| Opal-A is the main component of siliceous sinters in many terrestrial systems, and it commonly preserves silicified microorganisms. Opal-A from the Taupo Volcanic Zone (North Island, New Zealand) yielded reflectance (wavelengths between 0.5-25 mum) and absorbance spectra (wavelengths between 2.5-14 mum) consistent with the presence of isolated and hydrogen-bound water and silanol. Two classes (1 and 2/2A) reflectance spectra were detected between the wavelengths of 6-13 mum. The Class-1 spectrum is similar to existing opal-A and opal-CT spectra, and it was collected from well-consolidated samples that had low porosity. The Class-2/2A spectrum is unique when compared to the opal-A and opal-CT spectra, and it was collected from poorly consolidated samples that had high porosity. The Class-2/2A spectrum was also collected from opal-A crushed to a grain size of < 74 mum. Since fine grains dominate planetary surfaces, the Class-2/2A spectrum may be useful in the detection of hydrothermal systems on planetary surfaces. |
