ON THE ORIGIN OF 'MICRITIC' LAYERS IN SPELEOTHEMS: NOT OBVIOUS AND NOT SIMPLE

Speleothems are ubiquitously laminated at a variety of scales from <1 µm to centimeters. A minor, but significant, component in many speleothems are lamina that look like micrite in thin section. Micrite is calcite crystals that are <4 µm. It appears dark and opaque in thin section, because the tiny crystals stack many layers thick in the 30 µm thickness of the thin section. However, other materials can appear dark and opaque in thin section, requiring careful examination of any “micritic” layers at a finer scale.

Cave pearls are coated grains that form in caves and mines. Micritic layers are common in cave pearls and can be shown to have a variety of origins. In Carlsbad Cavern, NM, micritic layers include 1) <4 µm calcite, i.e. true micrite, 2) aragonite needle layers with 0.1 x 5 µm crystals, 3) detrital clays, likely a trioctrahedral smectite, often with biofilm and/or microbial reticulated filaments, and 4) authigenic clays, likely sepiolite, with biofilm and aragonite needles. Micritic layers in pearls from an underground limestone mine in Quincy, IL, include 1) <1 to 5 µm primary calcite, and 2) 5-10 µm secondary (recrystallized) calcite. Other workers have noted detrital (washed in) organic matter and autochthonous microbial communities in speleothems.

Thus, “micritic” layers in speleothems have a variety of possible origins including true micrite (primary or secondary), equivalent sized aragonite (and possibly other carbonates), detrital clays and/or organics washed in from outside, and even authigenic clays and/or cave community organics. In thin section these all have the same dark, opaque appearance. Additional closer examination (SEM, XRD, EPMA, etc.) is required to determine the origin.