THE OFTEN OVERLOOKED USE OF POLARIZED LIGHT MICROSCOPY IN THE MINING INDUSTRY

The use of the Polarized Light Microscope is a widely overlooked ‘art’ form which can be used as a powerful tool in the aggregate industry. A thorough and true understanding of the underlying principals as taught in Optical Crystallography is required to implement the effective use of the PLM. Knowledge and proficiency in working with grain mounts or the oil immersion technique in particular brings the ability to make quick and accurate mineral identifications possible. These determinations can be made in the office or field on a wide variety of sample types from settled dust to ledge rock or hand samples. Mineral identification can be done which represent a tremendous cost savings when it is necessary to rapidly assess asbestiform from non-asbestiform mineralogy in particular, thereby possibly reducing the requirement of numerous and more costly analytical methods. This rapid analysis allows multiple mineral site evaluations of aggregate operations to be completed in a timely manner. As a specific example, a particular shot at a quarry in North Carolina exposed a large (>4’) vein of an extremely white, “pointy”, and “fuzzy” mineral which also produced a dust cloud coating much of the immediate area. Workers were told to halt production until this mineral could be identified. One day passed before a sample could be sent to a lab, analyzed by PLM and results communicated back to plant personnel. The mineral was identified as Xonotlite Ca₆Si₆O₁₇(OH)₂. While certainly interesting for collectors, Xonotlite has distinctively lower refractive indices than any regulated amphibole asbestos. This could have been determined within 15 minutes if a PLM were used at the plant site. By use of these ‘traditional’ PLM techniques, aggregate companies can understand the risk or lack thereof with regard to potentially hazardous mineralogy in currently active operations or during evaluation of greenfield sites. Implementing the use of these fundamental PLM techniques have significant implications for the aggregate industry by allowing quick, accurate, and inexpensive evaluation of multiple sites with regard to regulated or otherwise hazardous mineralogy.