Mckelveyite-(Y) is a very rare mineral at MSH. This is a small specimen , from an analyzed find, with several “cones” of grayish mckelveyite-(Y).These are shown in the first pair of photos (FOV 2.6 x 2.2 mm). Individual cones are ca 0.25 mm in diameter. The largest aggregate is 0.5 mm. You will need 30-50X to view these crystals properly.
According to the analyst, this is likely mckelveyite-(Y). Based on the conventional formula for mckelveyite-(Y) - NaBa3(Ca,U)Y(CO3)6 · 3H2O – the EDS scan seems to be a very good match – much less ambiguous than most “donnayite group” scans that I have seen. (See the “Analysis” tab.) There are strong Ba and Ca peaks, as well as Y and U. The Sr peak is weak, ruling out donnayite-(Y). (The Na peak looks weak too, but Na is difficult to detect with this equipment – it barely shows up at all for sérandite and other “known” Na minerals.)
Nonetheless there is caveat. The conventional formula for ewaldite - Ba(Na,Ca,Y,Ce,K)(CO3)2 · 2.6H2O - is rather similar. In fact, according to Pekov and Podlesnyi ("Kukisvumchorr Deposit ... " Mineralogical Almanac, Vol. 7, 2004) ewaldite and mckelveyite should both have the same formula (Sr,Ba)(Na,REE,Ca)(CO3)2.•H2O - without the -(Y) - and differ only in their symmetry. The probe data presented by them certainly supports that contention. In particular, there are no significant differences in either Ca or Y content for the 6 ewaldite and 7 mckelveyite samples given, and there are even mckelveyites with no detectable Y! (The last observation is “good” for this specimen, because the Y peak isn’t very impressive. But bear in mind that peaks on a quick and dirty scan of an uncoated sample can’t really be used to make quantitative inferences, especially for REE. Also, the high energy Y peak is not visible in this scan.)
I have labeled the specimen as "mckelveyite" because, if one accepts the usual formulas, then that probably does make the best fit. But to be certain, one would need XRD to determine the symmetry. (It is nearly impossible to do this visually because, even though mckelveyite is triclinic, and ewaldite is hexagonal, crystals of both species look much the same.) In any case – this is not donnayite-(Y).
Also: The analysis was done on another specimen from this find. “Donnayite group” chemistry can be quite variable. But it isn’t practical to analyze every sample from a find.
Single item shipping weight (with TN case) is 3.0 oz (84g). For shipments outside the USA, up to a total weight < 8 oz (225g), this can be combined with other items from this or other auctions for the same postage. If you don’t need the TN case, you can save about 0.3 oz in weight which could be used for something else.
Within the USA, postage for total weights up to 13 oz ranges from $4.25 to $6.50 (including packing etc) regardless of destination. Above 13 oz, I will use “own box” Priority Mail. The rate, including packing, varies by destination from $7.25 to $9.50.
