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Open-File Report 2006–1366

Open-File Report 2006-1366

Mineralogical and Chemical Composition of Saprolite from South Fork Brokenback Run Watershed, Shenandoah National Park, Virginia

By Joy Pochatila, Blair F. Jones, Kathryn M. Conko, and Daniel M. Webster


This study of mineral assemblages within the weathering profile at South Fork Brokenback Run, Shenandoah National Park, Virginia, extends previous research that focused on mass-balance calculations employing idealized formulation for the dissolution of primary mineral reactants in the granitic bedrock and for resulting mineral products in the overlying saprolite. From two cores of the granite weathering profile (regolith) obtained in an earlier investigation, mineralogical and chemical analyses of five physically separated fractions of the saprolite were obtained for discrete depth increments. X-ray diffraction (XRD) analyses identified quartz, potassium feldspar, biotite, amphibole, plagioclase, illite, kaolinite, and goethite, in addition to a 2:1 expandable clay, considered an aluminous smectite. CLAYFORM, a computer program for calculating structural formulae of clay minerals from elemental chemical analyses, was used to characterize the composition of the finest fractions containing the most smectite, which appeared to range from K0.4(Si3.0Al1.0)4.0(Al2.0Mg0.3)2.33O10(OH)2toCa0.3(Si3.0Al1.0)4.0(Al2.0Mg0.2)2.33O10(OH)2. This clay was taken to be an important intermediate in the weathering of the granite.





Saprolite Sampling

Mineral Separation Technique

X-ray Diffraction Procedures

Chemical Analysis

Estimating Mineral Abundance

Use of CLAYFORM to develop a clay-mineral Formula

Mineralogical and Chemical Composition of Saprolite


Chemical Analysis





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