DESAUTELS MICROMOUNT SYMPOSIUM AND DESAUTELSITE

 

I am a member of the Baltimore Mineral Society and have attended most monthly meetings for the last five years. Now Baltimore is a far distance from either Wisconsin (current home) or Colorado (past home) and purchasing airline tickets every month would break my Social Security budget! However, since the “Covid Pandemic” the Society has met via Zoom and even today their meetings are a hybrid with both Zoom and in-person. At most meetings the Zoom attendees from about 7-10 states and a couple of international locations often “outnumber” the in-person event. I feel very much at home via Zoom and the group has very lively and participatory mineral discussions. In addition, I have presented ~4 Power Point programs and contributed several manuscripts to their Newsletter.  Interestingly, many of the members seem like “old friends” although I have not personally met any of them and most likely will never attend an in-person event (due to expenses).

Each year the Society sponsors the Desautels Micromount Symposium and is the home of the Micromounters Hall of Fame. The Hall honors those who have supported and promoted micromounting during their collecting career.  In the early years, the Society honored “modern awardees” and a few “old timer awardees.”  In 1981, the initial year, Paul Desautels was inducted along with “old timers” George Fiss (d. 1925) and George Rakestraw (d. 1904). In examining the Hall of Fame recipients I noted the names of Lazard Cahn, 1982, the Honorary President for Life of CSMS, Arthur Roe, 1993, a founding member of CSMS who studied under Cahn, Jim Hurlbut, 2011, from the Denver area and a force in the Rocky Mountain Federation, Shorty Withers, 1995, the Honorary Curator of Micromounts at the Denver Museum of Science, and Arnold Hampson, 2012, of Cortez who donated his micromount collection to the Colorado School of Mines. Two micromounters are inducted each year with the 2025 inductees being long time collector Ron Gibbs from Arizona and David Roe who has wandered around and collected in Devon, England, UK, for decades.

Paul Desautels (1920-1991), for whom the Symposium is named, held many professional positions in his career but perhaps is best known for his 25 years spent as Curator of Gems and Minerals in the Department of Mineral Sciences in the U.S. National Museum of Natural History, AKA Smithsonian Institution. And perhaps, he was “the most influential curator of the 20^th century”. Desautels was awarded the Carnegie Mineralogical Award for his mineralogical contributions, the Smithsonian Director’s Medal, and the Tucson Gem and Mineral Show created an annual award for “mineral collecting connoisseurship” named the Desautels Trophy. Above from Hall of Fame .

In 1979 Desautels was honored when a new carbonate mineral was collected from the Baltimore Mafic Complex (1.1-1.0 Ga) in a Pennsylvania aggregate quarry (Cedar Hill)  and was named desautelsite [Mg₆Mn₂(OH)₁₆[CO₃]-4H₂O]  (Dunn and others, 1979).  It is a rare, bright orange mineral associated with fractures and cracks in rocks called serpentinite. These exposed metamorphic rocks (layered ultramafic, mafic, and volcanic rocks) are composed of  magnesium silicates formed by hydration and metamorphism of mantle rocks along boundaries of tectonic plates.  Several other secondary magnesium minerals are usually associated with desautelsite, including artinite [Mg₂(CO₃)(OH)₂-3H₂O)], and hydromagnesite [(Mg₅(CO₃)₄(OH)₂-4H₂O] although desautelsite is the last mineral to form in the fractures.

 

Druse of orange desautelsite on matrix. Width FOV ~ 4 mm. 

Desautelsite is a very soft mineral (2 on Mohs) with translucent pseudo- hexagonal crystals forming an orange, druse-like, scaly crust.  Without a powerful microscope the individual crystals are exceedingly difficult to observe.

Desautelsite is an extremely complex mineral, especially to an old plugger like me. It is a member of the Hydrotalcite Supergroup defined by their “natural layered double hydroxides…characterized by layered lattices (metal hydroxide layers alternating with carbonate and/or sulfate anions).”  I have poured over the defining article by Mills and others (2012) and my meager knowledge of crystal chemistry is just not sufficing for an adequate understanding.

