MULTIPLE MECHANISMS TO RUTILATE QUARTZ

Rutilated quartz, a variety of quartz (SiO₂) containing acicular inclusions of rutile (TiO₂), is widely appreciated for its gemological beauty. Having been valued as a gemstone since antiquity, there has been considerable speculation about its origin, but there does not exist a fundamental understanding of the mechanisms responsible for its formation. With advances over the last decade towards determining the solubility and diffusivity of Ti in quartz, as well as developments in understanding the origin of rutile needle inclusions in other minerals (e.g., garnet, corundum), it is now possible to consider the thermodynamic and kinetic driving forces for forming rutilated quartz. From analysis of natural and experimental samples, we identify three distinct processes that result in the formation of rutilated quartz. Each process differs in the relative timing of quartz and rutile growth, the incorporation process for including rutile needles, and the thermodynamic and kinetic driving forces for formation. Experiments were designed to induce each mechanism under controlled laboratory conditions to evaluate their relative driving forces and to determine criteria for their discrimination in nature.