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samples have undergone exchange reaction with external
fluids at subsolidus
temperatures or formed from a pre-existing mineral such as leucite. According to
Karlsson and Clayton (1991) the whole rock
18
O value (7.6 %) is signi
cantly
higher than those of basanites and leucite basanites, and the high N content (6 ppm)
and low
δ
15
N value imply interaction with water either during magma genesis,
transport or post-extrusion. They observed that the glass matrix in the Colima
minette is also unusually H
2
O-rich (4 wt%) suggesting post-eruption, glass-
δ
uid
interaction. On the basis of above observation, Karlsson and Clayton (1991) con-
cluded that the analcites from Crowsnest and Colima are of secondary origin.
An experimental study of the replacement of leucite by analcime was made by
Putnis et al. (2007). Leucite and analcime have open framework aluminosilicate
structures, where ion exchange by cation substitution has been previously used to
explain the replacement of one phase by another. Using O
18
-enriched NaCl solu-
tions in a container, they made hydrothermal reactions and analysed the run
products using a scanning electron microscope, infrared and Raman spectrometer.
They also employed a time-of-flight secondary ion mass spectrometer, involving
cation exchange by volume diffusion. They showed that the replacement of leucite
by analcime takes place by solid-state reaction involving cation exchange by vol-
ume diffusion. Textural features such as nano-pores and cluster, as well as the
detection of high amounts of O
18
in the framework are exchanged and a new
analcime structure forms at the moving interface through the parent leucite crystal.
The characteristic high porosity (on a nano-scale) in the product leucite phase,
results in diffusion and some of the parent phase is lost to the solution to give a
volume de
cit reaction. However, pseudo-morphic replacement of an open
framework alumino-silicate structure takes place by a coupled dissolution and
reprecipitation mechanism.
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