GREEN ARSENATES II: LIBENTHENITE, CORNWALLITE, PSEUDOMALACHITE

CONTINUED: PART II

 

OK, so the zinc-copper cations may substitute for each other but what about the arsenate radical?  As noted in other Blogs (and numerous professional publications by mineralogists) the PO₄ and AsO₄ radicals are about the same size and with similar electron configurations and do substitute for each other in the Olivenite—Libethenite Series Cu₂(AsO₄)(OH)---Cu₂(PO₄)(OH). 

Libethenite then has the arsenate radical (of olivenite) replaced by PO₄ and the crystal symmetry has changed to Orthorhombic to Monoclinic.  It is a rather rare/uncommon mineral but again is found as a secondary mineral in the oxidation zone of copper ore deposits.  Some unnamed copper mineral supplies the copper while the phosphate is usually derived from the weathering of apatite or perhaps monazite or xenotime (rare earth phosphates) (MinDat.org). Cuprian adamite, Mapimi, Mexico.  Width FOV ~7 cm.

I have three different specimens of Libethenite displaying beautiful green (often emerald-green or olive-green), crystals with a vitreous luster that are translucent to transparent.  Although they have a hardness of around 4.0 (Mohs), individuals are quite brittle and fracture rather easily.  One specimen from near Ray, Arizona (Ray Mine), has sort of squat/equant crystals with some wedge-shaped terminations.  A second specimen from Morenci, Copper Mountain District, Arizona, has small sprays of elongated prismatic crystals.  The third, from near Lordsburg, New Mexico , is a large specimen (5.5 x 6.0 cm) with one side covered with a mass of individual, acicular, striated, bladed and terminated crystals---a really gorgeous specimen. 

Photomicrograph. Mass of libethenite. Individual crystals are ~1--2mm in length.  It is tough to provide quality photos of dark colored and vitreous crystals.

Photomicrograph of individual libethenite crystal.  Length of the centered individual crystal ~1 mm.

Spray of libethenite crystals.  Length of spray ~1 mm.

Spray of libethenite crystals.  Length of spray ~.4 mm.

Mass of small libethenite crystals that are non-acicular.  Each individual mass is ~1.4 mm in width.

As far as I can determine, there are no officially recognized intermediate minerals in the Olivenite-Libethenite Series.

Cornwallite, also a copper arsenate [Cu₅(AsO₄)2(OH)₄ [see Blogs January 3, 2015 and June 8, 2014], has an isostructural relationship (is in a series) with the phosphate pseudomalachite [Cu₅(PO₄)₂(OH)₄].   That is, they have the same crystal structure (both are Monoclinic) but different chemistries—the phosphate and arsenate radicals.  I cannot find evidence of an intermediate member in this series.

Crystals of green cornwallite.  Width FOV ~1.0 cm.

Cornwallite is a secondary mineral found in the oxidized zones of copper sulfide deposits where the ores contain both arsenic and copper and perhaps oxidized from something like tennantite (Cu₁₂As₄S₁₃) or enargite (Cu₃AsS₄).  Most cornwallite occurs as botryoidal to globular crusts of microcrystalline radiating fibers.  The dominate color is essentially an emerald green. 

Pseudomalachite is commonly a blue-green to emerald-green, usually compact or botryoidal, mass of non-descript and microscopic crystals. It has a hardness (Mohs) of 4.5 or less, and has a variety of habits: “commonly compact radiating and spherical, may be fibrous, in paintlike crusts and films, botryoidal, massive” (Handbookofmineralogy.com); and crystals in microscopic prismatic radiating clusters. With a strong light and magnification, the botryoids are translucent to transparent and have a vitreous luster.  

Crust of pseudomalachite.  Width of specimen ~5.5 cm.

Photomicrograph pseudo botryoids (P) encrusted on chrysocolla.Width FOV ~1.9 cm.

Pseudomalachite is a secondary phosphate found in the oxidized zones of copper ore deposits, at times in association with true malachite [Cu₂(CO₃(OH)₂].  The botryoids of pseudomalachite appear visually similar to botryoidal malachite; however, this type of malachite usually displays light and dark green banding.  In addition, as a carbonate, malachite will react to hydrochloric acid. 

So, this is today’s contribution to my elementary understanding of mineral series and nice green arsenates and phosphates.  It is not anything earth shattering, but I enjoyed the learning aspect!

You can only do your best.  That’s all you can do.  And if that isn’t good enough, it isn’t good enough.            Imelda Staunton

REFERENCES CITED

Braithwaite, R.S.W., 1983, Infrared spectroscopic analysis of the olivenite-adamite series, and of phosphate substitution in olivenite: Mineralogical Magazine, v. 47. 

Chukanov, N.V., D.Y Pushcharovsky, N.V. Zubkova, I.V. Pekov, M. merlina, S. Mckel, M.K. Rabadanov, and D.I. Belakovskiv, 2007, Zincolivenite CuZn(AsO₄)(OH): A new adamite-group mineral with ordered distribution of Cu and Zn: Doklady Akademii Nauk, V. 415, No. 3.

Trites, A.F., Jr., and R.H. Thurston, 1958, Geology of Majuba Hill, Pershing County, Nevada: U.S. Geological Survey Bulletin 1046-I.