The degree of aluminum saturation of an igneous rock may be described by its Aluminum Saturation Index (ASI) defined as the molar ratio Al₂O₃(CaO + K₂O + Na₂O). One suggested origin for mildly peraluminous granites (ASI between 1 and about 1.1) is by fractional crystallization of subaluminous (ASI < 1) magmas; hornblende, having ASI < 0.5, could be a major driving force in such a fractionation process. The efficacy of the process depends not only on precipitation of hornblende and its effective removal from the reacting system, but on the composition and nature of other coprecipitating phases, weighted by their modal abundances in the reactive system. Precipitation of feldspar (ASI = 1), for instance, would retard or even prevent aluminum enrichment in the melt if the ASI of melt is < 1, but would enhance such evolution if the ASI of the melt is > 1. Discussion of the efficacy of any mineral must be made in the context of the total reacting system.

For hornblende to effectively cause a melt to evolve into a peraluminous composition, it must be able to coexist with peraluminous magmas. Experimental phase equilibrium data show that at pressure > 5 kb hornblende can coexist with strongly peraluminous melts (ASI ˜ 1.5). Scantily phyric volcanic rocks show that hornblende can coexist with granitic magma having ASI ˜ 1.1 –1.2. The aggregate ASI of last-stage minerals of a typical granite is less than this value; therefore, even after hornblende has reacted out, the residual magma may be expected to continue to evolve toward more aluminous compositions.