Schorl, Pyrite, Muscovite

This softball-sized, 14 x 9 cm specimen of pseudomorphed schorl from the Maine Feldspar Quarry won hands down the "Ugliest G.D. Maine Mineral Specimen Ever Seen Award" during after hours wagering at the 1996 Maine Mineral Symposium. Field view 17 cm. Collected by Doug Watts in 1995.

This abomination to mineral aesthetics was originally a very big, well-formed schorl crystal section completely altered to olive green muscovite and 5-10 mm cubes and cubic-octahedra of pyrite, many of which themselves have weathered to limonite and goethite. The numerous small dark pits are where pyrite octahedra have fallen or weathered out and left clear, discernible casts lined with goethite in the muscovite.

It appears to have reached a much more advanced stage of muscovite replacement than the previous specimen, where some original, primary schorl was still present. This one has none, but still retains the overall shape and longitudinal fabric of a schorl crystal.

This specimen suggests to me that the late stage fluid which caused this replacement was fairly rich in sulfur, and in this case, converted much of the iron in the schorl into pyrite. The only examples of well-formed pyrite cubic-octahedra at the MFQ are embedded in pseudomorphs such as this one.

This specimen and others also suggest that the extensive sulfide mineralization of the MFQ pegmatite was a very late stage event, as confirmed by emplacement relations (amorphous blobs of sulfides appear to have invaded and corroded into primary quartz and feldspar). The most likely source of sulfur was the surrounding metamorphosed and melted sedimentary rock, which can contain lenses of what originally was euxinic (anoxic) deep sea muds rich in sulfur, iron, and arsenic.*

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* Euxinic is an unfathomably obscure and technical geologic term which comes directly from the Greek historian Herodotus. The Black Sea in Herodotus' time (500 BC) was named the "Euxine.' Because of the shallow straits at Bosporus, which are only 120 m deep, oxygenated water from the Mediterranean Sea cannot move into the Black Sea, making the entire Black Sea devoid of dissolved oxygen below a depth of 250 meters (it's 2,000 m deep in places). This means the organic sediment deposited in the Black Sea at depths below 250 m can only be broken down by anoxic, methanogenic bacteria, producing hydrogen sulfide, among other things. This results in a distinct type of 'black shale' which is high in carbon, iron, sulfur and arsenic. Western and southwestern Maine have thin but extensive beds of this euxinic black shale, presumably from an enclosed ocean basin similar to the modern day Black Sea.





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Photo added: 22nd Jul 2012