MINERALOGY OF ZELOITES FROM THE KEYSTONE BLUE QUARRY, ELBERTON BATHOLITH, NORTHEAST GEORGIA

Homogeneous fine-grained biotite granite of the Elberton Batholith (EB) is widely quarried in northeastern Georgia. The rock is undeformed and unaltered and is considered a post-tectonic pluton. In the Keystone Blue quarry, dikes of pegmatitic Elberton granite and diabase cut the fine-grained granite. Fractures within the granite are covered with various mineral assemblages including fluorite-chlorite-quartz, pyrite-quartz, and calcite-zeolites.

The mineralogy of the Keystone Blue zeolites was investigated by students and faculty in the Earth Materials class at UGA during the fall of 2007. The zeolites are fine-grained (individual crystals 0.1 to 1.0 mm), but well crystallized and are easily visible with a hand lens. Several crystal shapes were present on the fractures: cubic zeolite varied from colorless to orange, a radiating zeolite was composed of colorless to yellow crystals. Rhombohedral colorless to white crystals of calcite coated the zeolites and formed a crust that completely covered the zeolites forming veins in some fractures.

Samples were hand-picked from slabs of granite that exposed the zeolites. Several samples of veins were thin sectioned. Grain mounts were prepared and polished for electron microprobe analyses. Additional sample was ground by hand for X-ray diffraction and optical experiments.

The cubic zeolite is, in fact, rhombohedral with a rhombohedral cleavage and parallel extinction. Grains are uniaxial with a very low birefringence and a refractive index between 1.49 and 1.50. The radiating zeolite is biaxial with moderate birefringence, extinction parallel to the elongation, and is length fast. Refractive index is between 1.49 and 1.51.

The three strongest X-ray diffraction lines were: rhombohedral- 9.29, 4.32, and 2.92 Angstroms and radiating- 9.09, 3.34, and 4.05 Angstroms. Analysis of the XRD patterns showed the rhombohedral zeolite to be chabazite and the radiating zeolite to be stilbite. Optical characteristics are consistent with these identifications.

Electron microprobe analyses showed both zeolites to be calcic with 9 to 10 wt. % CaO. The chabazite is homogeneous and contains 1-2 wt. % K2O and 0.2-03 wt. % Na2O. Stilbite is heterogeneous, grains contain either 1-2 wt. % Na2O or K2O. Potassic varieties contain slightly elevated Sr (SrO = 04. - 0.7 wt. %) relative to the sodic grains (SrO = 0.2 - 0.3 wt. %).