Analysis of crystal growth as a function of temperature for synthetic granite and granodiorite compositions in the system KAlSi3O8-NaAlSi3O8-CaAl2Si2O8-SiO2 under H2O-saturated and undersaturated conditions yields quantitative data on the growth kinetics of quartz, alkali-feldspar, and plagioclase in these systems at 8 kbar and 400-900°C. Measured growth rates vary from 3 × 10−6cm/sec (about 3 mm/day) to 1 × 10−10cm/sec (about 1 mm/yr), while nucleation density (nucleation sites/unit volume) varies from 0 to over 1 × 108sites/cm3. Crystal-growth rates commonly increase with increased undercooling. Maximum growth rates are lower in systems which contain a H2O-rich vapor phase than in systems that are undersaturated with respect to H2O.

Number of nuclei (nucleation density), growth rate, and morphology for each of the minerals determines the texture of an igneous rock. Results of this study show that long periods of time are not necessary to produce the textures commonly associated with igneous rocks. A relatively small number of large crystals may be produced by crystallization at a low undercooling in a few days. Volcanic (fine-grained) textures may be produced by crystallization at large undercoolings, which produces a large number of small nuclei (high nucleation density, low growth rate). Coincidence of low nucleation density and high growth rate for alkali feldspar may explain the large alkali-feldspar crystals (phenocrysts or interstitial masses) sometimes found in granodiorites.

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