According to the label  (which see), this is sheldrickite on shortite, but according to László Horváth, the “sheldrickite” is most likely vaterite, which was much more common than sheldrickite in this association. This is borne out by the qualitative EDS scan, which is consistent with vaterite, but shows no Na.  (Na  would be expected for sheldrickite). Of course, the scan is also consistent with calcite, but given the association, appearance, and László’s comments, it is most likely vaterite. (In the absence of the shortite, the white stuff could be anything.) Vaterite is a very rare mineral at MSH – almost as rare as sheldrickite.

The first pair of photos (FOV 6.9 x 4.4 mm) shows the main area of vaterite. In this area, it looks somewhat flaky – i.e. like sheldrickite.

The third photo (FOV 4.1 x 5.0 mm) shows another area where some of the vaterite looks like a crust and some like little “droplets”. The green needles are aegirine. 

The fourth photo (FOV 6.9 x 4.6 mm) shows the only area where some prism faces and cross-sections can be seen on the shortite. Much of the specimen consists of shortite, but it is very inter-grown. There are no terminated crystals.

Single item shipping weight (no case) is 3.3 oz (94 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 (ca $9.25 including packing – varies by destination).