PEZZOTTAITE AND LONDONITE/RHODIZITE: MORE PESKY CESIUM MINERALS

The more I live, the more I learn.  The more I learn, the more I realize, the less I know.
           Michael Legrand

I have just about exhausted my knowledge on cesium with the Post on pollucite (March 15, 2018).  However, in further examining a list of minerals containing essential cesium, I looked through the back recesses of my mind and pulled out a couple of minerals in my collection!  I remember purchasing both at a past Denver Show due to the rareness and red-orange color of pezzottaite, and the rareness and crystal shape of londonite-rhodizite.  In addition, I thought it would be interesting to have these minerals containing cesium.  Perhaps some day I would get around to building an atomic clock!

MinDat.com has listed 18 minerals containing essential cesium as a positively charged cation that combines with various negatively charged anions: sulfur (sulfide), oxygen (oxide), boron + oxygen (borate), phosphorus + oxygen (phosphate), arsenic + oxygen (arsenate), vanadium + oxygen (vanadate), (silicon + oxygen (silicate).  All 18 minerals are relatively uncommon with pollucite (a silicate) being the best known (and the major ore source of cesium) since it contains a maximum weight percentage of ~43% cesium.  Pezzottaite ranges up to ~15% cesium by weight while londonite may hold ~8% cesium with less in rhodizite, a close relative.  There are also minor traces of cesium in beryl (silicate; particularly the morganite variety), the evaporitic chlorides carnalite and sylvite, and perhaps others. 

Pezzottaite is a member of the Beryl Group whose other representatives include: Beryl, beryllium-aluminum silicate; Indialite, magnesium-aluminum silicate and a dimorph of cordierite; Bazzite, beryllium-scandium-aluminum silicate; Stoppaniite, iron+++ dominant analog of beryl.  Pezzottaite was first discovered in ~2002 in Madagascar by rockhounds who thought it was a red beryl variety (bixbite) while gemcutters called it raspberry stone or raspberry beryl. It was displayed/sold at the spring 2003 Tucson Gem and Mineral Show and created much buzz. After discovering the presence of cesium in the mineral, a new moniker was awarded—cesium beryl.  But finally, mineralogists noted: 1) the presence of lithium in the mineral now giving the formula of Cs(Be₂Li)Al₂Si₆O₁₈; 2) that beryl is prismatic and Hexagonal (Mineral System) while pezzottaite is Trigonal (Mineral System); and 3) the Refractive Index of pezzottaite (1.60-1.62) is higher than beryl (1.56-1.59) (Laurs and others, 2003). 

This diagram show the crystal faces of a pezzottaite crystal.  Photo courtesy of Laurs and others (2003).

Regardless of exact name and chemical formula, gemologists jumped on the chance to facet an exciting new red to pink mineral.  Although pezzottaite is generally a mineral for collectors, a few good pezzottaite specimens are suitable for faceting and some gem dealers have beautiful cut stones.  In addition, less transparent specimens are cut and polished into cabochons.   

Pezzottaite is a hard mineral at 8.0 (Mohs), has a vitreous luster in raspberry-red to pink colors, a white streak, and ranges from translucent to transparent in clarity. The mineral exhibits conchoidal fracturing and is brittle and will fracture; therefore, faceted or cabbed specimens might be better off as a pendant rather than mounted in a ring (where it might get smacked).  Pezzottaite is also one of these minerals that may exhibit a “cat’s eye effect” and that magic line movement in the stone is due to the alignment of fluid-filled inclusions (rather than to a mineral such as rutile.

Pezzottaite (P) embedded in Albite (A), with Tourmaline Group (T).  Collected Sakavalana pegmatite near Ambatovia, Madagascar.  Width of specimen ~ 1.9 cm.

Pezzottaite crystal from above specimen (flipped 180 degrees). Note crystal faces m, d and c as diagrammed on specimen from Laurs and others (2003).

