Thursday, October 9, 2014

ATACAMITE: A RARE COPPER CHLORIDE



I attended a very small elementary school in central Kansas where the norm was two grades in one room.  At times both classes took the same course while at others (English and Math especially) the classes were separate.  During most of those years students studied geography of some sort—sometimes centered on the world, sometimes on the U.S. and in one instance we concentrated on Kansas.  Although we were required to memorize the capital cities of the U.S. states, I was more fascinated by physical landforms, or understanding the physiography of the continents.  I collected rocks and maps. That fascination continued on into my university studies until I caught the geology bug (there was not a major in geography).  As stated before, I have never regretted becoming a geologist (and sort of self-taught geographer).  Life has been good.

As I have gone through life, I’ve found that your chances for happiness are increased if you wind up doing something that is a reflection of what you loved most when you were somewhere between nine and 11 years old.”                          Walter Murch

Somewhere in the grade school curriculum I learned about the Atacama Desert in Chile (the “absolute desert”) as being the driest place in the world.  Now, it was dry in Kansas (most years) but we learned that some localities in the Atacama had not seen rain in hundreds of years—at least in recorded human history.  That aspect made a deep impression on my developing brain and I wrote a little “theme paper” on the Desert (using a trusty encyclopedia for some facts and most likely plagiarism)!  The best answer in the Funk and Wagnall’s for the dryness seemed to be a rain shadow created by the Andes Mountains.
 

The Atacama in South America.  Courtesy of Google Earth ©
Climatologists seem to know a little more today about the extreme aridity: The Atacama Desert represents an extreme habitat for life on Earth and scientists use it as an analogue for dry conditions on Mars. Aridity in the Atacama is primarily caused by the cold water of the Humboldt Current running parallel to the Chilean and southern Peruvian coast, preventing precipitation in the coastal areas. The hyperaridity is then intensified by the rain-shadow effect of the Andes Mountains to the east, which effectively block moisture transfer from the Amazon Basin.  (Paraphrased from Dunai and others, 2005).

Later in life I found that a mineral, atacamite, a copper++ chloride hydroxide [Cu2Cl(OH)2], was named for the Desert.  Atacamite is a member of the Halides where one of the halogens, something like bromine, iodine, chlorine, or fluorine are the major anions.  These anions (negative charge) combine with cations (positive charge) like sodium (NaCl, halite or salt), calcium (CaF, fluorite), potassium (KCl, sylvite), or copper with a hydroxal radical thrown in (atacamite).  The latter mineral is secondary mineral oxidized from other copper minerals and in Chile formed in an arid and saline condition.  However, atacamite is also known to form as a sulfide weathering product around subsea black-smokers, volcanic sublimates associated with fumarole deposits, and interestingly, crystals have been located as alteration products on very old copper and bronze (alloy: copper plus tin or arsenic) human artifacts.

Atacamite is a fairly rare copper mineral except in a few localities such as southern Australia and Chile.  The chisel-end crystals vary in color between a very dark, blackish green and a light, bright green.  The mineral is soft (3.0-3.5 Mohs) and brittle with a vitreous luster.  The transparency varies between translucent (in the dark green variety) and transparent (light green specimens). Collector crystals are slender prismatic and striated and/or tabular (others are massive or fibrous).  MinDat noted that atacamite may alter to malachite or chrysocolla.

In many short internet blurbs (for example, see Wikipedia) atacamite (Orthorhombic) is listed as being a polymorph (minerals with the same chemical composition but different crystal structures) of botallackite (Monoclinic), clinoatacamite (Monoclinic) and paratacamite.  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 drops its designation as a polymorph.  MinDat also has added anatacamite (Triclinic) as a polymorph.  All of this is sort of confusing to an ole stratigrapher like me.  What I do know is that all of these polymorphs are somewhat difficult to identify in hand samples without some sophisticated instrumentation.

In Chile, atacamite is a major constituent of the copper supergene deposits where circulating meteoric waters chemically weathered and oxidized the primary ore deposits (sulfides of the hypogene).  The circulating waters redistributed these secondary sulfides (the oxidized sulfides).  The early miners always valued the supergene zones since the newly formed minerals were greatly enriched with metallic elements, and easy to extract.

