The chemistry of tin and lead archaeological artefacts.
The present work explores the long term corrosion
phenomena of lead and tin in saline and other conditions.
Thermodynamic results are presented for the minerals
abhurite, blixite, mendipite, paralaurionite, stannite and
the compounds Pb706Cl2.2H20 and Sn30(OH)2S04 at 298.2K and P
= 105 Pa.
At this temperature laurionite is stable, rather than
its dimorph, paralaurionite, and mendipite is in fact
metastable, being thermodynamically stable above 29 0 C.
Kinetic influences are significant with respect to the
sequence of formation of solid phases in the PbO - H20 - HCl
system, and these have been elucidated for some important
reactions. Penfieldite and fiedlerite appear to be
metastable at all temperatures at 105 Pa. These results have
been used to develop a model for the formation of lead(II)
oxy- and hydroxy- chloride phases that are known as minerals
and as corrosion products of lead-containing artefacts. The
effect of C02 on the system is also discussed.
A new synthesis of sn30(OH)2S04 is described, and its
stability constant in aqueous solution has been determined
by direct ~easurement of the activities of Sn2+ and s04 2-
using ion selective electrodes. The true formula·of abhurite
is Sn2l06(OH)14Cl16' which corresponds to a synthetic phase
of known crystal structure. The results have been used to
assess the relative stabilities of these compounds in the
natural environment and their modes of occurrence in
relation to other secondary Sn(II) and Sn(IV) species.
Finally, some divalent metal hexahydroxystannates have
been made, and their role in the corrosion of tin and its
alloys assessed. The electrochemical oxidation of stannite
has been studied and some correlation has been made between
the supergene minerals formed and solution chemistry.