TIEMANNITE AND LUANHEITE: TWO RARE MERCURY MINERALS

 

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.

Tiemannite crystals (sub-millimeter in size). Notice the scattered individual crystals in center of photograph. Width FOV ~4.0 mm. Best my camera could produce of these microscopic crystals.   

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.

Dark metallic gray-black sheets of luanheite on a matrix of volcanic tuff. The specimen was collected in 1985, the year that information about the rare find was published. Width FOV ~9.0 mm.

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 11^th St. in Hays, the Brass Rail. Just a laid-back place with cold drafts of Coors for $.15 (in the early 1960s). 

 

The Brass Rail on 11th St. in Hays, Kansas. Parked in front is a great 1955 Chevy. Photo and car courtesy of Jeff Thisted.

 

A TALE OF TRACKING DOWN CORDIERITE VAR. IOLITE FROM SOUTH DAKOTA AND COLORADO

In the end, it's not the years in your life that count. It's the life in your years. A. Lincoln

I really was not very knowledgeable about the mineral cordierite until I looked at some gemstones labeled water sapphire and listened intently to the jeweler’s long-winded description of these blue-violet stones. That little experience caused me to start reading about these colored stones and trying to better understand them. Soon I discovered that water sapphire, also known as iolite, was not an “official” mineral but was the gemmy variety of cordierite. However, the only time I remember seeing cordierite was in my optical petrology course as we studied metamorphic rocks. I really don’t remember observing the mineral in hand sample but only in petrographic slides.

Later in life, after my move to Colorado, I was reading Dan Hausel’s book (2009) on Wyoming minerals and was fascinated by his descriptions of cordierite and gemmy iolite from the Laramie Range west of Wheatland. Hausel noted large deposits of cordierite gneiss that produced such gems as the Palmer Canyon Blue Star (1,750 Carats) and the 24,150 carat Grizzley Creek Blue Giant, the latter a specimen he believed was the largest iolite gemstone in the world. Hausel also located other iolite deposits in the Laramie Anorthosite cropping out near Sherman Mountain. Additionally, he noted that perhaps millions of gemmy iolites remained in these Precambrian rocks but lamented the fact that these gemstones were mostly untouched and off the market.

Since reading Hausel’s descriptions I have looked in many rock/mineral shows, and on the internet, trying to locate specimens of the Wyoming iolite. But Hausel also stated that most/all gemstone localities were off limits to “average” collectors and rockhounds. I even tried examining a few roadcuts of the Laramie Anorthosite, but no luck for any gemmy material. I have seen a few cabs of Wyoming iolite (at least noted as such) for sale on internet sites but could not locate jewels at shows or stores. It is my understanding that imported iolite is of better quality and less expensive than the Wyoming variety; however, it would be nice to have some local material!

Pendants faceted from Palmer Canyon, Wyoming, cordierite/iolite. Offered by Etsy seller Jane Reneau.  Now they are "Out of stock."    

After my failed search for Wyoming iolite, I decided to try Colorado localities and was somewhat more successful. Successful indicating that cordierite occurs in tens of localities across Colorado in metamorphic rocks, commonly in some relationship with sillimanite and/or staurolite and is often altered (?ugly). The less successful part of the equation is that few cordierite exposures seem to exhibit the nice blue to violet variety, iolite (Eckel and others, 1997). One somewhat major exception is the Grape Creek locality in Fremont County where glassy, clear, blue corundum (sapphire?) was noted by Finlay over a century ago (1907). Mark Jacobson (1988) later described the blue masses as cordierite, “essentially unaltered, usually less than 1.5 cm in diameter.”  Unfortunately, I could not locate photos on MinDat and did not have access to some older publications that could contain photographs. Never-the-less the hunt was on for Grape Creek iolite.

Since the collecting locality was only relocated in 1987 “with some difficulty” (Eckle and others, 1997), and by that time in my life bone joint replacements prevented serious hiking, I started looking at shows and asking dealers for information. Not much luck until about four years ago when I discovered a specimen at the Denver fall show. I consider myself lucky as I have not observed another “for sale” specimen.

Cordierite from Grape Creek locality, Fremont County, Colorado, Width FOV ~7 mm.

