The NSS Bulletin - ISSN 1090-6924
Volume 43 Number 1: 20-26 - January 1981
A publication of the National Speleological Society

Reflectance Spectra and Color in Speleothems
William B. White

Abstract

Calcite in its pure form is colorless. White speleothems are rare; most are colored shades of yellow, tan, gray and brown. Occasionally, blood-red dripstone is found, and still more rarely, green and blue speleothems occur. Possible sources of speleothem color include: iron group ions substituting for Ca++ in the calcite structure; included clay, silt and metal oxides (mostly of iron and manganese) between the calcite crystals; hydrated iron oxide stains along grain boundaries; and organic stains incorporated into the growing crystal. Gascoyne has challened the long-held (but unsupported) view that the common yellow to brown colors are due to iron oxides and advances organic stains as the dominant colorant. This hypothesis is tested by the use of diffuse reflectance spectroscopy, which permits one to extend his "color vision" into the near-infrared.

Pure calcite exhibits a complex spectrum in the 1.4 to 2.5 µm region due to incorporated water either as water-filled voids within the calcite crystals or absorbed on grain boundaries. Characteristic d-electron spectra of the iron-group elements were found in a few cases: Ni++ from yellow Timpanogos Cave calcite, Cu++ from several Arizona caves and from the Grotte Bleue in France, and Fe++ in a vein calcite.

The spectra of hematite, geothite, and other iron oxide hydrates all contain a characteristic band in the 850 to 950 nm region. This band also appears in the spectra of iron oxide speleothems from Porter Cave, Pennsylvania and Butler Cave, Virginia, and form the hematite-stained boxwork formations from Wind Cave, South Dakota. The spectra of most yellow and brown speleothems are featureless in the 700 to 1400 nm region. The characteristic 900 nm band of the iron oxides is systematically absent. The conclusion is that the almost universal yellow to brown colors of most speleothems are not due to iron oxides. The spectra suggest that a single colorant is responsible and the leading possibility is humic substances extracted from the overlying soils.

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