Precipitates were formed by titrating dilute, acidic solutions of
monomeric silicic acid [Si(OH)?] and copper (Cu) or zinc (Zn) to
various pH values. Those containing Cu formed above pH 6.0; those
containing Zn formed above pH 7.0. Periodically, during 2 years of
aging in their mother liquor, the liquid phase was sampled and
analyzed and the Cu/Si or Zn/Si molar ratios of the precipitates
calculated. The Cu/Si ratios of those containing Cu stabilized readily
near 0.78 and were unaffected by pH. The Zn/Si ratios of the Zn-containing
precipitates, however, changed as a result of Si enrichment
from near 1.80 soon after their formation to near 0.80 after aging 2
years. The systems with the lowest pH changed first and most rapidly
and became stabilized within 6 months; those above pH 8.0 changed
more slowly.
The Zn-containing precipitates gave diffuse X-ray diffraction
patterns indicative of a 2:1 layer silicate, whereas those containing Cu
were amorphous
