MSH is the type locality for both adamsite-(Y) and thomasclarkite-(Y). Adamsite-(Y) has also been found at Hundholmen, Norway and the Shomiokitovoe pegmatite (Lovozero Massif). Thomasclarkite-(Y) is know only from MSH. Both minerals are rare here. 

The two minerals are supposedly often found together, although that has not been my experience. In any case, this specimen appears to be very unusual in that the yellow thomasclarkite  looks as if it were a direct outgrowth of the pinkish adamsite blades. There is more adamsite  behind the thomasclarkite, so perhaps the thomasclarkite is epitactic on adamsite. (I don’t know if that is possible or not.)

The first two photos show the “transition zone” between adamsite-(Y) and thomasclarkite-(Y). Most of the thomasclarkite crystals are unusually elongated. However, the crystal at far right, which does not appear to be epitatic, is fairly typical of thomasclarkite. This crystal is about 0.6 mm long. In total, the thomasclarkite spans about 3 mm.

The next two photos show all of the thomasclarkite and most of the adamsite on the specimen. The thomasclarkite spans ca 3 mm, and the adamsite spans about 4.4 mm. They abut a nearly black aegirine crystal. The little reddish blebs are rhodochrosite.

The third pair of photos shows some very nice, free-standing, adamsite blades to 2.7 mm. These blades are attached to the main adamsite aggregate shown in the other photos but are out of view in them.

Note: The specimen may seem a bit “dusty”, but resist the temptation to “clean” it with a duster. The adamsite-(Y) is easily blown away. 

Single item shipping weight 3.6 oz (105 g). For non-USA destinations, up to a total weight < 8 oz (225 g), this can be combined with other items for the same postage. Within the USA, each additional ounce costs about $0.20; for a total weight of 13 oz or more, I will use “own box” Priority Mail ($9.25), which good up to 16 oz.