Thursday, February 26, 2015

PEROVSKITE: CALCIUM TITANIUM OXIDE

Western front of the Santa Catalina Mountains as seen from Catalina State Park, Oro Valley, Arizona (northernmost Tucson).  The massive granite "Cathedral" is left of center while folded gneiss is prominent on the left.


During my stay in Arizona for the Tucson shows I tried to keep abreast of the latest battles between rooftop solar vs. the power companies.  As in many states, monopolistic power companies seem frightened of the increasing number of individual homes installing solar panels.  There are substantial “for and against” arguments coming from both sides; however, with Arizona being a “sunshine state” solar panels would seem a reasonable option.  But, there is little cooperation between the two camps.  As a political junkie, I am fascinated, on a daily basis, with Arizona politics!

At any rate, in one of the numerous articles on solar power I stumbled on the term perovskite structure as it pertained to solar panels.  This piqued my interest since perovskite is a mineral, calcium titanium oxide [CaTiO3].  I wondered how this not-so-common mineral could be plentiful enough to be used in the solar industry.  But, further exploration and reading indicated my lack of understanding about: 1) the mineral perovskite; and 2) the perovskite structure.  I blame this little bit of ignorance on my propensity for breaking glassware in third semester chemistry and deciding that a chemistry major was not on my radar screen.  In addition, there were the rumors (mostly true I think) about a future college course, physical chemistry (P Chem in the vernacular), being a “killer course”, something that my college GPA did not need at that time! 

What I recently learned is that the term perovskites refers to a family of crystals, most of which are made synthetically, with the same type of crystal structure as the “real mineral” perovskite. These compounds have a chemical structure of ABX3 where A and B are two different cations of different sizes and the anion X bonds them together. It forms a cubic symmetry.

Structure of a perovskite with a chemical formula ABX3. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green spheres are the A-atoms (a larger metal cation, such as Ca2+). Pictured is the undistorted cubic structure.  Public Domain drawing and explanation. Original from Navrotsky, 1998.
The photo voltaic perovskite structure compound that first interested researchers was methylammonium lead tri-iodide although today tin is being examined as a less toxic replacement for lead.  “Solar cell manufacturers face a tricky trade-off between performance and cost. Most commercial solar cells rely on slabs of crystalline silicon that are more than 150 micrometers thick and take a lot of energy to produce (efficacy of 1-23 percent). Thin-film solar cells—those containing just a few micrometers of such semiconductors as copper indium gallium selenide (CIGS)—have lower material costs, but they are also less efficient. Cells using crystalline gallium arsenide, on the other hand, can reach 30 percent, but the materials involved are too costly for utility-scale solar power. Perovskites could resolve this quandary by matching the output of silicon cells (efficiency now at 19.3 percent) at a lower price than that of thin-film CIGS: Their ingredients are cheap bulk chemicals, and the cells can be built using simple, low-cost materials" (Peplow, 2014).

Kulkarni and others (2012) noted that besides photo voltaics, perovskite materials exhibit many interesting and intriguing and commonly observed features/properties: colossal ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermopower and the interplay of structural, magnetic and transport properties.  Sounds interesting.
 
But, back to the mineral perovskite.  In rummaging through some dusty trays at Tucson Show mineral dealer, I came across a small box labeled perovskite and containing a specimen with some small cubic crystals.  Wow, a little serendipity with a great price--$4.  So, I scooped it up for my collection.
Specimen containing cubes of perovskite.  Maximum width 1.6 cm.
The mineral perovskite actually belongs to the Orthorhombic Crystal System but usually appears as crystals with a cubic outline (BTW, that is something that still confuses me—the crystals appear cubic but are orthorhombic.  If it quacks like a duck………..:)  These small cubes come in a variety of colors from black to brown to shades of yellow, orange and red. My cubes have an adamantine to sub-adamantine luster, are fairly hard at ~5.5 (Mohs) and seem partially translucent (ranges from transparent to opaque).  At times some cubes have a metallic luster and look similar to galena.
Photomicrograph, cube of perovskite (P); width of crystal ~2 mm.  Some overgrowth of calcite (C).
Perovskite has several relatives listed in MinDat as belonging to the Perovskite Mineral Group with common substitution of Fe, Nb, Ce, La.  
Anthony and others noted perovskite forms as an accessory mineral in alkaline mafic rocks, as nepheline syenites, kimberlites, carbonatites, and can additionally form in calcium-rich skarns (such as Magnet Cove, Arkansas) and is a common accessory mineral in calcium and aluminum rich inclusions within carbonaceous chondrites.

My specimen was collected from the Malenco Valley, Valtellina, Sondrio Province, Lombardy, Italy.  MinDat lists 177 valid minerals collected from the locality.  The area is in the Italian Alps but I cannot locate references describing the specific geology.  The best known minerals from the locality are probably gemmy green “garnets.”

The USGS has noted that titanium is an important element mined for a variety of purposes and occurs primarily in the minerals anatase, brookite, ilmenite, leucoxene, perovskite, rutile, and sphene.   Of these minerals, only ilmenite, leucoxene, and rutile have significant economic importance.

