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

Tightly constrained determinative methods can be used to characterise the silica minerals (opal-A, opal-CT, opal-C, quartz, moganite) and physical properties of silica sinters. Optimal X-ray powder di€raction operating parameters indicate silica lattice order/disorder using untreated, dry, <106 mm powders scanned at 0.68 2y/min with a step size of 0.018 from 10±408 2y and an internal Si standard. Simultaneous di€erential thermal and thermogravimetric analysis of 15.020.1 mg sinter samples of <106 mm grain size, at a heating rate of 208C/min 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. Thermogravimetric-determined water contents of sinters di€er from Pen®eld determinations re¯ecting the di€ering nature of the two techniques. Laser Raman microprobe techniques can be used to explore the mineralogy of particular sinter morphologies and habits down to 10 mm 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 0375-6505/00/$20.00 # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0 37 5 -6 50 5 (9 9) 00 0 54 -1 Geothermics 29 (2000) 65±81 * Corresponding author. Tel.: +64-9-373-7599; fax: +64-9-373-7436. E-mail address: thermal@auckland.ac.nz (P.R.L. Browne). understanding of the hydrology and paleohydrology of a geothermal prospect. # 2000 CNR. Published by Elsevier Science Ltd. All rights reserved.