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].
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| Photomicrograph of boleite cube with lower left corner chipped. Width/length of cube ~ 5 mm. |
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| Photomicrograph of boleite cube showing smaller penetration cube. Width/length of large cube is ~3 mm. |
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| 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. |
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| 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!
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| Courtesy of picgifs.com |