo were rich in lead, copper, silver and a few other metals.  Oxidation and weathering of some sulfides allowed for the formation of the many colorful oxides, chlorides and carbonates.  MinDat lists 68 different minerals collected from the Boleo deposits.

So, the metals in the hydrothermal solutions are cations, positively charged molecules with more protons than electrons.  The metallic elements have a neutral charge but they emit electrons and lose their neutrality (and become  cations).  Chloride (Cl) is a negatively charged (more electrons than proton) anion (the anion of the neutrally charged element chlorine).  Cations, such as the lead, copper and silver in boleite are attracted to, and bond with, the negatively charged chloride anion. [KPb₂₆Ag₉Cu₂₄(OH)₄₈Cl₆₂]---I understand that concept.  But my question is---what is the source of the chloride in boleite?  Was it in the original hydrothermal solution, or was it completely secondary?  My guess, and this really is above my pay grade, is the chloride came from the host marine rocks and not from the original hydrothermal solution. I say this since MinDat notes that boleite also forms, rarely, when smelter slags are immersed in, and leached by, seawater (contains NaCl).  Boy, where are my mineralogy/petrology/ore genesis friends?  Whatever, chloride reacted with the sulfides and like magic----boleite and other halides.

Well, the above paragraph took a couple or three hours to write and I am not certain the sentences are correct.  I blame this lack of understanding on: 1) college chemistry befuddled my 18-19-year-old mind; 2) my first year in college was paid for by a basketball scholarship and that aspect was foremost in my thoughts; and 3) after struggling through those confusing chemistry courses I switched to geology and paleontology and as the saying goes, “found my niche.”  Now 50+ years later I am trying to relearn some basic chemistry---lifelong learning at its best! But this learning is tough, about like when I tried to master the German language ten years ago.  Well, not really master it but just try for some basic understanding.  I sort of achieved that aspect since I could order dark beer (dunkles Bier), bread (brot), sausage (wurst) and especially pig knuckles (schweinehaxen), as well as read the train schedules.  What more does one need in Germany?  I am still working on the chemistry aspect!

OK, so boleite is one of those uncommon hydroxyhalides (chlorine is cataloged into the Halide Group of elements) and the chemical formula [KPb₂₆Ag₉Cu₂₄(OH)₄₈Cl₆₂] shows the presence of a hydroxide ion (OH). Crystals have a beautiful indigo to prussian blue color; however, the color often appears much darker when examining a complete crystal in reflected light. It has a vitreous to pearly luster and is fairly soft (3.0-3.5 Mohs).  Thin pieces are transparent but larger crystal pieces appear opaque or at least translucent.  The powered form, streak, is a greenish-blue color.  Boleite belongs to the Isomorphic Crystal System and crystals are cubes although they commonly are twinned (Interpenetration Twinning) or have epitaxial pseudoboleite [Pb₃₁Cu₂₄Cl₆₂(OH)₄₈] or cumengite [Pb₂₁Cu₂₀Cl₄₂(OH)₄₀-6H₂O]. 

Photomicrograph of boleite cube with lower left corner chipped.  Width/length of cube ~ 5 mm.

Photomicrograph of boleite cube showing smaller penetration cube.  Width/length of large cube is ~3 mm.

Photomicrograph of boleite cube (upper) attached to a second boleite cube (B) surrounded by epitaxial pseudoboleite (P). Abdul-Samad and others (1981) stated "since pseudoboleite is never found without boleite upon which it is observed to geow epitaxially (Winchell, Ohio State University thesis, 1963), it is clear that boleite must form metastably prior to any pseudoboleite deposition."  Width of lower B-P section ~ 2.9 mm.

Photomicrograph of boleite cube (B) with epitaxial pseudoboleite. Width of photo ~4 mm.

Most boleite specimens on the market today come from the Boleo District and many/most seem to be floaters; crystals scattered over a matrix are rare and expensive.  According to one prospector/vendor I talked to in Tucson, boleite crystals of any kind are getting rather scarce.  In the U.S. the best-known boleite crystals are from the Mammoth-St. Anthony mine northeast of Tucson.

