It is not knowledge, but the act of learning, not possession but the act of getting there, which grants the greatest enjoyment.
Carl Friedrich Gauss
One of the interesting aspects of collecting minerals
is trying to find two minerals where radicals (an atom or molecule [usually the
case in mineralogy] with an odd number of electrons) are able to substitute for
each other in the chemical formula and create new species. Radicals are highly reactive since they are
always looking around for something to bond with (not unlike many high school
boys I knew) since they have a free electron wandering around in orbit. This free wanderer wants to meet up with an
atom looking to obtain an electron in order to stabilize.
Three of the more interesting radicals are PO4,
VO4, and AsO4 (phosphorus, vanadium and arsenic combined
with oxygen) and are usually organized as the Phosphate Class. All radicals operate as anions and have a
negative charge of -3. Therefore, they
easily (I think) combine with metallic cations with a positive charge. The resulting minerals are termed the
Phosphates, Vanadates, and the Arsenates and all are important in the mineral
world. For example, the mineral apatite
(with its many descriptive terms) is the backbone of vertebrate teeth and bones. In addition, vanadinite with the red
barrel-shaped crystals, and all of the colorful arsenates (like the pink
erythrite), are eagerly sought by mineral collectors.
The arsenate, phosphate and vanadate radicals are of
similar size and 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. This is an interesting situation since in
most solid solution series the substitutions are made by the positively charged
cations.
At the 2016 Tucson show I was able to pick up a nice
arsenate, lavendulan [NaCaCu5(AsO4)4Cl-5H2O],
a hydrated copper arsenate that forms as a secondary mineral in “oxidized zones
of some copper deposits” (MinDat). The specimen appears as sort of a crust
of intense electric blue on the matrix. However, on closer
examination under high power one can observe the crust is composed of very tiny
aggregates of thin platy crystals, some of which form rosettes. An older label
indicates collection at El Guanaco Mine, Santa Catalina, Antofagasta Province,
Chile, where MinDat noted 31 valid minerals and two type
localities. At the mine copper and gold veins are emplaced in Upper
Cretaceous and Paleocene volcanic sequences. I presume, but am
uncertain, that both the copper and the arsenic could be the result of
oxidation of the primary mineral enargite [Cu3AsS4].
I also have in my collection a small sample of
sampleite (a hydrated copper phosphate) that was picked up at the Denver
Show. Now this particular mineral is the
phosphate analogue of lavendulan—note the phosphate radical: NaCaCu5(PO4)4Cl-5H2O. As noted above, the similar size of the
radicals allows substitution of the phosphate radical for the arsenate radical.
Sampleite is blue to blue-green in color and is found
in a variety of habits from encrusting to rectangular tabular crystals to
rosettes. It has a hardness ~4.0 (Mohs),
is transparent with a light blue streak and has a pearly luster. Sampleite is a fairly rare mineral, compared
to lavendulan, but both occur in the oxidized zones of arid-region copper
deposits.
 |
| Small cluster of sampleite crystals, and "balls (upper blobs). Total width of cluster is ~1 mm. |
 |
| Cluster of small blades of sampleite. Width of large cluster ~1.5 mm. |
 |
| Cluster of nondescript sampleite crystals. Width of cluster ~.5 mm. |
My small mineral specimen came from the dumps associated
with the Endeavour 26 Mine in New South Wales, Australia. MinDat notes that
Endeavor 26 is both a surface and underground (currently) gold-copper complex
developed as a porphyry vein mineralization in a monzonite that is part of the
Goonumbla volcanic complex (Ordovician). The major primary (hypogene) minerals
are bornite, chalcopyrite and pyrite.
However, the most interesting thing about the mine is the presence of two
distinct and major oxidation zones, one of Carboniferous age and one of Cenozoic;
therefore, the host rocks have experienced prolonged weathering cycles (O'Sullivan and others, 2000).
The upper oxidized zone is dominated by the
secondary copper phosphate minerals libethenite (Cu2PO4OH),
pseudomalachite (Cu5(PO4)2(OH)4)
and the uncommon sampleite. However, this secondary phosphate mineralization
was preceded by the formation of another secondary mineral, atacamite (Cu2Cl(OH)3). Beneath the phosphates is a zone dominated
by malachite (Cu2CO3(OH)2), azurite (Cu3(CO3)2(OH)2)
and chrysocolla (CuSiO3--nH2O) that gives way at depth to
a thin native copper-cuprite (Cu2O)-chalcocite (Cu2S)
supergene enriched zone (Clissold and others, 2005).
So, the answers I needed to locate were to the
questions about: 1) the original source of the phosphorous; and 2) why are the
secondary phosphate minerals located in the upper zone only? Crane and others (2000) found that weathering
of apatite group minerals, especially hydroxylapatite, provided the phosphate
for the PO4 radical. In
addition, the zoning of the copper phosphate minerals is due to the
distribution of the apatite minerals in the host rock and the intensity of
weathering (Ollier, 1984). Ain’t learning fun?
For descriptions of pseudomalachite and libethenite
(January 4, 2016), lavendulan (March 21, 2016), and atacamite (October 9, 2014) see Blog postings.
REFERENCES
CITED
Crane, M.J., J.L. Sharpe, and P.A. Williams, 2001, Formation
of chrysocolla and secondary copper phosphates in the highly weathered supergene
zones of some Australian deposits: Records of the Australian Museum, v. 53.
Clissold, M.E., P. Leverett, and P.A.
Williams, 2005, Chemical mineralogy of the oxidized zones of the E22, E26 and
E27 ore bodies at Northparkes, New South Wales in Roach, I.C., ed., 2005, Regolith 2005-Ten Years of CRC LEME.
Ollier, C., 1984. Weathering: Longmans Publishing
Group, London.
O’Sullivan, P.B., D.L. Gibson, D.L Kohn, B. Pillans,
and C.F. Pain, 2000, Long-term landscape
evolution of the Northparkes region of the Lachlan Fold Belt, Australia:
constraints from fission track and paleomagnetic data: Journal of Geology 108.