V. GEOCHEMICAL FACTORS OF THE 
NATURAL PROCESS 
Summarized below are some of the more important 
parameters inferred for the geochemistry of the process of 
wood silicification, as it is postulated to proceed during natural 
sedimentary processes. 
Agent 
The most probable agent of petrifaction is molecular silicic 
acid — either the common monomer or some low molecular 
weight polymer thereof, in true solution. Colloidal silica sus- 
pensions and aqueous solutions of ionic silicate are improbable 
natural petrifactants for the reasons already noted in the previ- 
ous section. 
Site 
During petrifaction, the wood is probably buried in sedi- 
ment, in a topographical depression or some structural setting 
which allows water saturation of the sediment and water- 
logging of the wood. 
The cover of sediment protects the wood against rapid de- 
cay. Also, the sediment serves as an immediate and abundant 
source of available silica. Most commonly, this entombing 
sediment is volcanic ash (Murata, 1940). Petrified woods often 
occur in tuffs — fine-grained, compacted, atmospherically or 
water deposited sediments derived from volcanic ash. Vol- 
canic ash weathers rapidly, liberating large amounts of soluble 
silica (see Hunt, 1972). Less commonly, silicified woods occur 
in sediments with no history of volcanism. This is true for a 
number of pre-Tertiary occurrences of petrified woods. It is 
also true for two acid sulfate soils of Recent age, in Thailand 
and in the Netherlands (Buurman, 1973). In these situations, 
the petrifactant is released in silicate mineral diagenesis. A 
number of the phyllosilicate minerals, when transforming from 
one type to another, with a lower silicic to alumina ratio — such 
as in the case of montmorillonite diagenesis to kaolinite, liber- 
ate silica (Siever, 1962). 
It does not appear that siliceous thermal springs, whether 
acidic or alkaline, have been major sites of wood petrifaction in 
the past. The beginning stages of the silicification process, 
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