THE SELENIDE MINERALS: TOUGH TO ID--KLOCKMANNITE, CLAUSTHALITE, BERZELIANITE

 

This has been a tough subject to grasp--the selenides, and I have read countless hours trying to better understand. I apologize up front for all the technical jargon, much more than I try to incorporate into my postings.  With the selenides, I moved out of my comfort zone. Every time I get out of that zone, regardless whether I win or fail, I learn something new, and that is a personal enjoyment. Some people tend to think after a certain age, they are no longer allowed to start something new, and perhaps something scary. The truth is, not branching out is just an excuse to stay in the comfort zone. If I had stayed in the zone, I would have never learned about oxygen fugacity, fO₂, and its relationship to the formation of minerals. However, I suspect that rockhounds and readers on this list might wish I had never ventured out of my zone 😊

In this time of Covid-19, be brave, learn something new every day, get out of your comfort zone, read a book to relax, wear your mask, support local businesses, walk around the block and breathe deeply, check on your neighbors (especially the elderly), contribute to the local food bank, and be first in line to get vaccinated! May you enjoy the Holiday Season and wish for a new and better and safer New Year.  Thank you all for taking an interest in my Blog. Keep in contact. Stay well.  mike

And now, ON TO THE SELENIDES. 

Jöns Jakob Berzelius (1779-1848) was described by Wilson (1994) as one of the greatest mineral chemists who ever lived.  That is quite a statement!  Berzelius was born in Väversunda Sörgård, Sweden, a product of three generations of clergy on each side of the family.  Now, I presume that father, grandfathers, and great grandfathers all expected a great theological future for Jöns.  Surprise! His grades in theology were not so hot but he excelled in the natural sciences.  So, the entered the great University at Uppsala and confused the faculty.  Seems he was considered gifted but undisciplined (I was probably considered average and undisciplined). But all was not lost since he was very good in inorganic chemistry and mineralogy, and as an adult became one of the greatest mineral chemists who ever lived. He received that accolade, since MinDat.org noted that Berzelius was the father of analytical chemistry, inventor of chemical symbol notation. and discoverer of selenium (Se), cerium (Ce), silicon (Si), thorium (Th), titanium (Ti), and zirconium (Zr), in addition to other elements that he gave to his students to work on.  Those discoveries certainly would qualify him as the greatest, but my guess is that students do not learn these facts in modern mineralogy courses!

Jöns Jacob Berzelius, 1779-1848.

One of the elements first described by Berzelius was selenium, Atomic Number 34 on the Periodic Chart of Elements, collected from copper deposits in the mines near Falun, Sweden (see Posting July 1, 2020).  It was named after the Moon goddess Selene due to its similarity to the recently discovered tellurium (named for the Earth). Selenium is a metalloid with properties intermediate between a metal and nonmetals.  For comparison, other metalloids include silicon, boron, antimony, arsenic, tellurium, and several others. Selenium is a rare element and its abundance in the earth’s crust ranks the element 67th (0.05 ppm) while #1 oxygen has 461,000 ppm. Native selenium is rare as a mineral but does appear in some uranium-vanadium sandstone deposits. If selenium is available in hydrothermal or magmatic solutions it often substitutes for some sulfur in the formation sulfide minerals.

Commercially, selenium almost entirely (>80%) is obtained as a byproduct of copper refining (Brown, 2002).  A smaller amount comes from the refining of gold, silver, zinc, and lead, where high concentrations of selenium allow for profitable operations.  No ore deposits are mined for selenium alone. Although Germany and Japan are the leading producers of selenium, the raw products come from Africa, Asia, Australia, and South America (Bleiwas, 2010). The U.S. has about 8% of the world’s reserve, mostly tied up in copper resources in Nevada, Utah, New Mexico, and Arizona (Brininstool, 2015). Selenium is used in the manufacturing of glass and electronics, and in some color pigments.

