Image Credit: Malcolm Southwood
Mineral Species
Betpakdalite-CaMg
Type Locality
Yes
Composition
[Ca2(H2O)17Mg(H2O)6][Mo8As2Fe3+3O36(OH)]
Crystal System
Monoclinic
Status at Tsumeb
Confirmed (type locality)
Abundance
Extremely rare
Distribution
Second oxidation zone
Paragenesis
Supergene
Entry Number
Species; TSNB50
Type Mineralogy
Betpakdalite, sensu lato, was recorded as a rare species at Tsumeb by Schmetzer et al. (1984) and by Lombaard et al. (1986), but it was the reclassification of members of the betpakdalite supergroup within the broader heteropolymolybdate family that gave Tsumeb type locality status for a newly-defined species betpakdalite-CaMg, IMA 2011-034, with a published description by Kampf et al. (2012). Betpakdalite-CaMg was found on a specimen of scorodite, from 35 Level in the second oxidation zone at Tsumeb, in the collection of Bill Pinch; it was formerly in the collection of TCL mineralogist John Innes. Co-type material is conserved at Los Angeles County Museum of Natural History (catalogue numbers 63327 and 63328) and at Harvard University (catalogue number MGMH 2020.7.701).
General Notes
According to Kampf et al. (2012):
"Betpakdalite-CaMg occurs on and is included in scorodite, with associated quartz and djurleite. It is an oxidation zone mineral which is presumed to have formed as a result of the alteration of As, Fe and Mo sulfides by solutions rich in Ca and Mg. The crystals of betpakdalite-CaMg are yellow pseudo-octahedra up to ~1 mm in maximum dimension…. They exhibit the forms {001}, {11¯0} and {201¯}. In contrast to the other members of the betpakdalite supergroup in which twinning is very common, twinning is uncommon in betpakdalite-CaMg."
Betpakdalite-CaMg is much rarer at Tsumeb than betpakdalite-CaCa. It is interesting to note, however, that the material used for structure refinement of betpakdalite by Cooper and Hawthorne (1999b) had a chemistry corresponding to betpakdalite-CaMg.
A specimen offered for auction by Joy Désor in May 2023 consisted of a sulphide matrix with sparse microcrystals of betpakdalite-CaMg and sewardite (https://e-rocks.com/item/jyj978748/betpakdalite-camg-sewardite?sid=5e0cdc4079dccfb43afd4a598a25d6ff; accessed May 2023). Both species were identified by EDS.
Associated Minerals
djurleite; quartz; scorodite; sewardite