So, I have learned much from this little exercise—mostly that while perovskite seems like an innocuous mineral, it actually is quite complex.  For example, “the stability of perovskite in igneous rocks is limited  by its reaction relation with sphene.  In volcanic rocks perovskite and sphene are not found together” (Veksler, 1990).  And, perovskite also gives its name to a complex structure that is proving to be quite critical in many new industries.Life can provide great learning experiences!

REFERENCES CITED

Anthony, J.W.  R.A Bichard, A. Bideaux, K. W. Bladh, and M. C. Nichols, Eds., Handbook of Mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.

Kulkarni, A., F.T. Ciacchi, S. Giddey, C. Munnings, 2012, Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells: International Journal of Hydrogen Energy, v. 37, no. 24.

Navrotsky A., 1998, Energetics and crystal chemical systematics among Ilmenite, Lithium Niobate, and Perovskite Structures: Chemical Materials, v. 10, no. 10.

Peplow, M., 2014, Perovskite is the new black in the solar world: IEE Spectrum, 25 June. 

Veksler, I.V., and M.P. Teptelev, 1990, Conditions for crystallization and concentration of perovskite-type minerals in alkaline magmas:  Lithos, v. 26.

Monday, February 23, 2015

PARATACAMITE: TUCSON GEM SHOW REPORT: THE END




Lots of petrified wood, including some large polished slabs.
Multi-colored Banded Iron Formation from Australia.  Red and yellow jasper and chert and black hematiteSpecimen width is about35 cm.


I spent my last day at the 2015 Tucson Shows by heading back to the Main Street venues perusing the many small tents and stands selling a variety mineral specimens, petrified wood, Indian zeolites, banded iron formation, Moroccan fossils, etc.  There were some beautiful polished jasper specimens and really large slabs of petrified wood from Madagascar.  I also enjoyed visiting with several owners of the smaller “mom & pop” operations.  One of the owners noted my continued interest in a flat of small spheres of paratacamite.  I engaged him in a fascinating conversation about collecting the specimens from Baja California [Santa Rosalia, Boleo District, Mun. de Mulege, Baja California Sur, Mexico].  MinDat lists 47 valid minerals collected from area mines and three type minerals with the best known being boleite.


The Boleo Mining District is unlike most metallic ore deposits in western North America in that it is a sediment-hosted copper-cobalt-zinc-manganese ore deposit. These sediment-hosted deposits are known as manto ore deposits.  In this situation the metallic minerals replace sedimentary rocks, commonly limestones, and form bodies along the bedding planes.  What is questionable is the source of the ore—does it come from a sedimentary source within the basin, or from an adjacent intrusive pluton?  Or perhaps the minerals came from badly weathered primary deposits?  Or are there other possibilities?

To my surprise, and gratefulness, the owner let me pick out any individual sphere I would like to take home.  Rockhounds, especially those of small shops, are generally a nice group of people.

Last fall I put out a posting (October 9, 2014) on atacamite, a copper++ chloride hydroxide [Cu2(OH)3Cl] that is usually a secondary mineral oxidized from other copper minerals and forms in arid and saline conditions.  Atacamite (Orthorhombic) is a polymorph (minerals with the same chemical composition but different crystal structures) of botallackite (Monoclinic), anatacamite (Triclinic) and clinoatacamite (Monoclinic).  At one time paratacamite was thought to also be a polymorph.  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 dropped its designation as a polymorph.

Paratacamite [Cu3(Cu,Zn)(OH)6Cl2] looks quite similar to atacamite (at least to my untrained eye) so I asked the vendor how he knew it was the former rather than the latter?  He patiently explained that the nodules had been “x-rayed and confirmed to be paratacamite.”  That was good enough for me!  Later I noted MinDat had crossed atacamite off their Boleo District list and substituted paratacamite.

Crystals of the mineral is green to darker greenish-black, fairly soft at 3 (Mohs), have a vitreous luster, has some decent cleavage and a conchoidal fracture.  My specimen has very tiny crystals and it is quite difficult to observe specific physical characteristics.  So, I was really happy for the vendor’s identification.
A nodule of paratacamite (green) mixed with flakes of a light colored mineral, perhaps gypsum.  Width of nodule ~1.6 cm.

Photomicrograph of nodule section  above. Cluster of green paratacamite crystals ~4 mm.
In further reading, I learned that at times magnesium or nickel replaces the zinc and the mineral becomes paratacamite-(Mg) or paratacamite-(Ni). Welch and others (2014) noted that upon heating paratacamite reversibly transforms into herbertsmithite [Cu3Zn(OH)6Cl2] between 353 and 393 K.

REFERENCES CITED

Welch, M.D., Sciberras, M.J., Williams, P.A., Leverett, P., Schl├╝ter, J., Malcherek, T., 2014, A temperature-induced reversible transformation between paratacamite and herbertsmithite: Physics and Chemistry of Minerals, 41, 33-48.