This is an exceedingly strange specimen: Mckelveyite-(Y) as a uniform crust on glassy fluorapatite. Both species were identified via EDS. (See the “Analysis” tab.) EDS can’t really tell if “apatite” is fluorapatite, but the only “apatite” on the current MSH species list is fluorapatite.While mckelveyite-(Y) is generally impossible to distinguish visually from donnayite-(Y) and ewaldite, and even EDS scans are often ambiguous, in this scase the EDS scan gives a fairly clear indication for mckleveyite-(Y).
(More than you wanted to know: According to Igor Pekov – Mineralogical Almanac, V7, 2004, p. 77 – there is a complete isomorphous series between mckelveyite-(Y) and donnayite-(Y), so the presence of some Sr is not unusual. However, the Sr peak obscures the low energy Y peak. To absolutely prove that Y is present, one would need to look for Y peaks above 10 eV. That was not done in this case, so ewaldite can’t be absolutely ruled out, but I consider that possibility remote. Not that it really matters – either way this is weird stuff.)
It is very difficult to understand why mckelveyite (or ewaldite) would form such uniform crusts on fluorapatite. According to the analyst, under SEM the mckelveyite is composed of tiny saucers of typical shape.
The first pair of photos (FOV 2.3 x 3.5 mm) shows several encrusted apatite prisms. Note especially the prism just right of center, which is seen nearly end-on. The interior apatite crystal is clearly hexagonal. But so is the outline of the crystal plus crust. On another specimen, at least one of the apatite + mckelveyite combinations ahs a sharp triangular (trigonal) cross- section. But I don’t think this is epitactic growth. Very strange – quite possibly a unique find.
The mckelveyite crust appears to fall off readily, leaving behind lustrous “naked” apatite crystals. The second pair of photos (FOV 3.0 x 2.25 mm) shows a thin, glassy, apatite crystal ca 1.3 mm long near the center. Just below is another encrusted crystal seen nearly end-on. Once again, one can see the sharp hexagonal outlines of both the interior apatite crystal and the mckelveyite crust. The total cross-section is ca 0.25 mm.
The mckelveyite/apatite is sprinkled with unidentified,very tiny, black discs and lustrous, colorless, blocky crystals. The third pair of photos (FOV 1.2 x 1.65 mm) is an attempt to show these at very high magnification. Unfirtunately these crystals are really too small for my equipment to resolve properly. The colorless crystals are < 0.25 mm in size; the black discs only about 0.1 mm. Very often, in this environment (the “contact zone” of the Poudrette pegmatite, small, blocky, colorless crystals are gaidonnayite, but – to the extent that I can see them – these don’t look like gaidonnayite. They look more like hilairite. Colorlesshi;airite may seem surprising, but colorless, glassy, hilairite has been reported from this environment by others. However, I make no specific claims. The crystals are just too small to be sure. Even at 50-60X you will be hard pressed to discern their morphology.
As for the balckisih discs, one possibility might be UK109 (i.e. mckelveyite-2M), but two attempts to get EDS scans both failed. In any case, there are many other possibilities. Interesting - if you like a challenge.
Single item shipping weight is 2.7 oz (77 g). For shipments outside the USA, up to a total weight < 8 oz (225g), this can be combined with items from this or other auctions for the same postage.
Within the USA, postage increases by about $0.20 per ounce. Above 13 oz, I will use Priority Mail ($9.25 including packing).