Speaking of Mammoth-St. Anthony, the specimen of diaboleite purchased at the Show is from this famous Arizona mine.  Diaboleite is also a hydroxyhalide [Pb₂CuCl₂(OH)₄] with lead and copper as the cations along with a chloride anion.  It forms in a similar environment as boleite, in the secondary zone of copper-lead deposits, and at times is found in association with boleite.  Diaboleite is also blue in color that seems not as intense (the prussian blue) as boleite.  It is quite soft (2.5 Mohs), crystals have a vitreous luster and are more transparent than boleite.  The streak is about the same as boleite but seems a little greener.  The major difference between the two minerals is that diaboleite belongs to the Tetragonal Crystal System and produces crystals that are tabular although specimens are often a mixture of nice crystals and very tiny granular crystals packed together.

So again the question about the chloride ions!  Humphreys and others (1980) noted that “halide salts are generally soluble in water, which explains why halide minerals of the transition elements are usually absent from the vicinity of oxidizing orebodies. In such an environment low pH values also tend to facilitate the solution of most species. Nevertheless, many halide minerals are known and occur in considerable quantities at some localities. Two groups of halide minerals containing Cu(II) and Pb(II) are outstanding in their complexity and rarity. These are the boleite group…and diaboleite [+others]… These…minerals are thus apparently confined to systems where the availability of chloride ions is very high.”  They furthermore observed these oxyhalides and hydroxyhalides can only form from aqueous solutions as secondary minerals.  As to why specific oxyhalides or hydroxyhalide minerals form, Humphry and others (1980) and Ardul-Samad and others (1981) conducted a number of solution studies and constructed Stability Field Diagrams that indicate the parameters (especially pH) needed for deposition of these specific copper chloride minerals.  As to the origin of the chloride, Ardul-Samad and others (1982) stated that the minerals occurring at Mammoth-St. Anthony are “an extremely complex assemblage in the oxidized zone of a base-metal orebody…a diverse and complicated inorganic chemistry must have been responsible for the formation of the compounds…”  I presume this latter statement could also apply to the Boleo Copper District.

In my discussion of boleite, I suggested that perhaps the chloride in the oxidized zone came from the host marine rocks/sediments but that was sort of a guess from a non-mineralogist!  I am still looking for an answer; however, Frost and others (2003) noted that “several chloride minerals of the base metals is known from oxidised zones, especially those located in arid areas, or those which are associated with saline ground waters.”   I presume, but again a guess, that the chloride in the “saline ground waters” came from ground water percolating through the host rocks/sediments!

REFERENCES CITED

Abdul-Samad, F., D.A. Humphreys, J.H. Thomas, and P.A. Williams., 1981, Chemical studies on the stabilities of boleite and pseudoboleite: Mineralogical Magazine, v. 44. 

Abdul-Samad, F., J.H. Thomas, P.A. Williams, R.A. Bideaux and R.F. Symes, 1982, Mode of formation of some rare copper (II) and lead (II) minerals from aqueous solution, with particular reference to deposits at Tiger, Arizona: Transition Metallic Chemistry, v.7.

Frost, R., Martens, W. and P. Williams, 2003, Raman spectroscopy of the minerals boléite, cumengéite, diaboléite and phosgenite –implications for the analysis of cosmetics of antiquity: Mineralogical Magazine v. 61.

Humphreys, D.L., J.H. Thomas, P.A. Williams and R.F. Symes, 1980, The chemical stability of mendipite, diaboleite, chloroxiphite, and cumengite, and their relationships to other secondary lead(II) minerals: Mineralogical magazine, v. 43.

Wilson, I.F. and V.S. Rocha, 1955, Geology and mineral deposits of the Boleo copper district, Baja, California, Mexico: U.S. geological Survey Professional Paper 273.

 

Sprichst du Deutsch? Nicht wirklich. Gerade genug, um die Züge und um Essen zu fahren. Aber kein Kaninchen.

Haben Sie Chlorchemie zu verstehen? Nicht sehr gut, aber ich habe intelligente Freunde.

Das ist alles, Leute!

Courtesy of  picgifs.com

STICHTITE: PIKES PEAK GEM AND MINERAL SHOW

A wide view of the inside of the event center.  Several members of the CSMS sold items or had display cases.

