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