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Emerald Replaced Gastropod
Four years ago there appeared in Colombia an unusual emerald in the form of a sea shell. It was a mere 2 carats and of lower quality but, if genuine, would be of considerable importance in explaining the origin of emeralds from Colombia. It was unequalled as a rarity or a curiosity; however, being the only one known to exist, it was felt to be just too much of a freak occurrance to be significant scientifically. Recently however, three more emerald sea shells have appeared, giving validity to the first one and providing, along with some broken pieces, the basis for a preliminary study.
These emeralds are properly called emerald replaced gastropods. In the mineral kingdom it happens that the unit cells of a gem or mineral get petrified in the form of an organic material. Microscopic examination reveals that the emerald gastropod formation was one of carbonate destructive replacement; a molecule by molecule replacement of the grains of the shell structure by molecules, or unit cells, of emerald.
On the 2 carat gastropod pseudomorph the crystallization started at the outside of the sea shell and proceded inside. At the outside of the sea shell the grains of crystallization are very fine or microcrystalline. Then, as the environment for the crystallization stabilized, the crystals got grainier, or larger. The crystals resulting from this druze effect are an unusual 1/4 of a millimeter in size on the inside part of the shell.
According to geologists familiar with the Colombian mining region, the existance of these emerald gastropods clearly establishes the low temperature mineralization of the Colombian emerald. This is consistant with some of the newer theories of Colombian emerald mineralization as opposed to the high temperature metamorphic formation of most other emerald localities. Albite is found in the veins of the trapiche emeralds and is found in Chivor and Gachalá as well as Muzo and Coscuez. Albite is a feldspar, and Exploration Geologist Tony Mariano even found pseudomorphs of albite in Galacha of fossils.
Thanks to paleontologist Diana Gutierrez of INGEOMINAS for the fossil identification. Anthony Mariano Ph. D. supplied info on the albitized fossils fron the Gachala region of Colombia. Geologist William Rohtert identified the druze effect in the first gastropod.
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