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| Lots of petrified wood, including some large polished slabs. |
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| Multi-colored Banded Iron Formation from Australia. Red and yellow jasper and chert and black hematite. Specimen width is about35 cm. |
I spent my last day at
the 2015 Tucson Shows by heading back to the Main Street venues perusing the many
small tents and stands selling a variety mineral specimens, petrified wood,
Indian zeolites, banded iron formation, Moroccan fossils, etc. There were some beautiful polished jasper
specimens and really large slabs of petrified wood from Madagascar. I also enjoyed visiting with several owners
of the smaller “mom & pop” operations.
One of the owners noted my continued interest in a flat of small spheres
of paratacamite. I engaged him in a fascinating
conversation about collecting the specimens from Baja California [Santa Rosalia, Boleo District, Mun. de Mulege, Baja California Sur, Mexico]. MinDat lists 47 valid minerals collected from
area mines and three type minerals with the best known being boleite.
The Boleo Mining District
is unlike most metallic ore deposits in western North America in that it is a sediment-hosted
copper-cobalt-zinc-manganese ore deposit. These sediment-hosted deposits are
known as manto ore deposits. In this
situation the metallic minerals replace sedimentary rocks, commonly limestones,
and form bodies along the bedding planes. What is questionable is the source of the ore—does
it come from a sedimentary source within the basin, or from an adjacent
intrusive pluton? Or perhaps the
minerals came from badly weathered primary deposits? Or are there other possibilities?
To my surprise, and gratefulness,
the owner let me pick out any individual sphere I would like to take home. Rockhounds, especially those of small shops,
are generally a nice group of people.
Last fall I put out a
posting (October 9, 2014) on atacamite, a copper++ chloride hydroxide [Cu2(OH)3Cl]
that is usually a secondary mineral oxidized from other copper minerals and forms
in arid and saline conditions. Atacamite
(Orthorhombic) is a polymorph (minerals
with the same chemical composition but different crystal structures) of
botallackite (Monoclinic), anatacamite (Triclinic) and clinoatacamite
(Monoclinic). At one time paratacamite
was thought to also be a polymorph.
However, a recent study (Welch and others, 2014) noted that the crystal
chemistry of paratacamite indicates the presence of zinc and/or magnesium and
therefore dropped its designation as a polymorph.
Paratacamite [Cu3(Cu,Zn)(OH)6Cl2]
looks quite similar to atacamite (at least to my untrained eye) so I asked the
vendor how he knew it was the former rather than the latter? He patiently explained that the nodules had
been “x-rayed and confirmed to be paratacamite.” That was good enough for me! Later I noted MinDat had crossed atacamite
off their Boleo District list and substituted paratacamite.
Crystals of the mineral
is green to darker greenish-black, fairly soft at 3 (Mohs), have a vitreous
luster, has some decent cleavage and a conchoidal fracture. My specimen has very tiny crystals and it is
quite difficult to observe specific physical characteristics. So, I was really happy for the vendor’s
identification.
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| A nodule of paratacamite (green) mixed with flakes of a light colored mineral, perhaps gypsum. Width of nodule ~1.6 cm. |
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| Photomicrograph of nodule section above. Cluster of green paratacamite crystals ~4 mm. |
In further reading, I learned
that at times magnesium or nickel replaces the zinc and the mineral becomes
paratacamite-(Mg) or paratacamite-(Ni). Welch
and others (2014) noted that upon heating paratacamite reversibly transforms
into herbertsmithite [Cu3Zn(OH)6Cl2] between
353 and 393 K.
REFERENCES CITED
Welch, M.D., Sciberras, M.J., Williams, P.A.,
Leverett, P., Schlüter, J., Malcherek, T., 2014, A temperature-induced
reversible transformation between paratacamite and herbertsmithite: Physics and
Chemistry of Minerals, 41, 33-48.