Decrepitation studies of pegmatites
Pegmatites are often important sources of rare
elements or rare-earth elements and form an important class of
mineral deposits. However it has been difficult to understand how
these enigmatic rocks form and why they often contain gigantic
(metres across) perfect single crystals of many minerals. Many
models have been proposed to explain their formation.
good overview of pegmatite crystallization processes and models
by David London and George B Morgan is here.
(published in Elements magazine, August 2012 V8 #4)
Many, but not all, pegmatites formed from fluids
with substantial contents of CO2. In this
comparison plot, samples from Bynoe and Tanco have particularly
intense low temperature CO2 decrepitation, while
samples from Greenbushes and the massif central in France have no
Pegmatites also contain large feldspar crystals
which should preserve fluid inclusions. However, the feldspars
often show no decrepitation at all, and when they do show
decrepitation it does not usually correlate with co-existing
quartz analyses. Without further study, feldspars are a
problematical host mineral for fluid inclusion and decrepitation
Other less common minerals in the pegmatites can also show decrepitation. However, without a large suite of samples of the same mineral it is not possible to interpret the decrepitation plots. In the following plot, note that the pollucite result has been divided by 5 to fit this scale.
Samples of pegmatites have been collected from 6
different provinces and have been analysed by acoustic
decrepitation to try and understand the fluid environment in which
these deposits formed. Results from these provinces are compiled
and discussed in the following links.