Microstructures and Crystal Size Distribution (CSD) of Chromites from Gysian Silvana Ophiolite Serpentinites, Urmia, Iran

One of the components of Gysian Silvana ophiolite in the south of Urmia and the continuation of the Neo-Tethys in the northwesthern of Iran is serpentinites with lizardite/chrysotile and magnetite phases as the main mineral and chromite (Cr-spinel) and some rare olivine and orthopyroxenes. A variety of microstructures such as mesh, sieve, lamellar, fibrous, and hourglass were identified. The existence of lizardite plates (001) crosswise in the margin of a low crystallized mass nucleus of net texture is determined by microstructures. Due to the lack of antigorite in the serpentinites, the depth of subduction of the Neo-Tethys slab in this part of Iran may be low. The crystal size distribution (CSD) of the Gysian serpentinite chromites was studied.  We discriminate three types of CSD patterns for chromite crystals, extract nucleation, and growth parameters from intercept and slope values of the patterns. The first type is a linear steady-state pattern with a nucleation rate/growth rate (J/G) of about 12 and Gt=1.28; the second type has a non-linear concave downtrend which is the result of annealing or Ostwald ripening at a small size part with J/G=13-15 and Gt=1.33. The third type is complex and shows nucleation density with two separate parts. It could be modeled by two crystal populations, small and large sizes overlapping linear patterns. The J/G for small crystals was twice for large sizes and multiple nuclei have less growth. The Gt was 1.60 for small and 1.10 for those large sizes. Assuming the constant crystal growth rate in silicate materials is 10-9 cm/s, the time for the growth of large chromites is six times longer than that of small crystals. These two segmental CSD patterns probably present arising processes or alteration production of mafic minerals as small chromites.