The effect of second‐phase particles on stable grain size in regionally metamorphosed polyphase calcite marbles

Abstract

Samples of the calcite‐rich Shelburne Marble collected at the Pfizer Quarry in Adams, Massachusetts, show an order of magnitude variation in grain size. Calcite grain size ranges from 94 to 1101 μm. Because these calcite marbles share the same pressure, temperature and strain histories, some other factor must be responsible for the grain size variation.Grain size appears to be controlled by the concentration of impurity or second‐phase particles. Large calcite grain size occurs where the volume fraction of second‐phase particles is low and grain size decreases as second‐phase volume fraction increases. The relationship between calcite grain size (D), second‐phase grain size (d ) and second‐phase volume fraction ( f ) can be described by the power law D/d=1.4/f 0.36, a result that is consistent with models based upon short‐term (hours or days) laboratory experiments with metals and ceramics and computer simulations of grain growth. Grain growth appears to be greatly restricted by as little as a few per cent of second‐phase particles, with a transition from highly restricted to almost unrestricted grain growth occurring at ≈5% volume of second‐phase particles. These results indicate that second‐phase particles exercise an important control on grain size and can effectively inhibit grain growth in metamorphic rocks. The behaviour of second‐phases in short‐term laboratory experiments may closely approximate the behaviour of second‐phases in grain growth lasting several orders of magnitude longer in the metamorphic environment.