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.

FAIRFIELDITE: UNCOMMON PHOSPHATE

 

As I have noted in several other posts, phosphates are among my favorite minerals (along with relatives vanadates and arsenates).  Phosphate minerals contain the anion PO₄ with an oxidation state of 3- arranged in a tetrahedron: 1 phosphate with an oxidation state of 5+ and 4 oxygens each with an oxidation state of 2- (for a total of 3-). In some phosphate minerals chlorine, fluorite, or hydroxide may be added as a negative anion as in the most common phosphate mineral group, apatite:   hydroxylapatite Ca₅(PO₄)3OH; fluorapatite Ca₅(PO₄)3F; chlorapatite Ca₅(PO₄)3Cl.

Fairfieldite is a uncommon phosphate that occurs as an accessory mineral in granite pegmatites, especially those that are rich in lithium—the Type Locality in Fairfield County, Connecticut (Fillow Quarry), Cleveland County, North Carolina (Foote Quarry), Custer County, South Dakota (Tip Top Mine and others).  Fairfieldite is a calcium manganese phosphate, [Ca₂Mn(PO₄)₂-2H₂O] with transparent to translucent euhedral crystals that leave a white streak. Crystals range from prismatic to equant and commonly appear as aggregates or foliated masses, rarely as radiating or fibrous.  Crystals have a sub-vitreous (mostly) to pearly luster and are soft at ~3.5 (Mohs).  Their color ranges from colorless to white to light yellow to perhaps greenish white.  It is sort of a nondescript mineral that sort of looks like gypsum.  The best bet to help identify fairfieldite is its occurrence in granite pegmatites, especially lithium rich forms. 

Numerous white to colorless of fairfieldite crystals.  Width FOV ~ 7 mm.     
I have not collected many minerals east of the Mississippi River—not an astounding statement when you consider that my only living stint east of the River has been about 15 feet while residing in La Crosse, Wisconsin.  I have traveled extensively in the eastern U.S.; however, it seems I was always fishing, hiking, camping, seeing the sights, etc. and missed most minerals.

During one sojourn through North Carolina I did travel through King Mountain and read about the nearby Foote Lithium Company Mine.  My interest in the mine, at that time, was in the lithium due to the many lithium mines in the Black Hills of South Dakota.  I really could not locate much information on the Foote Mine (partially due to self- quarantine and lack of access to research libraries) except that the mine has not been active for several years, is partially reclaimed (a park), and collecting is strictly prohibited.  According to MinDat, the granite pegmatite mined for lithium, tin, beryllium, niobium, tantalum, and “mica” is hosted in the Cherryville Quartz Monzonite, a Mississippian age batholith in the Carolina Piedmont Belt.  The Mine has produced 161 mineral species including the Type Locality of 15 minerals, many of which are rare phosphates.

SMALL SPECIMEN; TINY ZEOLITES FAUJASITE-Na AND PHILLIPSITE-K

 I came home from the Bridling Estate sales with several micromounts stuffed into plastic containers holding 25 boxes. Some of the boxes contained pretty ordinary mounts of minerals like calcite, dolomite, and pyrite. However, several of the mounts contain interesting, and sometimes rare minerals.  I am uncertain who set the zeolite mount described today: faujasite with phillipsite.

Zeolites have always been sort of confusing to me (mineralogically) although the group is quite large in numbers, many members are quite common, and they are very important as industrial minerals.  In addition, zeolites like chabazite and heulandite are usually seen at almost every rock and mineral show. Some zeolites, such as the acicular/prismatic crystals of mesolite, natrolite, and scolectite, make interesting museum specimens, as well as beautiful objects de art. 

I would have trouble defining zeolites to my fellow rockhounds.  So, I leave that task to Lauf (2014): the zeolites are a large and highly diverse group of (generally) aluminosilicate minerals in which SiO₄, AlO₄, or other tetrahedra are arranged into a three-dimensional structural framework having cavities and channels that can host ‘extra-framework” cations and water molecules.  The important item in this definition are the words about cavities and channels: 1) these open areas are able to hold a variety of cations which may be exchanged with other cations found in contacting solutions. For example, in water softeners/purifiers where zeolites are the "salts" the sodium in the zeolites may be exchanged for magnesium and calcite in the water (the “hardeners”); 2) the channels and cavities also may act, in industrial chemistry, as molecular sieves to remove specific sized molecules.  Zeolites are incredibly important in various industries.  Today there are over 200 synthesized zeolites; however, natural zeolites are usually found in cavities of volcanic rocks deposited via post-depositional contact of the rock with alkaline groundwater.

