Twenty nine samples of silica sinter, ranging in age from modern to Miocene
, record temporal changes in both mineralogy and texture. When first deposi
ted, sinters consist largely of noncrystalline spheres (<1-8 mu m diameter!
of opal-A exhibiting varying degrees of close-packing. Particle densities
range from 1.5 to 2.1 g cm(-3), total water 4-10 wt%, and porosities 35-60%
. Changes over similar to 10,000 years following deposition are slight alth
ough the spheres ma!: be invested by an additional film of secondary silica
. For the next 10,000 to similar to 50,000 years, the silica incrementally
crystallises to become poorly crystalline opal-CT and/or opal-C; spherical
particles of thin-bladed crystals (lepispheres) replace opal-A particles an
d coalesce in microbotryoidal aggregates (similar to 10-30 mu m diameter).
Amygdaloidal fibrous clusters occur with lepispheres. As silica lattice ord
ering becomes enhanced, total water content drops to <7 wt%, particle densi
ty increases to similar to 2.3 g cm(-3), and porosity reduces to <30%. The
change from opal-A to opal-C takes place over a briefer periods (similar to
50 years) in silica sinters that contain other materials (e.g. calcite, su
lfur, alunite, plant remains). Sinters older than similar to 50,000 years h
ave recrystallised to microcrystalline quartz. With the onset of quartz cry
stallisation at similar to 20,000 years, total water is <0.2 wt%, particle
density approximates quartz 12.65 g cm(-3)) and porosity is <4%. The progre
ssive changes in silica species and texture yield ageing profiles for sinte
rs that may serve as guides to the paleohydrology of geothermal systems and
/or epithermal ore deposits in areas where surface thermal activity has dec
lined or ceased.