Opal-A, the main component of siliceous sinters in many terrestrial hydrothermal systems, is a hydrated silicate that commonly incorporates silicified microorganisms. The detection of opal-A on the surface of Mars, therefore, may carry important implications in the search for extraterrestrial life. Sinters from the discharge apron of Ohaaki Pool (North Island, New Zealand) yielded reflectance spectra (wavelengths between 0.5--25 ¿m) that indicate the presence of absorbed water, trapped water, and silanol in opal-A. Two classes (1 and 2) of reflectance spectra were detected between the wavelengths of 6--13 ¿m. The Class-1 spectrum is similar to existing opal spectra, and was collected from well-consolidated samples that had low porosity. The Class-2 spectrum is unique when compared to the spectra of other silica polymorphs, and was collected from poorly consolidated samples that had high porosity.