I do not have a specimen of native selenium but have acquired thumbnails of clausthalite [PbSe], klockmannite [CuSe] and berzelianite [Cu₂Se]; all are fairly rare minerals and in the selenide group.  Selenium can exist in the oxidation states of 2-, 2+, 4+, and 6+ and form selenates, selenides, and selenites; all are anions with a negative oxidation state.  In selenide compounds the selenium  has an oxidation charge of 2-; the group includes all ~125 naturally occurring selenium minerals (I think).  Selenites (not the gypsum variety) contain the SeO₃ radical (Se IV) while selenates contain SeO₄ (Se VI); both of these latter groups are quite soluble in water and do not form naturally occurring minerals, at least none that I could not locate. Once selenium becomes aqueous it can be taken up by organisms.  Selenates prefer well-aerated waters while selenites are more common in slow-moving waters such as lakes (Stillings, 2017).  There are a plethora of synthesized selenium compounds and solutions in the laboratory.

Black grains of fine grained berzelianite disseminated in calcite. From the Skrikerum Mine in Sweden. Width photo FOV ~ 5.7 cm.

Photomicrograph of disseminated berzelianite (and perhaps other selenides) in calcite.  From above specimen. Width FOV ~1.7 cm.

As above. Width FOV ~1.7 cm.

Berzelianite, a copper selenide, is rare in the mineral record and was first described from the Skrikerum Mine located in south central Sweden; a mine “famous” for producing rare selenides.  Besides berzelianite, the Mine is the Type Locality of three other rare copper-silver selenides: crookesite, eucairite, and selenojalpaite.  The minerals are found in fracture-fillings hosted by vein calcite associated with hydrothermal alteration. Copper was first mined ~1779 and some exploration and mining, mostly for selenium, continued until the end of the 1800s.

Berzelianite is a soft (~2.0-2.5 Mohs) metal with a metallic luster, a lead-gray to blue gray to black color (but shiny when fresh), an irregular fracture, and is opaque.   It occurs as microcrystals disseminated in a carbonate matrix (calcite at the Type Locality).  At times berzelianite [Cu₂Se] is incorporated within the calcareous matrix and is difficult to distinguish from other selenides at Skrikerum, especially klockmannite (CuSe].  The massive form of both minerals seems indistinguishable to a soft rocker like me; therefore, I am relying on previous identifications and published descriptions.

Klockmannite has a molecular weight of 44.59 % copper and 55.41% selenium (Webmineral.com) while berzelianite weighs in at 61.68 % copper and 38.32 % selenium. Klockmannite belongs to the hexagonal Crystal System while berzelianite is Isometric; however, in massive minerals there is little chance of observing mineral shape under a scope. Both are grayish black to dark gray in color, opaque, and soft at ~2.0+ (Mohs). Both have a sort of a dull metallic luster and are associated with calcite.  So, I am just taking my chances, especially since I do not have a reflected light microscope and polished sections. 

Blackish blue klockmannite "encased" in calcite.  Width of specimen ~9 mm. Skrikerum Mine, Sweden.

Photomicrograph of boundary of klockmannite and calcite (specimen above).
 Clausthalite is a lead selenide (PbSe) related to galena, the lead sulfide (PbS).  In fact, Förster (2005) has documented a solid solution series between the two minerals. Clausthalite forms in  low-sulfur hydrothermal deposits and may be the most common selenide. In many ways it looks like galena with a lead-gray color, a grayish black streak, and a metallic luster. It is soft (~2.5 Mohs), brittle and often granular but at times forms nice euhedral crystal (Isometric Crystal System). One of my specimens came from the famous Tilkerode Mining District, Mansfeld, Mansfeld-Südharz, Saxony-Anhalt, Germany (part of the Bohemian Massif, see below).  Here selenide minerals (including 3 Types) occur in veins with carbonate minerals, gold, hematite, platinum group minerals, and rare sulfides.  Mining hematite for iron occurred in the 1700s and 1800s (Stillings, 2017).  Simon and others (1997) studied the “why” of selenides forming from solutions rather than the simpler incorporation of selenium directly into sulfide minerals replacing some sulfur.  They believed that high fO₂ (oxygen fugacity: the pressure of oxygen that is available to react with other components) values helped separate selenium from sulfur and prevented incorporation into the sulfides! If I understand any small thing about mineral/ore deposits, and that is a real stretch, above the groundwater line there is a high fO₂ and oxide minerals form.  Below the water line there is a high sulfur fugacity (sO₂) and sulfide minerals form.  Don’t take that to the bank without checking.

