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[Cesar M. Salvan]

The center-north of Portugal hosts the richest pegmatite field in the Iberian peninsula. These pegmatites occasionally have a size and composition that makes them economically viable for the exploitation of some industrial minerals. The north Portuguese pegmatites are epigenetic and hosted by granites or metamorphic rocks (shales). They are Be, Nb-Ta, P and U rich pegmatites. The Assunçao mine is a good example of this type of pegmatite. It is a beryl-columbite-phosphate pegmatite, with a primary paragenesis where the big beryl crystals, the phosphates triplite, triphyllite, apatite and isokite, the columbite-Fe and copper sulfides are prominent minerals. A strong hydrothermal phase involved mobilization of Be with formation of secondary beryl and beautiful bertrandite, and alteration of primary phosphates. The remobilized phosphate left to the formation of good autunite specimens, thanks to the uranium richness of the deposit, and other secondary phosphates. The autunite, which generally forms beautiful crystals of yellow-green color, appeared initially in the market as uranocircite. We demonstrated, by analysis of a lot of specimens, that all these lots were autunite with high probability and the geochemistry of the pegmatite do not suggest an efficient barium source.

Now, in a recent study we performed on Assunçao samples, we can report the occurrence of phurcalite, an uranyl and calcium phosphate with different structure than autunite. Rarer than autunite, the phurcalite crystallized as a previous phase in the secondary assemblage of minerals in uranium-rich pegmatites. The phurcalite occurs as small spherules composed by the radial aggregation of acicular crystals and in the form of minute tabular crystals that resembles phosphuranilite, a mineral with similar distribution of uranyl cations in its structure.

It is possible that this mineral may have gone unnoticed due to its composition, similar to autunite, but it is easily distinguishable by morphology and by structural analysis, specially vibrational spectroscopy (Raman), the best tool to recognize the mineral.

In conclusion, another mineral species to add to the long list of the Assunçao mine.

[Jordi Fabre]

Superb. Thanks Dr.!

[Cesar M. Salvan]

Thanks Jordi! A pleasure to contribute.

[stef4412]

Very nice
Thank a lot
It"s interesting to see phurcalite from this locality, it's changing from autunite alone

[Cesar M. Salvan]

More specimens to add to this interesting locality!

[Roger Warin]

Hi,
Many thanks. A master class with great pictures of exceptional minerals.
Roger

[Cesar M. Salvan]

Thanks Roger!

[Cesar M. Salvan]

Adding more complexity to the uranyl phosphates from Assunçao mine. The saleéite was not previously observed in the deposit, but we found very interesting specimens of saléeite-torbernite intergrown. The saleéite shows greenish yellow to green color, depending on the copper content. The torbernite forms epitaxial intergrowhts and alternating layers with saléeite. The most interesting specimens forms beautiful centimetric crystal aggregates on partially dissolved autunite or covering previous autunite crystals. Occasionally, a later autunite generation crystallized out on previous saléeite-torbernite.

So, the current count of identified uranyl phosphates described in this thread is: phurcalite, autunite, saléeite and torbernite. The observation of specimens suggests an early phurcalite deposition followed by autunite and later saléeite/torbernite. Also, the intergrowths and partially dissolved or attacked crystals suggests remobilization and recrystallization processes.

The uranyl phosphate minerals from Assunçao mine are very interesting and beautiful. Unfortunately the minerals are isostructural and practically indistinguishable de visu. Apart of torbernite, the identification of these species always require an analysis of every specimens. The Raman spectroscopy is a fast and non destructive tool to test them and we regularly used it in all samples.

[yowanni]

Great photos and very interesting information. Thanks for posting!!

[Cesar M. Salvan]

More minerals from this interesting deposit