Now, the above paragraph sounds great to a non-mineralogist like me.  Most articles indicate that atacamite is a constituent of the enriched supergene zone and assumed to be “primary” and oxidized from a copper sulfide.  But, in doing some extra reading in order to learn more about ore genesis, I ran onto an interesting article (Cameron and others, 2007) explaining the critical difference between important and primary!  They go on to explain that: 1) atacamite requires saline solutions to form (to supply the Chloride) and supergene oxidation is caused by percolating meteoric (rainwater and fresh) waters; and 2) atacamite dissolves in fresh water or undergoes a phase change.  The problem in Chile is that the supergene enrichment lasted from ~44 Ma to ~9 Ma and was stopped by the onset of hyperaridity; however, during this period of time fresh water was present (not saline water).  So, what is the answer?   Cameron and others (2006) presented two possibilities: 1) the atacamite-bearing oxides resulted from the replacement of preexisting oxides after the onset of hyperaridity as this dryness allowed for the concentration of chloride in the meteoric waters; and/or 2) saline waters percolating  upwards along fault zones supplied  the chloride.  How about that!!  Chilean atacamite is an important mineral in the supergene but is not a primary constituent.
 
  

Atacamite (A) sprays on malachite (M) with a base of chrysocolla (C) and small "balls of halloysite (H).   Collected from the La Farola Mine, Cerro Pintado, Las Pintadas district, Tierra Amarilla, Copiap√≥ Province, Atacama Region, Chile. Width of specimen ~3.3 cm.

Photomicrograph of dark green, chisel-end atacamite crystals with “balls” of halloysite resting on malachite.  Width of mineral section ~7 mm.
The second specimen in my collection is from the “Moonta Mines in South Australia.”  Information about the formation of atacamite in this area seems a bit more difficult to acquire.  The Moonta copper deposits (including Wallaroo) operated between 1860 and 1923 when several hundred thousand tons of copper were produced (along with some gold).  The price of copper forced the mine closing until 1988 when some open- pit mining produced addition copper and gold (423 kg) until again closing in 1993 (Geological Survey of South Australia, 2014).

 

Layer of atacamite crystals, some translucent, (and some unknown minerals) collected from “Moonta, South Australia.”  Width of specimen ~4 cm.


Photomicrograph, width ~7 mm., of specimen above.

At Moonta there are a series of steeply dipping pegmatitic veins hosted by the Precambrian Moonta Porphyry (1.737 Ga) of probable volcanic origin. Primary mineralization includes chalcopyrite, pyrite and bornite in a quartz, feldspar, tourmaline, chlorite and hematite gangue.  However, there have been at least three, and most likely four, periods of later hydrothermal alteration.  The enriched secondary sulfide zone, composed of chalcocite and covellite, caps the primary lodes (Geological Survey of South Australia, 2014).  The atacamite seemed to form when copper-bearing solutions migrated upward into a previously barren Quaternary clay unit.  Early miners used the presence of shallow atacamite nodules to indicate the presence of ore bodies directly below their occurrence (Keeling and others, 2003).  Now, that is about total knowledge of atacamite!

However, I could not leave without a final bit of trivia.  A biologist I once worked with said something like “have you ever seen a bloodworm?”  I had to admit that perhaps night crawlers were better fish bait in my quest for Kansas catfish.  He noted that geologists ought to be interested in these little creatures since Glycera dibranchiate use copper mineral fibers to strengthen the outside of their teeth in order to help resist abrasion.  He pointed to a new article in Science (Lichtenegger and others, 2002) describing the mineral atacamite as the strengthening copper mineral—“ bloodworm jaws exhibit an extraordinary resistance to abrasion, significantly exceeding that of vertebrate dentin and approaching that of tooth enamel.”  Now, that is an interesting factoid that someday may win you a contest prize!  Isn’t learning fun?

There is no end to education.  It is not that you read a book, pass an examination, and finish with education.  The whole of life, from the moment you are born to the moment you die, is a process of learning.                          Jiddu Krishnamurti

REFERENCES CITED

Cameron, E.F., M.L. Leybourne and C. Palacious, 2007, Atacamite in the oxide zones of copper deposits in northern Chile: involvement of deep formation waters: Miner Deposits, v. 42.  
  
Dunai, T.J., G. A. Gonzalez and J. Juez-Larre, 2005, Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms: Geology, v. 33, no.4.

Geological Survey of South Australia, 2014, Minerals: Copper: http://www.minerals.dmitre.sa.gov.au/

Keeling, J.L., A.J. Mauger, K.M. Scott, and K. Hartley, 2003, Alteration mineralogy and acid sulphate weathering at Moonta Copper Mines, South Australia in  Roach, I.C. (ed.) Advances in Regolith, CRC LEME. 

Lichtenegger1, H.C., T. Sch√∂berl, M.H. Bartl, H. Waite, and  G.D. Stucky, 2002, High abrasion resistance with sparse mineralization: copper biomineral in worm jaws: Science, v. 298, no. 5592.