into subalkaline, alkaline, and peralkaline volcanic rocks. Subalkaline rocks are defined as rocks in which SiO2 < -3.3539 × 10−4 × A6 + 1.2030 × 10−2 × A5 - 1.5188 × 10−1 × A4 + 8.6096 × 10−1 × A3 - 2.1111 × A2 + 3.9492 × A + 39.0 where both silica and total alkali oxide content (A) are expressed as molar fraction. Because the TAS diagram uses weight fraction and the boundary between alkaline and subalkaline rock is defined in terms of molar fraction, the position of this curve on the TAS diagram is only approximate. Peralkaline volcanic rocks are defined as rocks having Na2O + K2O > Al2O3, so that some of the alkali oxides must be present as aegirine or sodic amphibole rather than feldspar. The chemistry of volcanic rocks is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most magmas tends to increase the silica (SiO2) content, mainly by crystal fractionation. The initial composition of most magmas is basaltic, albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are the tholeiitic, calc-alkaline, and alkaline. Mineralogy Most volcanic rocks share a number of common minerals. Differentiation of volcanic rocks tends to increase the silica (SiO2) content mainly by fractional crystallization. Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as phyllo and tectosilicates including the feldspars, quartz polymorphs and muscovite. While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as olivine and the pyroxenes. Bowen's reaction series correctly predicts the order of formation of the most common minerals in volcanic rocks. Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called xenocrysts. Diamonds found in kimberlites are rare but well-known xenocrysts; the kimberlites do not create the diamonds, but pick them up and transport them to the surface of the Earth. Naming Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte, and latite. Pyroclastic rocks are the product of explosive