MinDat (accessed 30 November) described the Super Group Hydrotalcite as containing, among others, desautelsite [Mg₆Mn₂(OH)₁₆[CO₃]-4H₂O], pyroaurite [Mg₆Fe³⁺₂(OH)₁₆[CO₃]-4H₂O] the iron analogue (whose Xray powder pattern is indistinguishable from desautelsite), and their solid solution group members stichite [Mg₆Cr³⁺₂(OH)₁₆[CO₃] - 4H₂, and hydrotalcite [Mg₆Al₂CO₃(OH)_{16 -}(H₂O)₄]. And then throw in iowaite [Mg₆Fe³⁺₂(OH)₁₆Cl₂ - 4H₂O], described in an October 19 Post, and which may weather to pyroaurite, and readers can understand my confusion (Defeat is not the worst of failures. Not to have tried is the true failure: G.E. Woodberry).

Although the Type Locality of desautelsite is in Lancaster County, Pennsylvania, the mineral is best known from two sites in California, especially the Artinite Pit in San Benito County, an open cut of exposed serpentinized rocks.  Secondary minerals like desautelsite grow on the altered surfaces of the spaces between the breccia blocks.  Other than these localities in Pennsylvania and California there is one locality in Maryland, and three in Japan, where the mineral occurs with other ultramafic rocks.  So, it is a rare mineral.

 

Acicular crystals of artinite, Mg₂(CO₃)(OH)₂-3H₂O, a low temperature alteration product in serpentinized ultrabasic rocks, that is associated with desautelsite.

REFERENCES CITED

Dunn, P.J., Peacor, D.R., Palmer, T.D.,1979, Desautelsite, a new mineral of the pyroaurite group: American Mineralogist, Vol. 64, No. 1-2.

Mills, S. J., Christy, A. G., Génin, J.-M. R., Kameda, T., Colombo, F., 2012, Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides: Mineralogical Magazine, Vol.76, No.5.

 

GOLDHILLITE: A NIFTY GREEN COPPER ARSENATE FROM THE TYPE LOCALITY

 

“Anybody want to head out on a field trip this weekend” was a clarion call during my stay at the University of Utah in the late 1960s. If our faculty members did not have a field excursion scheduled, then our small cadre of grad students could usually find somewhere to explore. As a result, I became decently acquainted with large parts of “rural” Utah--a much less populated state nearly 60 years ago! Couple that with the field work associated with my dissertation research, three years as a summer ranger at Dinosaur National Monument (with weekends to roam), my two sabbatical leaves and my two decades of summer research projects with students during my teaching career, and a large part of Utah became familiar territory. Throw in my many excursions with my close friend James Madsen, he of dinosaur fame, and together they remind me of fun, fun, fun, excitement, learning, digging out of mud holes and snowbanks, running out of gasoline, finding nifty fishing holes, teaching my children to locate fossils (and cow bones) and identify rocks, traveling with my spouse to remote localities as she became the chef for my field crew, having an ice-cold brew around a campfire after work, and having the time of my life. I suppose this is not much different than the lives of other field geologists. Most of us chose geology as a career so we could spend time outdoors and experience hiding from high altitude thunderstorms in the mountains to profusely sweating while hiking through the desert looking for just the right outcrop that could contain fossils. I would not change those experiences for anything.

 

The Deep Creek Mountains of western Utah. Photo courtesy of Qfl247, AKA Matt Affolter.

