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
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgg2yztSj7FHCHlBaFcIuhmw002jqz9k6QQwdS4dcZYb22oeV87DkYk80oRw8TLaF8zY2cK06NVQuyDx3Bp4EhHeA25rOQxW5Li4fP2GmU8IVQQ8hQnsFklg-ZhwJvcMFQLMH2Hn_POHEP5/w322-h241/selen2.jpg)
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 (Å kácha 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.
Lifelong
learning is a form of self-initiated education that is focused on personal
development. While there is no standardized definition of lifelong learning, it
has generally been taken to refer to the learning that occurs outside of a
formal educational institute…Lifelong learning …is best described as being
voluntary with the purpose of achieving personal fulfillment. valamis.com.
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,
Å kácha, Pavel, Sejkora, JiÅ™Ã, Plášil,
Jakub, 2017, Selenide mineralization in the PÅ™Ãbram 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.