Mineral replacement in bone

On behalf of Dr. Clare Ellis, Amanda Clydesdale
<mandyc [at] aocscot__co__uk> writes

>    A medieval site in Edinburgh has produced animal bone from all
>    levels from 14th to 18th century that has had the calcium
>    phosphate partially replaced with crystalline quartz. How did
>    this happen? The pH of the soils hovers around neutral. Is it
>    post-depositional or diet? No stone cleaning has taken place on
>    site. The site after about 1800s was occupied by a coal shed for
>    the gas works.

Silica mineralisation of bone or wood is most commonly known as
petrification. Water with dissolved minerals infiltrates the
interstices and pores of the material and crystallisation occurs
within the structure. Bone can be mineralized in a number of ways.
When the original interstices and pore space is infilled with
minerals, this is known as permineralisation. This is often confused
with petrification which is the replacement of the original bone
material with minerals. Permineralisation is by far the more
commonly observed preservation, and almost invariably where
petrification has occurred, permineralisation will have occurred
earlier.

Experimental evidence has shown silica mineralisation leading to
permineralisation or petrification will occur within the pH range
and chemical compositional range of most surface waters. Immersion
experiments (in dilute solutions of silica or ethyl silicate)
demonstrate petrification of wood could occur in as few as several
years with suitable conditions. A related reference, although not
specifically on bone, but which still may be useful is "Organic
geochemistry of silicified wood, Petrified Forest National Park,
Arizona" from the September 1978 issue of Geochimica et Cosmochimica
Acta (vol. 42, pp. 1397-1405).

Deborah Lau
Analytical and Conservation Scientist
CSIRO
Material  Environment Interactions
Building Construction and Engineering.
PO Box 56
Graham Road
Highett Victoria
Australia 3190
+61 3 9252 6403
Fax: 61 3 9252 6253


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                  Conservation DistList Instance 15:59
               Distributed: Wednesday, February 27, 2002
                       Message Id: cdl-15-59-003
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