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| Highlights |
Amie
Garcia
Research Training Program, 2004
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ABSTRACT The Morefield pegmatite of the Amelia pegmatite district, Virginia is a mineralogically complex, internally zoned, rare-element granitic pegmatite enriched in Be, Nb, Ta, and F. Microcline is a major component of the primary zones and varies in color from tan to green. Tan-colored microcline occurs in the 1st intermediate zone, and green-colored microcline (amazonite) develops in the 2nd and 3rd intermediate zones and the core. Bi-colored microcline crystals with tan and green regions occur locally in the intermediate zones. The green color of amazonite increases in intensity from the 2nd intermediate zone to the core. Cathodoluminescence
(CL) was used to study the textural relationships between tan-colored
microcline and amazonite from the pegmatite. Amazonite samples show
a blue-green color in CL whereas tan-colored microcline displays a blue
color. Within bi-colored microcline samples, both CL colors are visible.
Microcline immediately adjacent to albite lamellae is sometimes non-luminescent.
Microcline exhibiting the non-luminescent areas contain significantly
lower concentrations of Na2O, Ti4+ and Fe2+ than the blue-green luminescent
areas. Textural evidence obtained from the CL images confirms that amazonite
from the Morefield pegmatite developed as a result of a post-magmatic
replacement process. Aqueous fluids that were introduced along fractures
and cleavage planes of the original primary tan-colored microcline,
aided in the transformation to amazonite.
In CL, tan- and green-colored microcline reveal different shades of blue luminescence. The albite lamellae appears as black or dark blue in CL and are typically oriented in the same direction within the microcline.
The non-luminescent microcline contains 0.3-0.4 wt % Na2O whereas blue or blue-green luminescent microcline has 0.6-0.8 wt %. Typically, this textures occurs surrounding the albite lamellae in the microcline.
This fluid either had Pb in it or activated Pb that was already present within the microcline.
The color of amazonite in CL shows blue-green luminescence whereas the tan-colored microcline is blue. This variation in color is key to understanding the transformation of tan-colored microcline to amazonite. The microcline textures seen under CL indicate that the formation of amazonite in the Morefield pegmatite is the result of a replacement process that involved the interaction of a water-rich fluid that migrated along fractures of microcline crystals and Pb ions that were likely incorporated into the microcline structure during crystallization of the pegmatite melt. Want to ask a question? Visit the Message Board Virtual
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Research
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Biology |
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INVERTEBRATE
ZOOLOGY "Intergeneric Hybridization between the sand dollars Encope michelini and Mellita isometra (Echinodermata: Echinoidea: Mellitidae)" |
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VERTEBRATE
ZOOLOGY "Morphological
adaptations for terrestrial habits of the Scimita-Billed Woddcreeper (Drymornis
bridgesii)." |
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VERTEBRATE
ZOOLOGY "Identity
of an atypical bear from Alaska: The first documented wild hybrid bear
(Ursus americanus X Ursus arctos)?" |
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from
Paleobiology |
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Mineral Sciences |
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Anthropology |
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