The 53^rd annual Pikes Peak Gem and Mineral Show was held at the Mortgage Solutions Financial Expo Center in Colorado Springs on June 3-5, 2016.  For the first time in many years the Show was entirely “inside” and that move was a nice change of pace compared to past shows---no rain, no high winds, plenty of room for displays and vendors, and no forest fires!  In fact, the 50+ vendors represented an amazing number for our venue, and a wide variety geology-related items were offered “for sale.”  I was really impressed with the diversity of minerals, rocks, gems and jewelry offered by vendors.  Kim and Bodie, along with a host of club volunteers, are to be congratulated for producing this awesome Show.

As usual, I was on the lookout for a few less expensive and rather uncommon minerals----no Brazilian amethyst or Arkansas quartz!  And, I consider my hunt successful as my modest collection picked up several nice specimens of interesting and colorful minerals. 

Stichtite encrusting chrysotile?, a serpentine mineral.  Specimen width ~6.5 cm.

One purchased item was a mineral that was completely unfamiliar to me: violet- to lilac-colored stichtite encrusting a green serpentine mineral (probably chrysotile or perhaps clinochrysotile---precise identification above my pay grade).  Stichtite is a hydrated carbonate of magnesium and chromium: Mg₆Cr₂(OH)₁₆[CO₃]-4H₂O and is some sort of an alteration product of serpentine.  Although many of us routinely refer to serpentine as a mineral, it actually is a “group” of minerals with the generic formula of D₃[Si₂O₅](OH)₄ where D is magnesium, iron, nickel, manganese, aluminum or zinc or a combination of cations.  Several members of the “group” are asbestiform in nature and have health warnings about breathing in the fibers. I believe my specimen is the serpentine mineral chrysotile: Mg₃(SiO₅)(OH)₄.  In addition to the chrysotile and stichtite, the specimen contains tiny grains of magnesiochromite [Mg(Cr,Al,Fe)₂O₄] and a somewhat substantial amount of magnetite---it readily attracts a magnet.

Bands of magnetite crystals (<----M) are interlaced in the serpentine.  Crystals of lilac-colored stichtite are replacing tiny crystals of magnesiochromite (<---------).  Crystal size less than 1 mm.

However, I purchased the specimen due to the beautiful colored stichtite contrasting with the green serpentine. It was collected from Stichtite Hill, Dundas Mineral Field, Zeehan District, Tasmania, Australia.  Most people know the island of Tasmania as home to the Tasmanian Devil, an endangered carnivorous marsupial.  This little critter is much better known that the mineral stichtite!

The Tasmanian Devil.  This cute little fellow is carnivorous and most likely would nip your finger.  Public Domain photo.

Taz, the Tasmanian Devil ©, the hero/villian in Loony Tunes cartoons.   Created by Robert McKimson and owned by Warner Bros.

Most stichtite specimens on the market were collected from a few localities (about five) with the best known being Stichtite Hill.  According to Bottrill and Brown (2000), specimen mining at the Hill produced material for carving—lilac colored stichtite on green or green-black serpentine.  Bottrill (MinDat.com, 2009) described the occurrence of stichtite deposits at Dundas as “hosted by generally massive serpentinite bodies, probably derived from altered chromite-rich dunites [olivine-rich rocks] within mid-Cambrian ultramafic complexes [rocks with low silica content and high magnesium/iron content], particularly the Dundas ultramafic complex (Brown, 1986; Burrett and Martin, 1989). These complexes are a series of thrust-emplaced [faulted], dismembered ophiolitic bodies along the Dundas Trough, which bisects western Tasmania. They were deformed and intruded by mineralised granites during the Devonian era (Brown 1986). The occurrences are erratic…”  Ashwal and Cairncross (1997) noted stichtite “occurs exclusively in Cr-rich serpentinites of ophiolites or greenstone belts…with stichtite formation invariably [post-dating] serpentinization.”  

