2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 210-74
Presentation Time: 9:00 AM-6:30 PM


WEST, Kristen T. and GOSNELL, William T., Geosciences, University of West Georgia, Carrollton, GA 30118, kwest8@my.westga.edu

Sentinel Rock is an impressive granite/pegmatitic intrusion of the Pikes Peak Batholith near Colorado Springs, Colorado. Sentinel Rock is one of two impressive granite/pegmatite exposures visible from Colorado Springs. The main objective of our research is to compare and observe the numerous varieties of feldspar present at and below Sentinel Rock.

On June 11, 2015 students from the University of West Georgia on an experiential learning fieldtrip to Wyoming, Utah, and Colorado visited Sentinel Rock, Colorado. They hiked to the base of Sentinel Rock and collected samples of pegmatite below Sentinel Rock. Collected were several types and variations of feldspar, micas, smoky quartz, and hematite. Fluorite and zircon have been identified in thin sections of the rocks.

Research techniques include collection of hand sample and field note observations. Using UWG labs, samples are further examined with thin section analysis, scanning electron microscope (SEM), and X-ray powder diffraction (XRD) analysis. Much of the research is concentrated on samples of different variations of feldspar including alkali feldspars. These samples were collected 150 yards below the base of Sentinel Rock; GPS coordinates (N 38°48.702’ W 104°54.973’). The most interesting include microcline, quartz, and mica. Samples collected show the transition from graphic granite to open pocket with fine crystals of amazonite, smoky quartz, mica, goethite, and some minerals coated with hematite, which cause a slight distortion of crystal faces and striations. Colors of the microcline crystals range from light pink to beige. Several of the microcline samples collected demonstrate upward growth with variation in direction, best determined through observation of striations. During the crystallization process in certain cavities, an overabundance of lead (Pb) in the chemical make up of microcline resulted in an alteration, forming the amazonite. The resulting alteration of the crystal chemistry produced a vibrant blue-green hue in many crystals while other varieties exhibit a slightly more dull color.