A CATHODOLUMINESCENE STUDY OF MICROCLINE FROM THE MOREFIELD PEGMATITE, VIRGINIA

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 sometime displays dark gray luminescence. This feature is very prominent in amazonite samples, and occurs infrequently within tan microcline. Microcline exhibiting the gray CL color contains lower Na₂O contents (0.3-0.4 wt. %) compared to the microcline with blue or blue-green luminescence (Na₂O=0.6-0.8 wt. %).

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.