Thursday, April 27, 2017

PARISITE: CERIUM RARE EARTH MINERAL



Learning is an everyday event for a slumer like me.  For example, take the element cerium!  Before today, what did I know about cerium?  Turns out not much.  I knew that: it was named after the dwarf planet Ceres; 2) cerium oxide is used in polishing high quality glass lenses, and as a final polish for some lapidary specimens; 3) it is “probably” a Rare Earth Mineral (REE); and 4) somehow it is used in gas (the lantern gas, like Coleman) mantles.   Other than those factoids my knowledge about cerium was pretty sparse.
OK, Ceres is the largest planet, or “object” that is positioned between Mars and Jupiter in the asteroid belt (that factoid came from my early 1960s era astronomy class).  In turn, Ceres was named after the Roman Goddess of agriculture and fertility. 

Cerium oxide is both unstable Ce2O3 and stable CeO2 and the latter is often referred to as cerium(IV) oxide and is the type used in polishing compounds. 

Cerium is a lanthanide element—one of the 15 metallic elements with atomic numbers 57-71.  This group also contains elements not overly familiar to non-chemists such as neodymium and europium.
Pretty amazing to me is that cerium is much more abundant in the earth’s crust than lead or tin and despite the REE moniker is not very rare.  As for lantern mantles, I fail to understand the chemistry but mixing thorium oxide with cerium oxide and coating the silk mantles produces a very white light.  I suppose today very few “younger persons” know how to properly install a mantle in a “gas lantern!”  Of course, as a kid I knew next to nothing about burning whale blubber oil but did know how to “trim a wick” in a kerosene lamp!

I have learned that in nature cerium seems to never occur as a stand- alone element and that most cerium is produced from mining and refining the Rare Earth Minerals monazite (lanthanide + thorium + PO4) and bastnäsite (lanthanide carbonate fluoride).  Several months ago, I offered a post on bastnäsite (May 9, 2013).  Knowing a little more, but not much, about REE and REM after a later posting on a field experience to a REM mine near Colorado Springs (October 17, 2015), I was surprised to see a specimen of the REM parisite offered for two bucks at a mineral store in Tucson.  I did not have the slightest idea what parisite was when I found the specimen in an isolated drawer of thumbnails.  But due to what Phillip Caputo (2013) calls the MD (Magic Droid), I soon learned much and for that low price will pick up most anything that is not in my collection!

So, parisite is a REM (and actually rare) composed of calcium, cerium, lanthanum combined with a fluoro-carbonate, or Ca(Ce,La)2(CO3)3F2.  Well, that last statement is sort of a generality because mineralogists “in the know,” and with an XRD or some other gizmo providing analytical information, use parisite as a general term and designate the minerals parisite-(Ce), parisite-(Nd) and parisite-(La) depending on the “domination” of these REE.  And then it becomes quite easy to confuse parisite (any type) with lanthanide-dominant specimens of bastnäsite, synchysite, and röntgenite,  And, so its goes.
 
Photomicrograph of a banged-up and chipped parisite crystal.  Note partial hexagonal faces at arrow.  Height of crystal ~1.3 cm.
Therefore, the question then becomes, what is the correct name for my purchased specimen.  I am going to settle on parisite-(Ce) [CaCe2(CO3)3F2], not because I have a XRD in my back pocket, but because MinDat has stated the cerium variety is the mineral species at the Snowbird Mine in Montana, the home of my small collected crystal.
Parisite is not what one would call a spectacular mineral, nor a museum piece, but one that is mainly of interest to rockhounds.  It seems only to occur as definite crystals as not as massive or encrusting forms.  Parisite belongs to the Hexagonal Crystal System and commonly occurs as double hexagonal pyramids that at times appear to be prismatic.  However, the crystals are not prime specimens and often are striated and very “rough” looking.  Crystals are often brown or amber in color but at times range down to brownish-yellow to yellow.  Specimens are very brittle, are transparent to translucent with a yellow-white streak, and have a sub-vitreous to greasy luster.  Hardness has been measured as ~4.5 (Mohs).  Almost all crystals are small in size and I suppose 3 cm. would represent a very large specimen.  

Metz and others (1985) describe the Snowbird Mine as occurring in “a lenticular, rare earth- and fluorite-rich quartz-carbonate body with a pegmatitic texture, which intrudes Belt Supergroup metasediments [late Precambrian] at the Idaho-Montana state line west of Missoula, Montana…the U-Th-Pb parisite ages (71.1 + or - 1.0 m.y.) indicate emplacement during the Late Cretaceous, probably associated with the intrusion of the nearby Idaho batholith.”

REFERENCES CITED

Metz, M.C., D.G. Brookins, P.E. Rosenberg and R.E. Zartman, 1985, Geology and geochemistry of the Snowbird Deposit, Mineral County, Montana: Economic Geology, vol. 80 no. 2.

