Characterization of garnets in a Typic Cryoboralf (Gray Luvisol) from Saskatchewan, Canada

Garnets separated from the 250- to 2000 micrometer sand fractions of a Typic Cryoboralf (Orthic Gray Luvisol) and beach sands by a liquid magnetic separator, heavy liquid, and an optical microscope were used to study the composition, surface morphology, and distribution with regard to garnet weathering in a present day Boreal environment. Garnets constituted 0.2 to 0.3% of the 250- to 2000 micrometer sand fractions and 16 to 29% of the heavy mineral fractions in the profile. The sand garnet/total soil quartz ratios for soil horizons were relatively constant, ranging between 3 X 10-3 to 4 X 10-3 within the profile. There were no striking differences in weathering among the garnets within the profile or between the soil and lake garnets, suggesting very limited pedogenic weathering since the last deglaciation of the area. The pits that were attributed to dissolution were likely produced by ancient weathering. Despite the high Fe content, and evident surface weathering that had taken place before the last glaciation of the area, it is concluded that these garnets are highly resistant to chemical weathering in present Boreal soils. The association of diatoms with soil and lake garnets suggests that the parent material is of lacustrine origin. Light pink and dark pink garnets were the two varieties present. X-ray diffraction and chemical analyses revealed their composition was close to the end member almandine. Scanning electron microscopic studies showed extensive evidence of crescentic and several other morphological pits in all the samples. Different type of hillocks were also present, but they were less frequent than the pits. The type of pits and hillocks varied from grain to grain and also within individual grain surfaces. Limited occurrence of vein-like poorly crystalline tubules were observed in the primary fissures of some garnet grains.