The NSS Bulletin - ISSN 1090-6924
Volume 46 Number 1: 5-9 - April 1984
A publication of the National Speleological Society

A Spathite Occurrence in Virginia: Observations and a Hypothesis for Genesis
David A. Hubbard, Jr., Janet S. Herman, and Richard S. Mitchell

Abstract

Spathites, associated with chiefly aragonitic soda straws, have been found in a new locality, the third for these speleothems. They occur in Roberts Caves, developed in the Shady Dolomite (Lower Cambrian), in Smyth County, Virginia. Microscopy and X-ray diffration have shown spathite mineralogy to be complex. The outer shell of the spathite form originates as aragonite, but some spathites have altered to calcite. Calcite and argonite forms were identified for both spathites and soda straws. Observations of these speleothems show that the water drip rate from mainly calcite specimens greatly exceeds that from aragonite specimens. Water samples were subsequently found to be supersaturated with respect to calcite, aragonite, and dolomite. Lower calcium to magnesium rations were found in water smaples from aragonite specimens, but calcite specimens contain more magnesium than aragonite specimens.

Spathite development proceeds from an initial aragonite form not unlike a soda straw. The aragonite shell flairs as it grows until the termination reaches a critical size too wide for a drop of water to extend across. Water dripping from one side results in the development of an opening or "window" into the formation as the next stage is initiated. The admission of cave air into the window is hypothesized to result in a microenvironmental change and subsequent development of a calcite partition. This partition isolaties the window from the internal "plumbing" of the speleothem. Sequential development of several stages leads to complex spathites. External encrustation may eventually obscure windows such that multistage specimens appear to be knobby oversized soda straws.

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