One of my “experiences” with Jim Madsen was chasing some Miocene vertebrate fossils after a tip from a couple of local rockhounds. This trip took us out to one of the more desolate areas I experienced during my “tromping around” career. First of all, it was an early “up and at em” morning since we needed to head west and cover the 120 miles of Bonneville salt flats before arriving at the oasis of Wendover, Utah, and Wendover, Nevada, a stop for gasoline, coffee and a snack. In those days (life has now changed in Wendover)  the burg was home to a single large establishment, the Stateline Hotel and Casino (currently operating assume other name) out in the middle of the desert. Seems like an entrepreneur by the name of Bill Smith operated a gasoline station in Utah, right east of the state line.  Never one to miss an opportunity,  when gambling was legalized in Nevada in 1931 Bill added a casino across the state line!  During the years that I was stopping in for a refreshment before heading to the desert, the vehicle was parked in Utah, we crossed a white painted line, and voila---we were in a different world with tinkling slot machines, blackjack tables, hard liquor at ten in the morning (a different approach than Utah), and an establishment open 24 hours a day.

But in those early years  we could not afford to succumb to the demons, so we grabbed large coffees and headed south driving  along the west flank of the Deep Creek Mountains.  After a rather unexciting journey we came to the tiny hamlet of Gold Hill, an old mining community that in the 1970s seemed unoccupied except for perhaps ghosts and goblins.

Later in life I developed that earlier hunt into a vertebrate fossil collecting project near the western edge of the southern Deep Creeks but located in Nevada (Nelson and Madsen, 1987).  I remember the area as still being one of the most desolate collecting sites that my students and I worked and made certain that my crew remained in visual site of each other. 

But we could never quite get Gold Hill out of our mind and decided a little exploration was in order. The diggings, both open pit and subsurface, at Gold Hill were mined for gold, copper, zinc, lead, arsenic and tungsten from the mid to late 1800s until the late 1940s.  The peak activity was in the early 1900s when a spur railroad reached the area in 1917.  There was only sporadic mining after World War I although World II brought a mini boom as tungsten was mined for use in the hardening of steel.

Gold Hill, also known as the Clifton District, is located near the northwest end of the Deep Creek Mountains, perhaps Utah’s most isolated and unknown mountain range.   Peaks do reach 12,000 feet—Ibapah Peak at 12,087 and Haystack at 12,020 and are the major topographic feature in western Utah.  The range has a Precambrian core surrounded by Paleozoic sedimentary rocks with later Mesozoic intrusions—mostly quartz monzonite and granite/granodiorite, and later Tertiary volcanics. 

Mineralization began at Gold Hill approximately 152 Ma as magma was intruded into the overlying Paleozoic rocks (Robinson, 1993). This intrusion created three different types of ore deposits: skarn deposits, replacement type bodies, and vein deposits. At Gold Hill skarns are small structures but contain a rich suite of metals that includes copper, iron and tungsten (Ege, 2005).  

 Replacement deposits occur along the contact between the cooled magma (granodiorite) and limestone. The host rocks for the ore deposits are Mississippian to Pennsylvanian (360 to 330 Ma) limestones (Ege, 2005; Robinson, 1993).  These deposits are rich in silver, gold, arsenic, copper, and lead.

 Vein deposits are found within the intruding igneous rocks. All of the veins are small and represent only a small portion of the ore mined from the district (Ege, 2005). 

 A second stage of mineralization commenced in the early to middle Tertiary (perhaps ~38 Ma) when magma again intruded into Mississippian to Pennsylvanian sedimentary rocks.  These deposits are rich in silver, lead, copper, gold, and other metals.  In addition, extrusive volcanic rocks, ~8 Ma, created some low-grade beryllium mineralization in veins associated within the granodiorite (Ege, 2005; Robinson, 1993).

