Abstract

A specimen of flowstone, three cm thick, contains an important variety of carbonates. Ordinary length-fast calcite forms large, clear columnar crystals. Length-slow calcite forms bundles of fibers, some in bands resembling coconut meat, and also zoned crystals; these crystals appear to be the first type of calcite formed after halts in crystallization. Areas thought to have formerly been fibrous aragonite are now converted to calcite; they are recognized by ghost square-ended rays 0.2-0.7 mm wide and 2-4 mm long, now replaced by a mosaic of equant calcite spar crystals 0.05-0.2 mm across. Some fibrous crystals remaining within this spar probably represent surviving original aragonite. Some solution also took place in this zone, as well as inversion of aragonite to calcite. Inversion probably took place when the bathing solutions had the proper intermediate pH to just dissolve aragonite and precipitate calcite, the ions "hopping across" the liquid film from one lattice to the other. Solution took place at lower pH, where ions left the system. Examples of similar square-ended rays once aragonite but now converted to sparry calcite, are shown from the Triassic of the Italian Alps, thus these fabrics can be fossilized. Presumably all aragonite rays were precipitated by waters with a Mg/Ca ratio greater than 2.

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