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.