This is a fairly large specimen from a hornfels seam with lots of good dolomite crystals and pyrite. But the real interest here is the blue-gray “donnayite” shown in the close-up photos. This stuff is somewhat lumpy but relatively abundant on the specimen.
According to qualitative EDS (see the “Analysis” tab) on an another specimen from a similar,near-by, find, the donnayite is mixed in with FeSO4 and/or pyrite. It is typical at MSH for donnayite to be sprinkled with and to include pyrite. It isn't clear just from EDS whether any or all of the Fe and S is coming from such pyrite, or if (at least some of) the pyrite has altered to an FeSO4 mineral. The "donnayite" on this specimen has less visible pyrite on it, so perhaps it would have produced a purer donnayite EDS scan.
The first two photos (FOV 6.9 x 4.5 mm and FOV 6.7 x 4.5 mm resp.) are slightly different views shows the most heavily “donnayite” populate area – and a relatively large, but rather twinned, pyrite crystal, plus a couple of small dolomite crystals.
The third photo (FOV 4.9 x 6.9 mm) shows a few more “donnayite thingies” and dolomite crystals. The dolomite crystals are sharply formed and translucent, but they are encrusted by oxidized pyrite which does make them look dirty.
The fourth photo (FOV 6.9 x 4.5 mm) shows another typical dolomite crystal 2.6 mm on edge with a few of the more attractive pyrite cubes on the specimen. There are one or two larger cubes, but most of the pyrite is heavily twinned.
In addition to the “donnayite” crystals shown in the photos, there are several similar crystals/blobs sparsely scattered over the specimen.
Note: The “dolomite” is a visual ID. Possibly there is an epitactic crust of another carbonate on top of dolomite. Ankerite epitactic on siderite is known from the carbonate pegmatites at MSH, but I don’t think that’s what this stuff is. In any case – not analyzed.
Found 5/98 a hornfels seam near the Poudrette pegmatite
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