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Percussion Marks on Quartz Grains
Donald H. Campbell
Preliminary investigation shows that percussion marked quartz grains are common in sediments and rocks of many areas. The percussion mark is a crescentric crack on the surface of a particle; the conoid of percussion is commonly produced by sand grain impact in the eolian environment and by collision of gravel in an aqueous environment. In thin-section the percussion marks appear as gently to steeply curving, inwardly projecting cracks in the periphery of the grain. Iron oxide in the crack sharply defines the mark. From the small number of grains counted in this study the percussion marks are systematically oriented on approximately 37 percent of the marked grains and on 31 percent the marks are oriented parallel to the long dimension of the grain. Percussion marks on 14 percent of the marked grains are oriented parallel to the "c" crystallographic axis; 63 percent of the marked grains show marks that are randomly scattered on the surface of the grain. Oriented percussion marks may be the result of aerodynamic orientation of the grain whose shape may be effects of original shape, differential hardness, and/or preferred fracture. The presence of percussion marks on a particle of sand-size may be used to indicate an eolian environment but because the marks are very durable other evidence such as ventifacts and dune cross-bedding should be combined in the interpretation.
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