Pezzottaite is a mineral of cesium-rich, pegmatitic granites and is usually found as isolated crystals in vugs. Collecting localities are scarce and pezzottaite is only recorded from:  10 the Sakavalana pegmatite near Ambatovita, Madagascar, where Laurs and others (2003) believed it formed when pollucite was corroded and released cesium into late-stage pegmatite formation;  20 the Deva mine and Dara-i-Pech pegmatite field, Kunar Province, Afghanistan (Panzer and others, 2010);  and 3) near Molo, in the Momeik area of the Mogok district, Burma (Devourd and Devidal, 2007).

Londonite is a nifty looking mineral crystal in my collection that seems to  rank third on the list of weight-percent of cesium, coming in at ~8%.  However, there is a small problem. It is almost impossible to visually identify the borate minerals londonite (dominated by cesium and including rubidium) from the potassium-dominated rhodizite; they are in a solid solution series: (Cs,K,Rb)Al₄Be₄(B,Be)₁₂O₂₈  to 

(K,Cs)Al₄Be₄(B,Be)₁₂O_28.  So, londonite is the cesium analog of rhodizite and “specimens from Madagascar [as mine is] without analysis should be labelled as londonite—rhodizite series. Many crystals contain complex patchy zones of both londonite and rhodizite which are only visible by back-scattered electron imaging” (MinDat.org). 

Most specimens of londonite-rhodizite were referred to as rhodizite until 2001 when Simmons and others declared that the cesium rich variety would be separated out and named londonite.  But even today, 17 years later, most of the crystals at the rock and mineral shows from Madagascar are marketed as rhodizite. The rubidium quantity in londonite is low and seems absent from rhodizite.  The following phase diagram shows the distribution of cesium, potassium and rubidium in londonite-rhodizite.
 

Plot of representative compositions of Madagascar rhodizite and londonite in terms of K (Potassium), Rb (Rubidium), and Ce (Cesium).  Diagram from Simmons and others, 2001.

Londonite-rhodizite belongs to the Isometric Crystal System and often occurs as nice individual crystals that are mostly dodecahedrons.  Specimens are usually vitreous, hard (~8.0, Mohs), with pale colors of yellow and white ranging to colorless, mostly translucent but rather transparent if colorless, a white streak, brittle, and with a conchoidal fracture.

Diagram (Public Domain) of a 12-sided dodecahedron crystal.

It is difficult to tell this is a mounted specimen of a dodecahedron crystal of londonite-rhodizite from Madagascar.  Width of crystal as oriented ~6 mm. 

Both rhodizite and londonite seem to occur in more highly evolved pegmatites, often in miarolitic cavities.  Like pezzottaite, collecting localities for londonite-rhodizite are few and far between:  mostly in Madagascar, the Type Locality in Russia, one questionable site in England, and rhodizite in the Animikie Red Ace pegmatite in Florence County, Wisconsin.

REFERENCES CITED

Bradley. D.C., A.D. McCauley and L.L Stillings, 2010, Mineral-deposit model for lithium-cesium-tantalum pegmatites: Chapter O in Mineral Deposit Models for Resource Assessment: USGS Scientific Investigations Report 2010-5070-0.

Devouard, B., Devidal, J.L., 2007, Pezzottaite from Myanmar: Gems & Gemology, v 43, no. 1.

Hawthorne, F.C., Cooper, M.A., Simmons, W.B., Falster, A.U., Laurs, B.M., Armbruster, T., Rossman, G.R., Peretti, A., Günter, D., Grobéty, B., 2004, Pezzottaite Cs(Be₂Li)Al₂Si₆O₁₈ A spectacular new beryl-group mineral from the Sakavalana pegmatite, Fianarantsoa province, Madagascar. Mineralogical Record: v. 35, no.5.

Henrich, E. W. 1985. A Mount Antero postscript. Rocks & Minerals v.60.