Faujasite is a rare zeolite, or actually a series of minerals with three endmembers named after the dominant cation: calcium, sodium or magnesium.  MinDat writes the chemical formula as:  M_3.5[Al₇Si₁₇O₄₈]-32H₂O where M is Ca, Na₂ or Mg present in different percentages while potassium and or strontium may partially substitute for one of the cations. Faujasite-Na is the most common composition with the type example from Sasbach, Germany; faujasite-Ca has a type example from Ilbeshausen, Vogelsberg, Hessen, Germany; and faujasite-Mg has the only known example from an “old museum specimen” collected from Sasbach, Kaiserstuhl, Germany (Faujasite Series, 2005).  

The specimen in my collection came from the Type Locality of faujasite-Na in the Linberg Quarries (seven of them),  Kaiserstuhl Volcanic Complex, Southwest Germany.  The volcanic rocks at the quarries are Miocene in age, ~19-16 Ma, and are a type termed limbergite: dark-colored rock resembling basalt but with an absence of feldspar. The glassy groundmass is composed of augite and olivine often with another generation of euhedral augite crystals. The zeolites usually appear in vugs of the volcanic rocks and are post depositional.

The crystals of faujasite are octahedrons, colorless to gray to white in color, transparent to translucent, vitreous and often are intergrown with other zeolites such as phillipsite and offretite.

Radial aggregates of phillipsite-K along with a mass of individual crystals. Width of vug with crystals ~2 mm.  Width of largest aggregate(middle left) is ~0.3 mm.

A second zeolite present in my specimen is phillipsite-K.  Originally there was a single phillipsite, essentially the potassium variety although the sodium species was the original Type. In 1997 phillipsite was divided into species with names based on the dominant channel cation. The type examples for the new phillipsite species compositions are as follows: phillipsite-Na has the composition from the original phillipsite type locality at Aci Castello, Sicily, Italy; phillipsite-K, Capo di Bove, Rome, Italy; and phillipsite-Ca, Lower Salt Lake Tuff, Puuloa Road, Oahu, Hawaii (Phillips Series, 2005).

MinDat.org writes the chemical formula for the potassium species as: (K,Na,Ca_0.5,Ba_0.5)_4-7[Al_4-7Si_12-9O₃₂] - 12H₂O.  So there is a mixture of potassium, sodium, and calcium cations combined with aluminum silicate and lots of water.  Also notice the barium (Ba) in the formula, an element present in all of the phillipsite series minerals.  In fact, if barium is the chief cation, the mineral is known as harmotome.

Phillipsite-K crystals are colorless and transparent to white, prismatic, vitreous, hard (4-5 Mohs), and often terminated.  They occur as terminated individual crystals or spherical radiating aggregates—both are visible in my specimen and often are intergrown with faujasite and another zeolite named offretite.

Phillipsite-K is the most common of the species and is found  in cavities of basaltic lavas and as a diagenetic alteration product in volcaniclastic sediment. The species of phillipsite formed is usually controlled by the type and mineral composition of the host rock, and the nature of the contacting solution Phillipsite Series, 2005).

  REFERENCES CITED

Faujasite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

Lauf, R.J., 2014, Collector's guide to the zeolite group: Schiffer Earth Science Monographs Volume 17.

Phillipsite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

The photomicrographs below are at the very limit of my skills with the digital microscope.  Many are somewhat fuzzy.  As you can observe these crystals are less than 1 mm in size. 

 

Aggregate of radiating phillipsite-K crystals with large faujasite octahedrons on either side of the mass.  Width of phillipsite ~0.3 mm.  

 

Radial aggregate of phillipsite-K water-clear crystals (pointing arrow).  The arrow bisects a large fuzzy crystal of faujasite. The aggregate is embedded in a mass of tiny, terminated phillipsite-K crystals along with nondescript faujasite.  Length of aggregate ~0.8 mm.      

Two "larger" octahedrons of faujasite situated in a vug on a layer of what appears to be an earlier generation of faujasite crystals.  Width: small, see below. 

 

Black, or darkest green, euhedral crystals of augite embedded in the limburgite groundmass.  The two vugs to the right hold distinct individual octahedrons of faujasite on an earlier generation of what I assume is faujasite. The photomicrograph above is an enlargement of the lower right quadrant of the complete vug. Length of vug ~ 2.7 mm.

Another vug containing faujasite octahedrons (F) and a sphere of perhaps offertite (O), another rare zeolite.  I am way out on a limb here with that designation, however, under my microscope it appears similar to those identified in MinDat.  Width of vug ~ 3.0 mm 

This is an interesting vugs with groundmass containing euhedral augite, numerous octahedral faujasite crystals and in the middle there are numerous small spheres that are most likely Opal-AN.  Also scattered around are individual prismatic crystals of phillipsite-K.  The orange mineral surrounding the vug is weathered olivine.  Width of vug ~4.0 mm.