The morphology and growth of garnet, (Fe (sub 0.74) Mg (sub 0.13) Mn (sub 0.09) Ca (sub 0.04) ) (sub 3) Al (sub 2) Si (sub 3) O (sub 12) , in Archean schist near Yellowknife, Northwest Territories, Canada

Compositionally zoned crystals of garnet, on average (Fe (sub 0.74) Mg (sub 0.13) Mn (sub 0.09) Ca (sub 0.04) ) (sub 3) Al (sub 2) Si (sub 3) O (sub 12) , occur locally in the cordierite zone of an Archean thermal dome in the southern Slave Province of the Canadian Shield, Northwest Territories. In one sample, crystals range in diameter from 0.19 to 1.7 mm and are bound by six-sided trapezohedral {211} faces and by smaller four-sided dodecahedral {110} faces. The 110 faces advanced more rapidly than 211 by a factor of 1.1. Topographic features revealed by an SEM examination of forty crystals are 1) irregular steps on 211 and 110, 2) a ridge-and-valley topography on 211 with ridges parallel to [111], 3) conical hills on 211 and 110, approximately 0.4 mu m high (determined by AFM), 4) rare ringed and simple mounds <0.4 mu m in diameter with long axes parallel to [111], and 5) circular and elongate depressions with diameters 1.0 to 0.1 mu m. The ridge-and-valley topography on 211 is viewed as a high density of alternating {110} planes [e.g., (101) and (110)]; the hills could be points of emerging screw dislocations (as found on some synthetic crystals of garnet), and the depressions are interpreted as vacated fluid inclusions. As the chlorite-->garnet reaction was in progress, a complex rearrangement of atoms occurred within a crystal-boundary phase, and for the given increase in temperature (to approximately 550 degrees C), the rate of garnet growth was determined by the placing of atoms on garnet surfaces, which presumably occurred by the Frank mechanism of the spreading of layers anchored at emerging screw dislocations. This mechanism is expected to produce a rate of increase of crystal volume proportional to surface area, as deduced earlier from the nature of compositional zoning in the crystals.