On the formation and occurrence of saddle dolomite

Saddle dolomite is a variety of dolomite that has a warped crystal lattice; it is characterized by curved crystal faces and cleavage, and sweeping extinction. Perfect saddle forms have trigonal symmetry, with crystal elongation at high angles to the 'c' axis. Saddle dolomite occurs as both a void-filling cement and a replacement mineral and is commonly associated with hydrocarbons, epigenetic base-metal mineralization, and sulfate-rich carbonates. These associations imply late diagenetic formation by sulfate reduction processes. Saddle dolomite is slightly enriched in calcium and has significant variations in composition within individual growth laminae. Calcium is more abundant in the lattice at crystal apices and face edges that are at high angles to the 'c' axis, than towards face centers. These composition gradients along growth laminae cause the lattice distortion which has trigonal symmetry corresponding to the saddle morphology. The cause of selective ion adsorption during crystal growth is open to speculation but must be associated with the crystal as an entity. Surface-charge effects, the most probable cause, may be produced by either a pyro-electric phenomenon at elevated temperatures or pH and ionic concentrations of the precipitating fluids. Saddle dolomite has potential as a geothermometer, being indicative of elevated temperatures (60-150 degrees C).