BLUE SAMPLEITE: COPPER PHOSPHATE

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 (CuSiO_3--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.