方解石与蛤蜊化石 Calcite in Clamshell Fossil

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ere’s a fine matrix specimen of GOLDEN CALCITE inside a FOSSIL CLAMSHELL from RUCK’S PIT, NEAR FORT DRUM, OKEECHOBEE COUNTY, FLORIDA. The size is about 8 x 6.5 x 3 cm。The calcite crystals are SHARP SCALENOHEDRONS (“DOGTOOTH” CRYSTALS) and and are covered a lovely GOLDEN YELLOW. These fossils date back to the PLEISTOCENE (1 to 1.5 million years ago).

    This specimen is a great example of gemmy honey-amber calcite crystals filling and replacing Mercenaria permagna clams. First discovered by Dr. Thomas Scott of the Florida Geological Survey, the clams are found in living position, and are largely confined to a stratigraphic zone 0.3 – 0.6m thick throughout the pit. Because these Mercenaria are found in hinge-up living position, it has been suggested that they died in place due to a catastrophic event, such as sudden burial or freshwater poisoning resulting from a hurricane, tsunami or other causes. The age of these Mercenaria fossils is late Pliocene to early Pleistocene (1 – 1.5 million years old).

    The size of the crystals (to over 3 cm in length) and the occurrence of many complete articulated crystal-filled clams make this occurrence unique. In all crystal-bearing specimens, a portion of the sides (and sometimes tops) of the clam shells have been dissolved by ground water. This creates “natural windows” into each clam, allowing the internal crystallization to more easily be seen. The largest crystal clusters grow downward from the hinge area (top) of the clams. When found broken away from the rest of the fossil clam, these hinge pieces make outstanding thumbnail to small cabinet-sized crystal specimens.

    The calcite from this site fluoresces a light yellow-green color under shortwave and longwave UV light, and also phosphoresces for a second or two once the black light is extinguished.

    This site produces other calcified pelecypods, including Busycon rucksorum (Whelk), Anadara (Ark shells), and others. Most calcite crystals within these mollusks are drusy, and lack the deeper color and transparency of the larger crystals found growing within Mercenaria permagna clams.

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