Andyrobertsite and calcioandyrobertsite are two fascinating mineral species present in a single very famous specimen from Tsumeb. This superb specimen was featured in the cover of the Mineralogical Record vol.30, num. 3, May-June 1999, jointly with the original article describing these new minerals in the same issue (p.181-186).
This outstanding specimen has a long and interesting history. It was mid-September 1996, when the well known mineral collector William W. Pinch recently deceased (1940-2017), stopped to chat with Carol Smith in the parking lot of Holiday Inn, in Denver Show. Carol mentioned a large and spectacular “keyite” specimen for sale in the room of Carter Rich. An immediate visit to his room revealed a beautiful electric-blue crystal cluster on matrix, perhaps the most spectacular rare-mineral specimen I have seen, in words of W.Pinch. The label stated “Keyite crystals, with Zn-deficient analysis provided, from Tsumeb , S.W.A. “ but the mineral did not resemble keyite. The absence of Zn and presence of K indicated that this must be a new mineral!
The specimen first belonged to Richard Baughart, a mining engineer working at Tsumeb; it was reported to have been found in the early 1950’s. At a later time, it was adquired by Ben Staskun, a close friend to Baughart. Staskun donated it to a well-known U.S.university. In the 1990’s, the university sold the mineral for a few dollars (!) in a dispersal sale to Dr. Steven Kuitems. Dr. Kuitems then exchanged the specimen with Carter Rich for a zincite crystal from Franklin. Carter Rich put up the piece for sale, that was finally purchased in Denver Show on September 15, 1996 by William W. Pinch.
The single specimen and the only one known worldwide for many years consists of an aggregate of overlapping plates with radiate outward from a common center. The aggregate is pyramidal, 1,4 cm long and is perched on cuprian adamite and zincian olivenite. The overall specimen is 4,1 cm long. Andyrobertsite and calcioandyrobertiste from a lamellar intergrowth, that is crystallograpically continous. They are a characteristic electric blue, with pale-blue streak.
It seems that today only two “macroscopic” andyrobertsite specimens exist, see the other one in Mindat, that was in display at the 2012 Munich Show. In addition to these two big specimens, a very few small single crystals/fragments exists of this extremely rare mineral species, one of them is on e-Rocks this week offered by Purple Sky Minerals (see SKY672908 link below) .
Andyrobertsite - Tsumeb, Namibia - Purple Sky Minerals
It is interesting to note that andyrobertiste is a cadmium mineral species. Only 27 of 5450 approved mineral species are cadmium minerals!
...... 28 if you include manganoshadlunite - (Mn,Pb,Cd)(Cu,Fe)8S8. - designated as probably valid.
Of the the known cadmium bearing minerals, 4 are arsenates with combined metallic cations; keyite and andyrobertsite are 2 species endemic to Tsumeb, (though keyite has apparently been discovered at Lavrion).
Vanackerite (Pb4Cd(AsO4)3(Cl,OH) ) is a variation on mimetite with a cadmium cation in substitution to a lead (also endemic to Tsumeb).
The other cadmium arsenate mineral is nyholmite (Cd3Zn2(AsO4)2(HAsO4)2 · 4H2O) in the Hureaulite Group, endemic to Broken Hill, NSW, Australia.
For completeness calcioandyrobertsite is the calcium analogue of andyrobertsite - does not contain cadmium. (Ed)
Andyrobertsite and calcioandyrobertsite are named for Andrew C. Roberts (1950-) mineralogist of the Geological Survey of Canada.
Thanks to David Hospital for providing this blog.