less than the rate of crystal growth. In other words, molecules in the fluid from which the crystals are growing are much more likely to join an existing crystal than to come together to start a new crystal. This favors growth of a few very large crystals. In normal igneous rocks, coarse texture is a result of slow cooling deep underground. However, the extraordinarily coarse texture of pegmatites must be a result of more than just slow cooling. In fact, it is not clear if pegmatite forms by slow or rapid cooling. Pegmatites are widely believed to form from the last part of a magma body to crystallize. This final fluid fraction is highly enriched in volatile and trace elements. The residual magma undergoes phase separation into a melt phase and a hydrous fluid phase saturated with silica, alkalis, and other elements. Such phase separation requires formation from a wet magma, rich enough in water to saturate before more than two-thirds of the magma is crystallized. Otherwise, the separation of the fluid phase is difficult to explain. Granite requires a water content of 4 wt% at a pressure of 0.5 GPa, but only 1.5 wt% at 0.1 GPa, for phase separation to take place. The volatiles (primarily water, boron, fluorine, chlorine, and phosphorus) are concentrated in the hydrous phase, greatly lowering its viscosity. The silica in the hydrous phase is completely depolymerized, existing almost entirely as orthosilicate, with all oxygen bridges between silicon ions broken. The very low viscosity promotes rapid diffusion of molecules through the fluid, enhancing crystal growth rates and allowing very large crystals to grow. When this hydrous fluid is injected into the surrounding country rock, minerals crystallize from the outside in to form a zoned pegmatite, with different minerals predominating in concentric zones. A typical sequence of deposition begins with microcline and quartz, with minor schorl and garnet. This is followed by deposition of albite, lepidolite, gem tourmaline, beryl, spodumene, amblygonite, topaz, apatite, and fluorite, which may partially replace some of the minerals in the earlier zone. The center of the pegmatite may have cavities lined with spectacular gemstone crystals. Some pegmatites have more complex zoning. Five distinct zones are recognized in the Harding Pegmatite in the Picuris Mountains of northern New Mexico, US. These are: A white border rind of fine-grained quartz-albite muscovite pegmatite. A continuous layer of very coarse quartz, albite, and muscovite. This