Igneous ore deposits can also form when magma migrates away from newly formed minerals, as opposed to the minerals settling out of the melt. This process is believed to occur later in the cooling process when the remaining magma resides between the mineral grains-similar to how water exists in the pore space of sediment. Pressure changes can then force this residual magma to leave the magma chamber where it is then injected into fractures within the surrounding rock. As illustrated in Figure 12.6, the magma then cools and forms elongated bodies of igneous rocks. These small intrusive bodies sometimes contain valuable ore minerals because the crystallization process leaves the residual magma enriched with certain elements. One type of mineral deposit that forms in this manner is unusually coarse-grained deposits of silica-rich rock called pegmatites. These pegmatites are similar to granite in composition, but will sometimes contain concentrations of minerals composed of rare elements (e.g., lithium, beryllium, niobium, and tantalum), which are normally found dispersed throughout an igneous rock body. Some pegmatites are mined solely for their exceptionally large grains of quartz and feldspar.