Selenium, Number 34 on the Periodic Chart of Elements, has properties that are intermediate between the elements above (sulfur) and below (tellurium) in the periodic table. It is often described as 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’s sulfide minerals.
I do not have a specimen of native selenium but have acquired and described, in a previous posting (September 19, 2020), 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 (not the gypsum variety). In selenide compounds the selenium has an oxidation charge of 2- and this group includes all ~125 naturally occurring selenium minerals (I think).
My most recent collection addition to the selenides is a specimen of tiemannite acquired at the 2022 Tucson Show. Tiemannite is a rare mercury selenide [HgSe] where the selenium anion has an oxidation state of 2-. Mercury, in nature, has three possible valence states. Elemental mercury has no valence state (Hg0), mercurous mercury has a 1+ state (Hg+), and mercuric mercury comes in at 2+ (Hg++). So, tiemannite has a nice balance of a 2+ cation and a 2- anion. It is related to coloradoite, a mercury telluride (HgTe) (see Posting January 10, 2021).
Tiemannite has a steel gray to black color, a metallic dull luster, and a black streak. Like most metals it is opaque and soft (~2.5 Mohs); however, it does exhibit a brittleness. Although some collecting localities produce small tetrahedral crystals most specimens of tiemannite are massive to granular and compact. It commonly occurs with other tough-to-identify selenides in hydrothermal veins.
Tiemannite crystals (sub-millimeter in size) forming dendrites and other features. A large cluster of crystals may be observed in the lower right quadrant. Width FOV ~7.0 mm.
My specimen came from the Lucky Boy Mine, Mount Baldy Mining District on the east flank of the Tushar Mountains of Piute County near Marysvale, Utah. The district is a large gold-silver producer having significant zinc-lead deposits and covers part of the Marysvale volcanic field in the transition zone from the Basin and Range Province to the west and the Colorado Plateau to the east. Upper Paleozoic and Mesozoic sedimentary strata occur along the eastern base of the range and are unconformably overlain by rocks of the Marysvale volcanic field (Chenoweth, 2007) (see Posting May, 11, 2012). The Lucky Boy mine was not a gold-silver mine but was producing mercury (213 flasks) by retort during 1886 to 1887 and, as far as is known, is the only U. S. deposit of the selenides of mercury to be operated commercially (Callaghan, 1972).
The second mineral that joined my collection in 2022 (Tucson) is also a mercury mineral that, at first, confused me to no end! With the tiemannite described above I was able to observe mercury as a “normal” cation with an oxidation charge of 2+ and balance with the selenium anion of 2-. My new specimen was luanheite with a formula of Ag3Hg and I nabbed it due to mercury appearing as an anion—and so it came home with me. Last week while sorting and looking at minerals (a constant joy) I pulled out the two perky boxes and suddenly my mind hit a brick wall. Something was wrong, or so I thought. As noted above, mercury has oxidation states of 0, 1+, and 2+ so how could it be an anion? How could it match with the positive oxidation states of silver, 1+, 2+, 3+? Confused was I!
As noted before in this blog, I am trying to remain a lifelong learner and therefore relearning “basic chemistry as I advance in age. My three semesters of chemistry as an undergraduate in Hays, Kansas, were completed over 60 years ago and much of the “learning” in these classes did not stick in my brain for the following decades. That is one reason I commonly mention oxidation states in discussions—pushing, pushing my mind to try and understand. So now perhaps I have an answer.
Luanheite, according to MinDat, belongs to the silver amalgam group (yes, the same as your tooth fillings) and therefore is an amalgam mineral. So far, so good. An amalgam is an alloy and a combination of mercury with another metal, in this case silver. Most minerals, other than the native elements, are chemical compounds and held together by chemical bonding (several types of bonding) and may be transformed by chemical reactions. Amalgams, and most metals, are held together by metallic bonding where electrostatic forces are in play. This bonding is quite strong and therefore “holds together” the silver and the mercury.
With some continued interaction examining chemistry books, I found the answer. All elements in an amalgam are in an elemental state and have oxidation charges of zero. So, in the mineral luanheite, an amalgam, both mercury and silver are in oxidation states of zero. I suppose any student enrolled in CHEM 100 would know that; however, if I learned such, it “slipped my mind.”
Robert Cook wrote a great article in Rocks and Minerals (2002) describing the discovery and naming of luanheite. The crux of the story was your work is not finished till the paperwork is done. Cook posed the question, “if one discovered a pocket of this material [luanheite in Chile], a mineral unknown until the 1980s, its peculiarity and obvious rarity would suggest that timely formalization as a new species was not an urgent matter. Why then rush to publication?” As you might guess, a group of Chinese scientists had identified and published a description of the new mineral luanheite located in a gold-bearing alluvial gravel, a completely different environment from the Chilean volcanic tuff-hosted silver and mercury mine. Although the Chilean luanheite locality produces the finest specimens in the world, the Type Locality is an obscure river in China and the Type Specimen is a rounded small pebble; no photograph is included on the MinDat description.
As Cook described, luanheite closely resembles native silver, ranging in color from gray, white to black, has a metallic luster, a hardness of ~2.5 (Mohs), and is soft and malleable. It usually is massive, granular, or sheetlike; however, at the Chilean Elisa de Bordos mine it may occur in arborescent growths. The chemistry remains constant across mineral grains indicating it is a mineral and not just a jumbled mixture of silver and mercury.
This
has been a tough assignment but perhaps something sort of perked me up---Richard
Faynmann: Study hard what interests you the most in the most undisciplined, irreverent,
and original manner possible. I can, at times, be quite irreverent and
undisciplined!!
REFERENCES CITED
Callaghan, E. (1973) Mineral Resource Potential of Piute County, Utah, And Adjoining Area: Utah Geological & Mineralogical Survey Bulletin 102.
Chenoweth, W.L., 2007, History of uranium production, Marysvale district, Piute County, Utah, in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, Central Utah―Diverse geology of a dynamic landscape: Utah Geological Association Publication 36.
Cook, R.B., 2002, Connoisseur's Choice: Launheite: Elisa de Bordos Mine, Northern Chile, Rocks & Minerals, vol. 77 no. 2.
TRIVIA. Although my mind might have lost some knowledge once learned in chemistry, one thing that has not disappeared in those 60+ years is a small tavern on 11th St. in Hays, the Brass Rail. Just a laid-back place with cold drafts of Coors for $.15 (in the early 1960s).
