Routine instrumental procedures to characterise the mineralogy of modern and ancient silica sinters

Tightly constrained determinative methods can be used to characterise the s ilica minerals (opal-A, opal-CT, opal-C, quartz, moganite) and physical pro perties of silica sinters. Optimal X-ray powder diffraction operating param eters indicate silica lattice order/disorder using untreated. dry, < 106 mu m powders scanned at 0.6 degrees 2 theta/min with a step size of 0.01 degr ees from 10-40 degrees 2 theta and an internal Si standard. Simultaneous di fferential thermal and thermogravimetric analysis of 15.0 +/- 0.1 mg sinter samples of < 106 mu m grain size, at a heating rate of 2 theta degrees C/m in in dry air, identify thermal events associated with dehydration, organic combustion, and changes of state. Where abundant organic matter is present , nitrogen is the preferred atmosphere for thermal analysis. Thermogravimet ric-determined water contents of sinters differ from Penfield determination s reflecting the differing nature of the two techniques. Laser Raman microp robe techniques can be used to explore the mineralogy of particular sinter morphologies and habits down to 10 mu m diameter. The nature of the silica species present can assist in characterising individual sinter deposits and , combined with textural, density and/or porosity determinations, can lead to a better understanding of the hydrology and paleohydrology of a geotherm al prospect. (C) 2000 CNR. Published by Elsevier Science Ltd. All rights re served.