Manganoneptunite is a rare mineral, known mostly from the Khibiny and Lovozero massifs, MSH, and a few other localities in the Motérégian hills, including the Saint-Amable sill, where it is relatively common. The crystals here are small (mostly < 4 mm), but uniformly well formed and attractive (except when heavily weathered). The largest crystal on this specimen is 2.1 mm, which is a good size for the locality.
From the point of view of rarity however, the most interesting things on the specimen are two pseudomorphs of monazite-(Ce) or rhabdophane-(Ce) after an unknown precursor. The material was analyzed via qualitative EDS (see scan #337 under the “Analysis” tab), but the chemistry of these two minerals is identical. EDS serves only to eliminate other possibilities. Both minerals are known from this quarry, and, according to László Horváth, they form very similar aggregates. I posted this as monazite-(Ce) because rhabdophane-(Ce) is supposedly very rare at this locality. But visually, it looks much the same as the rhabdophane-(Ce) posted by László elsewhere. Monazite-(Ce) is also rare in the Saint-Amable sill, but not as rare as rhabdophane-(Ce).
Note: The presence of Ca in scan #337 is not unusual. In the case of rhabdophane, if Ca > Ce, it becomes brockite (an isostructural mineral). But Ca can also be present in monazite-(Ce). I have included scan #279 for monazite crystals (from another specimen) which is nearly identical (apart from the K and Zn wich came from the substrate) to scan #337). (“Crystals” of rhabdophane larger than a fraction of a millimeter are invariably pseudomorphs. Anything that has the same chemistry and forms visible, gemmy, crystals is probably monazite.)
The first pair of photos (FOV 4.2 x 5.8 mm) shows the manganoneptunite crystals and, on the far right, one of the monazite/rhabdophane PSMs. This one is only partly visible in the photo, and some of it has crumbled away. The brownish fibers are yofortierite (or perhaps tuperssuatsiaite – they were not analyzed).
The second pair of photos (FOV 2.55 x 2.25 mm) shows the better “monazite” PSM. It is ca 1.25 mm tall. The brown fibers at top are yofortierite (or possibly tuperssuatsiaite).
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