PETROGRAPHIC EXAMINATIONS AS A TOOL IN AGGREGATE EXPLORATION

Crushed stone (aggregate) is a multi-billion dollar industry comprising 1,550 companies operating 4,091 quarries and underground mines in the U.S. More than 80% of concrete and 90% of hot mix asphalt is composed of aggregate. New sources of aggregate must be found as mineable reserves become exhausted and as new markets develop. Aggregate also must meet increasingly demanding specifications set by state departments of transportation, the U.S. Army Corps of Engineers, Federal Aviation Administration, and others. Specifications include limitations on specific gravity and absorption, sulfate soundness and LA-abrasion loss, content of deleterious materials, and identification of possible alkali-aggregate reactions in concrete.

Exploring for a potential greenfield site is expensive. It requires drilling closely spaced core holes and testing to determine if rock meets specifications, which can cost hundreds of thousands of dollars. If the stone does not meet specifications then the money spent on drilling and testing is lost.

Petrographic examinations used in the initial stages of exploration provide a low-cost alternative. Petrographic samples may be obtained from outcrops or from rock chips produced by percussion drilling, which is much cheaper than core drilling. Petrography can be used to determine types and amounts of deleterious materials, identify potentially alkali-aggregate reactive components, determine if environmentally unfriendly minerals such as asbestos or asbestiforms are present, and to estimate the specific gravity and absorption of the rock. Grain boundary relationships, mineral cleavage, grain size, and alterations that form deleterious products (e.g., saussurite, epidote, etc.), may correlate to how the aggregate responds to testing. Potential savings for aggregate companies is considerable but requires a petrographer with the required experience, training, and proper equipment to conduct these examinations.