The primary minerals (hypogene) in and near the cooked limestone include galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, tetrahedrite, and arsenopyrite.  The overlying oxidized supergene includes all of those beautiful arsenates: adamite, austenite, arsenosiderite, beudanite, conichalcite, clinoclase, mimetite, olivenite, pharmacosiderite, scorodite, and veszelyite  (listed by El-Shoutoury and Whelan, 1970).  Since that publication, there have been other supergene minerals identified—see MinDat.com.  The arsenic in the arsenates probably was derived from the primary (hypogene) mineral arsenopyrite (FeAsS).  Shoutoury and Whelan (1970) noted the arsenates of zinc, lead, copper, iron and calcium were produced when primary chalcopyrite, galena, sphalerite and pyrite were oxidized in close proximity to arsenopyrite. 

Goldhillite in a nest of a Mixite Group Mineral, probably Mixite. With of emerald green goldhillite is ~0.8 mm.

A closeup of goldhillite showing the flattened tabular crystals in the rosettes. The photo is somewhat fuzzy due to magnification by camera. Again width of specimen ~0.8 mm.

One quite rare copper arsenate mineral from Gold Hill is goldhillite that was named and defined in 2022 (Ismagilova and others, 2022). Celestian (2022), while describing new mineral names in the American Mineralogist, noted that this new Ismagilova study redefines the structure and chemical composition of philipsburgite, Cu₅Zn[(AsO₄)(PO₄)](OH)₆⋅H₂O,  to show that it has ordered phosphate-arsenate sites in its structure and then defines the arsenate end member (no PO₄) of the goldhillite-phillipsburgite-kipushiote solid solution series to be goldhillite, Cu₅Zn (AsO₄ )₂ (OH)₆ ·H₂ O.  In other words, phillipsburgite was restudied and “split into” two different members in the solid solution series since new classification schemes [now] open the door to new mineral descriptions [and names].

Although usually a micromineral, goldhillite is beautiful under the scope and is a bright  emerald green in color with a vitreous luster. The crystals are usually tiny flattened tabs forming small rosettes. Crystals are soft (~3.5 Mohs) and brittle with an irregular fracture. Goldhillite is often found with cornwallite, mixite, and/or conichalcite and my specimen exhibits a neat bed of mixite surround the goldhillite rosette. One of the problems associated with goldhillite splitting off from phillipsburgite is that the two minerals are very difficult to visually distinguish between, and many localities that originally listed phillipsburgite in their mineral list must now use an electronic gizmo to determine if their tiny green mineral has phosphate (PO₄) substituting for some of the arsenic (note chemical formulae in above paragraph).  Ain’t life a bowl of cherries. My specimen was located in the Middle Pit at Gold Hill and that locality is the Type Locality for the mineral so goldhillite it is. 

You can't fight the desert.. you have to ride with it.    Louis L'Amour  

 REFERENCES CITED    

 

Ege, C.L., 2005, Selected mining districts in Utah: Utah Geological Survey, Miscellaneous Publication 05-5.

Celestian, A.J., 2022, New mineral names: American Mineralogist,. vol. 107. 

El-Shatoury, H.M., and Whelan, J.A., 1970, Mineralization in the Gold Hill mining district, Tooele County, Utah: Utah Geological and Mineralogical Survey Bulletin 83.

Ismagilova, R.M., Rieck, B., Kampf, A.R., Giester, G., Zhitova, E.S., Lengauer, C.L., Krivovichev, S.V., Zolotarev, A.A., Ciesielczuk, J., Mikhailova, J.A., Belakovsky, D.I., Bocharov, V.N., Shilovskikh, V.V., Vlasenko, N.S., Nash, B.P., and Adams, P.M., 2022, Goldhillite, a new mineral species, and redefinition of philipsburgite, as an As-P ordered species: Mineralogical Magazine, vol. 1, issue11. 

Nolan, T.B., 1935, The Gold Hill Mining District Utah: USGS Professional Paper 117.

Staples, L.W., 1935, Austinite, a new arsenate mineral, from Gold Hill, Utah: American Mineralogist, v. 20.

 Robinson, J.P., 1993, Provisional geologic map of the Gold Hill quadrangle, Tooele County, Utah: Utah Geological Survey Map 140, scale 1:24,000.