Sunday, January 11, 2015

ARSENATES AND MINERAL SERIES



If I were again beginning my studies, I would follow the advice of Plato and start with mineralogy!  Apologies to Galileo Galilei

I did not start my geology studies until mid-semester in my second university year—later than most students.  And mineralogy (first semester third year) was a very difficult course for me.  In fact, I almost dropped the geology program since the first six weeks of the course was spent trying to learn crystallography and I seem unable to visualize in three dimensions.  We spent class time looking at wooden crystal models and trying to relate these to beautiful mineral crystals that really had no resemblance to the models (at least in my mind).  Monoclinic, dipyramidal, miller indices---really did not make much sense to me.  Finally, we moved out of crystals and into groups of minerals—sulfides, carbonates, phosphates—and my mind begin to function, at least some of the gears started to slowly click. 
As I experienced “going to the field” and actually collecting rocks and minerals, I got a terrible sinking feeling in my stomach that minerals observed in the outcrops did not look anything like either those crystal models or the examples in the drawers.  So, I shoved crystallography into the deep recesses of my mind and immersed myself in fossils, something that I could better understand.  That was a good feeling since I was now on my third major and really needed to “settle in.”


A
As a student more interesting in pokking around in the field, and maybe catching snakes and bats, how was I supposed to remember that Tetragonal-dipyramidal Class crystals have a single 4-fold axis perpendicular to a mirror plane?  This results in 4 pyramid faces on top that are reflected across the mirror plane to form 4 identical faces on the bottom of the crystal. Sketch and description courtesy of Tulane University.

So here I am, many decades later in life, trying to muddle through chemistry and crystallography and really appreciating the skills of people like Pete up at the USGS and Tom at Dakota Matrix.  Both are willing to help with any questions that pop into my mind.  In addition, I note that colleagues out in that cyberland called www.mindat.org and rockhounds@drizzle.com are a tremendous source of information and comfort.


I have spent several hours composing this small posting.  I am trying to understand the mineral dynamics, and at the same time post a legitimate and hopefully informative offering.  It has been tough, but intellectually engaging.  I just hope that I got it correct---I am still having trouble understanding isostructural relationships and mineral series!
As I have stated before---I really love the arsenates and phosphates.  Many are colorful (in fact, many of the green ones look very similar), most are small and easily stored, and there are hundreds to choose from (including many uncommon minerals).

Olivenite is a rather common copper arsenate [Cu2(AsO4)OH] that gets its name from the characteristic olive-green color.  However, many specimens are composed of aggregates of acicular crystals that appear almost black or very dark green, in color.  Upon observing individual crystals under magnification, the olive-green color becomes quite apparent.  These tiny crystals have a nice vitreous luster and many are transparent to almost translucent.  The hardness is ~3.0 (Mohs), and they seem quite brittle and easily fracture in an irregular manner.  What I have described above is the olivenite desired by most collectors.  However, the mineral also occurs as earthy, granular or fibrous masses that may not be olive-green in color but other shades of green to gray to yellow to pale white; crystals in these habits generally are opaque.  As with most of the secondary arsenates, olivenite forms from the oxidation of ores containing arsenic (arsenopyrite, tennantite, enargite) and a copper mineral—take your pick but at Majuba Hill chalcopyrite, pyrite and arsenopyrite are the major hypogene minerals with chalcocite being the enriched copper ore mineral of the supergene.


Photomicrographs of olivenite crystals from Majabu Hill Mine, Nevada.  Each individual crystals average about 1.5 mm in length.
My specimen is from the Majuba Hill Mine in Pershing County, Nevada, where in a Blog posting on October 21, 2014 I noted the mining geology as a copper-tin-arsenic deposit that Trites and Thurston (1958) described as a complex plug of rhyolitic rocks intruding Triassic sedimentary rocks.  Copper (27,000 tons of copper ore shipped between 1916 and 1949) and tin (350 tons of shipped ore) were the major commodities with small amounts of gold, lead, arsenic and silver.

One of the interesting aspects of many arsenates is their ability to interact with other minerals in a series where there seems to be a gradual substitution of one element with another element or radical when the two substituting entities are about the same size and with similar electron configurations.  There are at least two end members in a series and there may be recognized (officially named) or unrecognized intermediate members.  Some of the arsenates also have an isostructural relationship with other minerals.  In this relationship each mineral has a similar chemical structure (in the same crystal system) but with a different chemical makeup.
Radiating crystals of adamite.  Width FOV ~7 mm.
For example, in olivenite the copper arsenate [Cu2(AsO4)(OH)], zinc sometimes replaces the copper cation and a new mineral is formed: adamite, a zinc arsenate [Zn2(AsO4)(OH)].  But there are intermediate forms—zincolivenite is a structurally distinct, recognized mineral (Chukanov and others, 2007) containing both zinc and copper: CuZn(AsO4)(OH).  Each of the cations specifically ranges from 25% to 75% with the optimum distribution being 50%-50%.  Adamite and zincolivenite belong to the Orthorhombic System while crystals of olivenite are Monoclinic; the changeover to Orthorhombic is at ~20% zinc (Braithwaite, 1983). 
Cuprian adamite, Mapimi, Mexico.  Width FOV ~7 cm.
In addition to zincolivenite, there are informal members of the series such as cuprian adamite, a nice bright green variety of adamite with an unspecified amount of copper (as I understand it, less than 25%)—(Zn,Cu)2AsO4OH.  And, zincian olivenite [(Cu,Zn)2(AsO4)(OH)], containing less than 25% zinc, is known, but rare. However, two of the “prettiest” adamites are: 1) the violet-purple crystals from Mapimi, Mexico, where cobalt acts as a chromophore; and 2) the bright lime-green crystals where uranium salts are present in small amounts.  In this latter variety of adamite an ultraviolet lamp really lights up the specimens and they bright glow a yellow color.  Try as I might, I could not get a photo of my specimens under UV light.
.

Radiating crystals of adamite, lime-green in color.  Width of bundle ~2.0 cm.

Olivenite, adamite, zincolivenite, and other intermediate forms such as cuprian adamite are found at the Gold Hill and East Tintic Districts in Utah; both areas are well-known for their arsenate minerals. 

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.