Figure 8.01. Crystal Settling Movie. This video shows sylvite crystals growing in a concentrated H₂O–KCl solution that was boiling at 110°C, poured into a rock-chip lined glass box made of 26-mm-wide microscope slides glued together, and allowed to cool to room temperature (20°C). As the brine cools, sylvite crystals nucleate homogeneously at first, then heterogeneously on the surface, sides, and other crystals. Many crystals sink, although some are carried upward by thermal convection currents. Sylvite crystals that nucleate on the upper brine surface are held there by surface tension, growing as a crust until it is large enough to break free and sink. The movie is a collection of five-second clips beginning at 5, 20, 45, 80, 125, 180, 245, 320, and 405 seconds after the solution was poured into the box. The movie and description are modified from Brady (2009).

This movie may help you think about magma fractionation by crystal setttling. However, because the viscosity of the brine (~6x10^-4 Pa^.s) is much lower than the viscosity of the basalt magma (~2x10¹ Pa^.s), crystal settling in a basalt magma chamber will be much slower and require larger crystals. Furthermore, the density contrast of sylvite and brine (1.99/1.16 = 1.72) is considerably higher than the density contrast (3.44/2.76 = 1.25) of Fo₈₅ olivine (3.44 g/cm³) and dry basalt magma at 0.5 GPa and 1200°C (~2.76 g/cm³), which will also reduce settling rates in magmas relative to this movie.