Obtaining Quartz Concentrates by Low-Temperature Ashing of Coal

Leslie F. Ruppert, Ronald W. Stanton

Pure concentrates of quartz were obtained from upper Freeport coal from west-central Pennsylvania to determine grain size and shape. Although quartz concentrates from coal can be removed by low-temperature plasma ashing (LTA) followed by nitric and acetic acid leaching, the sheet silicate minerals tend to adhere to and obscure the surface of the quartz grains. The adherence is caused by static charges that are not easily eliminated.

Using a sodium pyrosulfate fusion technique to destroy the octahedral sheets within the clays, we produced quartz concentrates from the LTA of the coal. These concentrates were collected on a silver membrane filter and photographed stereoscopically by use of an SEM. Quartz grains from the concentrates range in size from 1 × 1 to 34 × 30 µm and have a mean maximum dimension of 7.8 µm. In contrast, quartz grains in polished blocks from the same coal range from 2 × 3 to 27 × 20 µm and average 9.8 µm. Small (<2 µm) quartz grains are common in the concentrates but are rarely observed in the polished blocks. Grain shapes for the concentrates range from angular to subrounded; some euhedral grain shapes, surface etching, and quartz overgrowths were noted. Grain shapes for the polished blocks ranged from subangular to subrounded.

Concentrates of quartz obtained by the sodium pyrosulfate fusion procedure can allow for more reliable size and shape measurements than do polished sections in which only exposed cross sections of quartz grains can be observed. In addition, pure concentrates of quartz from coal and carbonaceous shale can be obtained for isotopic analysis.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.