In reference to the above paragraph, one that is tough to understand by a non-student of geology, consider the following abbreviated explanation:

The earth is composed of a number of moving plates and the plates include oceanic crust (rocks deposited in a marine environment) and continental crust (rocks that make up the continents).  When a continental plate collides with an oceanic plate, rocks of the oceanic plates are forced under (subducted) the continental plate since the later has rocks of a higher density.  Sometimes upper rocks of the oceanic plate are scraped off by the continental plate during subduction.  These rocks are termed ophiolites.  The collision of plates are sites of earthquakes, volcanism and mountain building.  During earlier studies geologists referred to the moving and collision of plates as “Continental Drift” but today we know the process as “Plate Tectonics.”  The entire process is quite complex and not quite as simple as noted here.

Many ophiolites have a high percentage of olivine and pyroxene group minerals that started out as mantle or near mantle rocks (deep seated rocks below the earth’s crust).  As these mantle rocks are brought toward the surface by tectonic forces during mountain building events along subduction zones, the minerals in the rocks begin to destabilize (oxidation, metamorphism, input of water) and the rocks change into serpentine group minerals.”   Ashwal and Cairncross (1997) then believe the stichtite formed by a “reaction between serpentine and altered chromite during addition of substantial fluid.”  That fluid is either water (H₂O), some phase of CO₂ or carbonic acid (H₂CO₃)

So, the rocks at Dundas were subjected to the process of serpentinization and then invaded by fluids (post-serpentine formation) that interacted with the serpentine and altered chromite resulting in the lilac mineral stichtite along with the original green chrysotile (or some serpentine mineral).

That is the best that I can do with the process. Geologists first learned about seafloor spreading (think Mid-Atlantic Ridge) ca. 1960 although the idea of Continental Drift was thrown around in the earlier part of the 20^th Century by Alfred Wegener.   After the validation of sea floor spreading the theoretical model of plate tectonics was put forth amid much debate.  My university “schooling” was during the 1960s and needless to say the plate tectonics theory was widely discussed in almost every geology class.

But back to stichtite.  The lilac- to violet-colored mineral is very soft (1.5-2.0 Mohs) and may be scratched by a fingernail.  It appears translucent to transparent in very thin “slices” I could peel away.  These thin slices almost appear micaceous or fibrous under the high power of a microscope but under eye observation the mineral appears as encrusting.  It is subvitreous to waxy in luster. It is non-magnetic but this can be deceiving.  Intermixed with the mineral and seemingly in layers beneath the encrustation are magnetite crystals and fibers.  Stick a magnet to the lilac mineral and it will adhere due to the underlying magnetite therefore appearing to indicate that stichtite is magnetic!  I could not locate identifiable stichtite crystals; however, MinDat notes it belongs to the Trigonal System.  Stichtite has a polymorph, stichtite-2H, that belongs to the Hexagonal System but has the same chemical formula (again above my pay grade for identification).

A very thin crust of stichtite.  Width of photo ~5.1 cm.

Reverse side of photo above.  The color is brighter and the encrustation appears as micaceous or fibrous plates.

Photomicrograph of a section of above photo.  Width ~ 1.4 cm.  Note bands of magnetite.

Photomicrograph of same specimen showing the fibrous nature of both the stichtite and chrysotile. Width ~ 1.2 cm.

Minerals with chromium acting as the chromophore are fun to collect since the element produces a variety of colors: the red or pink in rubies, the green in emeralds and purple or red in alexandrite.  In pink or red spinel some chromium ions replace aluminum and voila. So, is chromium the pink/lilac coloring agent in stichtite?  Maybe, but that is a question for my mineralogist friends, maybe Tom up at www.dakotamatrix.com or Pete up at USGS.

But, it was nice to add stichtite to my collection.  

REFERENCES CITED

Ashwal, L.D. and B. Cairncross, 1997, Mineralogy and origin of stichtite in chromite-bearing serpentinites: Contributions to Mineralogy and Petrology, v. 127.

Bottrill, R. S., and G. Brown, 2000, Rare Australian gemstones: stichtite: Australian Gemologist, v. 20.

Brown, A.V., 1986, Geology of the Dundas–Mt Lindsay–Mt Youngbuck region: Bulletin Geological Survey Tasmania, no. 62.

Burrett, C.F., and E.L. Martin, (ed.), 1989, Geology and Mineral Resources of Tasmania: Special Publication Geological Society of Australia, no.15.