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Cathodoluminescence
Study of Feldspars from the Black Mountain Pegmatite
Abstract
The
Black Mountain pegmatite, located in Oxford county, Maine, is an internally
zoned granitic pegmatite that contains many replacement features that
are caused by late-stage fluids. The reaction of the residual fluids
with other minerals can be observed using the cathodoluminescence (CL).
Albite commonly replaces pre-existing minerals, and occurs in three
varieties in the pegmatite: saccharoidal albite, cleavelandite, and
blocky albite.
With
the exception of blocky albite, each variety replaces other minerals
(potassium feldspar, quartz, muscovite, lepidolite, and spodumene) in
all zones of the pegmatite. The principal feature that is observed in
the albite is the reduction of luminescence wherever replacement occurs.
The exact reason for this is still not known, but there are several
possibilities which can be considered: 1) the incorporation of a quenching
agent in the albite during the replacement process, 2) the reduction
of activators (Ti4+, Mn2+. Fe2+, Fe3+) in albite during the breakdown
of replaced minerals, and 3) an increase in the amount of activating
elements leading to concentration quenching in the albite.
Black Mountain Pegmatite
The
Black Mountain pegmatite is located in Oxford County Maine, near the
city of Rumford. It is an internally zoned, granitic pegmatite that
contains a variety of alteration and replacement features that are only
visible using cathodoluminescence.
Cleavelandite
is the most common variety of albite present when alteration and replacement
occurs.
Zoning and Replacement
-
Primary
zoning occurs when the equilibrium between a mineral grain and the
melt is disrupted.
-
Secondary
zoning of a mineral is caused by its interaction with late-stage fluids.
-
Replacement
occurs when two coexisting minerals are not stable and one mineral
replaces the other.
-
Many
of the textures and features of replacement and zoning can not be
viewed by any other imaging methods. They are visible under CL.
-
Replacement
and zoning are important because they show the effects of late stage
fluid interaction that occurred during the crystallization history
of the pegmatite.
-
Every
zone and unit in Black Mountain displays replacement.
Conclusions
Black
Mountain experienced complex mineral replacement evidenced by extensive
fluid penetration of mineral grains and by mineral replacement and alteration.
All
of the replacement that occurs in the Black Mountain pegmatite involves
albite.
The
decrease in luminescence of the albite during the replacement process
may be due to the acquisition of trace elements. These elements may
either quench the luminescence, remove the activator, or cause concentration
quenching.
Further
studies are needed regarding the concentration quenching of feldspars
and their quenching elements.
Acknowledgements
I would
like to express my sincerest gratitude to the National Science Foundation,
the Department of Mineral Sciences, Cathe Brown,Tim Rose, and Mary Sangrey
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The
information presented here represents preliminary research as
the result of ten-weeks of investigation in-residence at the
National Museum of Natural History. This is not an official
publication of the information.
As preliminary information, results and/or findings should not
be cited as part of conclusive work. Please contact the authors
first if you wish to utilize the information presented here.
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