 

Laurs, B.M, W.B. Simmons, G.R. Rossman, E.P. Quinn, S.F. McClure, A. Peretti, T. Armbruster, F.C. Hawthorne, A.U. Falster, D. Günther, M.A. Cooper, and B. Grobéty, 2003,  Pezzottaite from Ambatovita, Madagascar: A New Gem Mineral: Gems and Geology, v. 39, no. 4.

London, D., 2008, Pegmatites: The Canadian Mineralogist Special Publication 10. 
 

Modreski, P. 2005. Colorado mineral collecting localities. Rocks & Minerals v. 80.

Panzer. G., M. Graft, D. de Ligny, M. Boudeulle, and B. Champagon, 2010, Luminescent centres in pezzottaite, CsBe₂LiAl₂Si₆O₁₈ : European Journal Mineralogy, v. 22.

Simmons, W. B.; Pezzotta, F.; Falster, A.U. and  Webber, K. L., 2001, Londonite, a new mineral species: The Cs-dominant analogue of Rhodizite from the Antandrokomby granitic pegmatite, Madagascar: Canadian Mineralogist v 39.

POST SCRIPT 

I have spent a large amount of time reading scientific articles pertaining to this short article.  As stated before, I am merely an ole rockhound and am nothing close to a mineralogist or petrologist.  But, learning is a real joy to me and I certainly appreciate the many resources available, especially Pete Modreski from the USGS in Denver who explained the ideas behind the term "highly evolved pegmatites" (highly enriched in any or all of the light, or volatile, or high-field-strength elements).

Anyone who stops learning is old, whether at twenty or eighty.  Anyone who keeps on learning stays young.

          Henry Ford

 

Since this posting has much geology-speak I will try, my best, to offer some explanations.
 
CRYSTAL SYSTEMS: geologists define seven mineral systems (Isometric, Triclinic, Monoclinic, Trigonal, Orthorhombic, Tetragonal, Hexagonal) defined by: 1) imaginary internal lines termed crystallographic axes: 2) the length of the said axes: and 2) the angle at which these axes meet.  There are usually three axes named C (the vertical and the longest), B and C (both horizontal) and all pass through the center of the crystal.  In the Hexagonal System there are three horizontal axes (add D).

There is too much information about crystal systems to insert into this post.  I would suggest that interested readers check out any of several web sites such as: www.gemsociety.org/article/mineral-habits/.

All of the Systems have subdivisions termed Crystal Classes.  Each Class is defined by shape and symmetry (morphology).  A good web site with explanations and diagrams is: www.mineralogy4kids.org/all-about-crystals/crystal-systems.

PRISMATIC:  Refers to shape/habits of minerals.  Prismatic minerals are elongate with well developed crystal faces parallel to the C axis (the long axis)--think a quartz crystal.  See the web site www.en.wikipedia.org/wiki/Crystal_habit

LONDONITE, RHODIZITE, PEZZOTTAITE, THE NAMES:  Minerals are named in a variety of ways; today many/most are named after well-known mineralogists, for example bobjonesite after the Senior Editor of Rocks and Gems.  All proposed names must be approved by teh International Mineralogical Association (https://ima-mineralogy.org/). Londonite was named after Dr. David London, a major contributors to our understanding of pegmatites.  Pezzottaite honors Federico Pezzatta, a petrologist who has extensively studied pegmatites in Madagascar.  Rhodizite was given this name back in 1834 when a worker, G. Rose, tried to identify this new mineral from the Ural Mountains by using a flame test and/or blowpipe flame test (see posting on pollucite).  The resulting red color in the flame test was used to name the mineral (Greek, rhodizein, rose colored (MinDat).  However, the red indicated the presence of lithium, an element later found to be absent from rhodizite.  But, lithium was present in the accompanying matrix and evidently contaminated the sample.  However, the name stuck (Simmons and others, 2001). 