Clausthalite, Tilkerode Mining District, Germany. Width FOV ~ 1.5 cm.  Photomicrograph below.

The second specimen of clausthalite I obtained is labeled as being collected from Petrovice, Russia—I believe this is incorrect.  First, I could not locate mines in or near a Russian locality named Petrovice.  In fact, not even a locality was noted.  Second, there is a mining district in the Czech Republic named Petrovice, a major locality noted for the occurrence and collection of rare selenides, including petrovicite. The old mines are located in one of the uranium-vanadium-polymetallic mining districts in the Bohemian Massif (structurally controlled block of highlands).  The Massif encompasses parts of the central Czech Republic, eastern Germany, southern Poland, and northern Austria. In the Czech Republic, the Massif has a core of plutonic granites surrounded by metasedimentary rocks of late Precambrian and early Cambrian age.  Mineralization, as dated by the uranium, is Permian in age and is associated with the Varisican Orogeny (the mountain building event associated with the collision of Laurasia and Gondwana producing the Pangaea Supercontinent) (Škácha and others, 2017).

Clausthalite, Petrovice, Czech Republic. Width FOV ~1.0 cm.

Photomicrograph below.

The selenide occurrences in the Bohemian Massif are amazing as the rocks have produced at least18 newly discovered species (since around 1970; and 5 more before that date).  These selenides seem fascinating to me and minerals worth studying if I were a young student in a major research institution!!! Those selenides…are characterized using wavelength-dispersive spectroscopy, reflected light, powder X-ray diffraction, single crystal X-ray diffraction, Raman spectroscopy, and electron backscatter diffraction (Škácha and others, 2017). As I said, if I were a bright young student well versed in electronic gizmos!!!!  Now I am just an ole soft rocker trying to have fun and do my best with some pretty complex, and hard to identify, minerals.

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REFERENCES CITED

Bleiwas, D.I., 2010, Byproduct mineral commodities used for the production of photovoltaic cells: U.S. Geological Survey Circular 1365.

Brininstool, Mark, 2015, Copper [advance release], inMetals and minerals: U.S. Geological Survey Minerals Yearbook 2012, v. I, p. 20.

Brown, R.D., Jr., 2002, Selenium and tellurium, in Metals and minerals: U.S. Geological Survey Minerals Yearbook 2000, v. I, p. 67.

Förster, H.J., 2005, Mineralogy of the U-Se-polymetallic deposit Niederschlema-Alberoda, Erzgebirge, Germany, IV—The continuous clausthalite-galena solid-solution series: Neues Jahrbuch für Mineralogy—Abhandlungen, v. 181, no. 2, April.

Simon, Grigore, Kesler, S.E., and Essene, E.J., 1997, Phase relations among selenides, tellurides, and oxides; II, applications to selenide-bearing ore deposits: Economic Geology, v. 92,

Škácha, Pavel, Sejkora, Jiří, Plášil, Jakub, 2017, Selenide mineralization in the Příbram Uranium and Base-Metal District (Czech Republic):: Minerals, 7, no. 6: 91.

Stillings, L.L.2017, Selenium, Chapter Q of Critical mineral resources of the United States—economic and environmental geology and prospects for future supply: USGS Professional Paper 1802-Q, Editors K.J. Schulz, J.H. DeYoung, R.R. Seal II, and D.C. Bradley.

Wilson, W.E., 1994, The history of mineral collecting1530-1799: The Mineralogical Record, v. 25, no. 6.