Caputo, P., 2013, The longest road: overland in search of America from Key West to the Arctic Ocean: Henry Holt and Company, New York.

I had only one hard-and-fast rule:  avoid interstates.  They are predictable and boring and their uniformity somehow erases changes in landscape; you can drive six hundred miles, from forests into deserts, and fell that you haven’t gone anywhere.  In a sense, you haven’t.  You have no idea about the lives of the people in the towns and cities you’ve bypassed at seventy miles an hour.      Phillip Caputo

Saturday, April 22, 2017

CHENEVIXITE: A GREEN ARSENATE



I continue to be fascinated by minerals containing the element arsenic (As).  The arsenate minerals are those minerals containing the anion AsO4- - -  and are often grouped/studied together with the phosphate minerals [PO4 - - -] and the vanadate minerals [VO4- - -].  Since these three anions are about the same size with the same charge, minus-3, they often replace and substitute for each other and a new mineral is born. I have written many posts about the arsenates and they include a metallic cation plus the AsO4 anion: annabergite (nickel), austenite (copper and zinc), clinoclase (copper), conichalcite (calcite and copper), cornubite (copper), cornwallite (copper), erythrite (cobalt), chenevixite (copper and iron), mimetite (lead), and olivenite (copper).  An example, annabergite: Ni3(AsO4)2-8H2O

The arsenite minerals are those containing arsenic in a metallic role and cation (As) and often combing with other metals which in turn combine with sulfur (the anion) to form a sulfide: arsenopyrite (iron), cobaltite (cobalt), enargite (copper), orpiment (arsenic), realgar (arsenic), proustite (silver), tennantite (copper). Example, enargite: Cu3AsS4

The arsenide minerals have arsenic (As) as its major anion: algodonite (copper), domeykite (copper), nickeline (nickel), skutterudite (cobalt, nickel), lollingite (iron).  Example, nickeline: NiAs

The arsenates are the most common minerals containing arsenic while the arsenides are relatively uncommon.  The arsenites are somewhere “in-between.”

Last summer at the CSMS Show I was rummaging around and came across a specimen of chenevixite, a mineral completely unknown to me.  However, the green color indicated the possible presence of copper so I scooped it up.  The price tag in a broken-down specimen box said $2, a good bargain. 
Chenevixite is a hydrated copper iron arsenate with copper and iron as the major cations and the arsenate ion as the anion [Cu(Fe)(AsO4) – (OH)2].  It is a rare mineral and found in the secondary oxidized zone of polymetallic ores.  Chenevixite represents the oxidation product of the primary sulfides enargite and tennantite (both copper arsenic sulfides).   
Coating of green chenevixite on matrix.  Width of specimen ~5.5 cm.
Chenevixite is tough to recognize in hand specimens without knowing something about the mining location—the crystals are much too small to see without help of a magnification device.  The mineral is some sort of a green color from yellow-green to olive green to dark green.  Chenevixite appears as a massive coating on matrix and the crystals are cryptocrystalline, much too small to be picked up with my camera equipment.  Luster is hard to distinguish, not really earthy but certainly not bright, perhaps “oily.”  Hardness is ~4.0 (Mohs) and the massive form appears opaque but that is difficult to determine; it may be semi- translucent.  It does produce a yellow-green streak.

Photomicrographs of massive chenevixite coating matrix.  Individual, submillimeter,  "globs" may be observed in some masses.  Width of specimen ~1.1 cm.
With a name like chenevixite I suspected the name came from a French locality or perhaps a French scientist. It was named for Richard Chenevix (1774-1830) an Irish chemist born in Dublin but who later lived and died in Paris.  Its Type Locality is from Wheal Gorland in Cornwall, England. Chenevixite forms a solid solution series with luetheite as aluminum replaces the iron.

My specimen came from the Chuquicamata Mine in the Atacama Desert of Chile (west Coast).  The scarcity of chenevixite in the world may be due, at least partially, to the fact that it is one of the few arsenic minerals that is stable in arid regions but often leaches in more humid region.  The minerals of the Desert are usually rare in other environments.

Now, here is a question above my pay grade: bronze is a combination of copper and another metal, usually tin, whose discovery, was a great metallurgical feat since it allowed the construction of “harder” implements and weapons. However, the first bronze was made with copper and arsenic and termed arsenical bronze.  I wonder if chenevixite ever provided both copper and arsenic, was ever smelted into bronze? Another one of one of life’s persistent questions!


Albert grunted. Do you know what happens to lads who ask too many questions?
Mort thought for a moment.
No, he said eventually, what?
There was silence.
Then Albert straightened up and said, Damned if I know. Probably they get answers, and serves 'em right.          Terry Pratchett