REFRACTIVE INDEX:  The speed of light differs in various substances.  When light travels from one dimension to another (say from air to a mineral), the light bends or refracts (the angle of refraction). All gemstones have a diagnostic angle of refraction termed the Refractive Index and can be measured by an instrument called a refractometer.  Pezzottaite has a refractive index of ~1.6 meaning that the speed of light in the mineral is 1.6 times slower than the speed of light in air.  See www.minerals.net/resource/property/optical.aspx#Refractive%20Index. 

CATION and ANIONS: Ions are atoms or molecules which have lost or gained electrons from their original atomic configuration.  Think back to the old high school science course and remember protons (positive charge) and neutrons (neutral charge) in the center of an atom with the electrons (negative charge)  circling around in outer shells. Originally the number of electrons equaled the number of protons. However, sometimes an element gains an electron or two or three (it steals it away from another element) and therefore it has more electrons than protons and is said to be positively charged and termed a cation.  The poor ole fellow who lost his electron(s) is negatively charged and termed an anion.  Most metals, say silver, assume the role of cations while most nonmetals, like oxygen, are anions.  Web Browse Ions to locate a plethora of explanatory sites.

IRON+++:  Iron is a metal with two common positive oxidation states.   Iron ++ (ferrous) nabs and shares two electrons with an anion and may form the mineral pyrite (FeS₂).  Iron+++ (ferric) nabs and shares three electrons with an anion as in the mineral goethite (FeO(OH).  Web Browse iron ions and hold on.

CAT'S EYE EFFECT:  In some cabochon gemstones a narrow band of light moves back and forth across the stone.  This chatoyancy is caused by a band of very fine, parallel fibers or inclusions (often rutile) that reflect light in a single band.  Check out www.geologypage.com/2017/01/cats-eye-gemstones.html

PEGMATITES:  London (2008) defined pegmatite as: “an essentially igneous rock, commonly of granitic composition, that is distinguished from other igneous rocks by its extremely coarse but variable grain size, or by an abundance of crystals with skeletal, graphic, or other strongly directional growth-habits.”  Pegmatites often contain gem minerals, rare earth minerals, and a wide variety of uncommon elements/minerals.

HIGHLY EVOLVED PEGMATITES: These pegmatites are often enriched with lithium, cesium and tantalum and/or rare earth elements. The USGS noted "Lithium-cesium-tantalum (LCT) pegmatites comprise a compositionally defined subset of granitic pegmatites. The major minerals are quartz, potassium feldspar, albite, and muscovite; typical accessory minerals include biotite, garnet, tourmaline, and apatite. The principal lithium ore minerals are spodumene, petalite, and lepidolite; cesium mostly comes from pollucite; and tantalum mostly comes from columbite-tantalite. Tin ore as cassiterite and beryllium ore as beryl also occur in LCT pegmatites, as do a number of gemstones and high-value museum specimens of rare minerals. Individual crystals in LCT pegmatites can be enormous" (Bradley and others, 2010).

NYF (niobium, yttrium, and fluorine) pegmatites show enrichment in rare earth elements (REE), U, and Th.  The granitic rocks are characterized by Fe-rich micas, amphiboles, and pyroxenes. The pegmatites are compositionally granites, with quartz, K-feldspar, and albite as major minerals but without either biotite or hornblende. Other minerals may include muscovite, rare metal minerals, and volatile element bearing minerals such as phosphates and tourmaline (www.proexmin.com/deposit-models/rare-element-pegmatites/).  Modteski (2005) and Heinrich (1985) noted that both the the Mt. Antero and Pikes Peak granites are NYF types.

Pegmatites are the current subject of numerous scientific publications as researchers have shown that their formation likely cannot be pigeon-holed into nice boxes. For a great description of pegmatites  see http://minsocam.org/MSA/Special/Pig/PIG_articles/Elba%20Abstracts%2018%20Simmons.pdf      

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