My next attempt at locating cordierite var. iolite was to explore the Precambrian rocks of the Black Hills of South Dakota, one of my favorite places to wander. I remembered: 1) that Roberts and Rapp (1965) had stated that “cordierite occurs chiefly as a microscopic constituent of highly aluminous metamorphic rocks.” They also noted a couple of localities west of Custer; and 2) several years ago I was “exploring” the metamorphic rocks west of Custer trying to figure out what sort of a rock was described as amphibolite. Although at that time in my life I was hot into sedimentary rocks and vertebrate fossils, my curiosity had popped up while reading USGS papers describing the geology of the Four Mile and Berne Quadrangles immediately west of Custer and noting the large number of times “amphibolite” was mentioned. So off I went to explore, and to try and understand.

Life seems a quick succession of busy nothings. J. Austen

If I remember correctly, I located the amphibolite unit as it is exposed over several square miles. There were also “lots of” other rock units that I noted were really “gneiss” (pun intended). I collected a few hand samples (why???) because I was practicing being a geologist. Most were later discarded in one of my many rock gardens although a few were retained including one that I thought might be amethyst. But before you giggle, remember I have never claimed to be a mineralogist or petrologist!

So today I have a “hunk” (~4 x 5 cm) of metamorphic rock that appears to be part gneiss and part schist with layers of glassy blue or blue-violet cordierite var. iolite collected, as my label states, “west of Custer.”

Cordierite from "west of Custer County, South Dakota, near the amphibolite unit."  Width FOV ~7 mm.

By-the-way, I never really completely understood amphibolite. As defined by Wikipedia (retrieved 3 January 2023): “Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure.

Amphibolite frequently forms by metamorphism of mafic igneous rocks, such as basalt. However, because metamorphism creates minerals entirely based upon the chemistry of the protolith, certain 'dirty marls' and volcanic sediments may also metamorphose to an amphibolite assemblage. Deposits containing dolomite and siderite also readily yield amphibolite (tremolite-schist, grunerite-schist, and others) especially where there has been a certain amount of contact metamorphism by adjacent granitic masses.”

Cordierite collected in "Madagascar" (top and middle figures) purchased 2022 from Geofossiles in Colorado Springs. Bottom figure: Purchased, but collected Eminiminy (Anbinany), Androy Madagascar. Width FOV ~8 mm.

Thin, glassy, translucent blue-violet fragment of cordierite without matrix, etching is natural. Maximum width ~1.0 cm. Purchased, but collected by Luiz Menezes, 2001, Coroaci, Minis Gerias, Brazil.

Cordierite [(Mg,Fe)₂Al₃(AlSi₅O₁₈)] occurs in a variety of colors: gray, yellow-brown, greenish, colorless, blue, and bluish violet. It has a hardness of ~7.0+ and a vitreous luster while thinner crystals are translucent to transparent while the massive material seems rather opaque. Cordierite belongs to the Orthorhombic Crystal System although some twins resemble pseudo-hexagonal prismatic crystals; other material appears as massive to embedded grains. It has a white streak and a subconchoidal fracture.

Cordierite/iolite is also quite pleochroic, that is there are changes in color depending on the angle at which you view the specimen. Gemmy iolite may have pale blue color or a violet color or even a pale-yellow color. This pleochroism is quite easy to observe in my specimens as the mineral is rotated. In some case the blue color almost disappears into a gray-blue color.

The variety iolite/water sapphire is a blue to blue-violet to a blue-gray color and can be quite gemmy. I assume that lapidaries are experts in cutting the gems correctly so that the stones bring out the brightest blue color. It is softer than natural sapphire and has a lower refractive index (less brilliance). However, the cost of using iolite in jewelry is substantially less than mounting sapphire and most casual observers of a well-cut stone (cabs or faceted) would likely not notice the difference.

So, that is my tale of tracking down a mineral that was of interest to me but without the chance to tromp through the mountains.  It just took a little sleuthing while remembering the words of Dr. Suess: You have brains in your head. You have feet in your shoes. You can steer yourself any direction you choose.

REFERENCES CITED

Eckel, E.B., 1997, updated and revised by R.R. Cobban and others, Minerals of Colorado: sponsored by Friends of Mineralogy. Colorado Chapter, Denver Museum of Natural History, Fulcrum Publishing, Golden CO.

Finlay, G.I., 1907, On an occurrence of corundum and dumortierite in pegmatite in Colorado (near Canon City): Journal of Geology, vol. 15, no. 5.

Hausel, W.D, 2009, Gems, Minerals & Rocks of Wyoming: Private Publication, Gilbert, Arizona.

Jacobson, M.I., Part II: 1988, Corundum in pegmatite, or is it?, Rocky Mountain Boy claim, Grape Creek, Fremont County, Colorado: Mineral News, vol. 4, no. 2.