[1] They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics that indicate, for example, magma hybridization in the deep crust.
[2] I-type granites are saturated in silica but undersaturated in aluminum; petrographic features are representative of the chemical composition of the initial magma.
In contrast S-type granites are derived from partial melting of supracrustal or "sedimentary" source rocks.
The biotites in I-type granites are greener in general than those in S-type, both in hand sample and in plane polarized light.
In more evolved I-Type granites, calcite occurs as a late stage and/or a subsolidus mineral.
Color index, or the modal abundance of minerals other than quartz, plagioclase and alkali feldspar (e.g., mafic silicates, oxides, sulfides, phosphates, etc.
Amphibole is a diagnostic feature on the hand sample scale between S-type and I-type granites.
This is expressed mineralogically by the presence of amphibole and accessory minerals such as sphene and allanite in the metaluminous I-type granites.
Note that weakly peraluminous fractionated I-type granites may crystallize primary muscovite and rare spessartine-rich garnet.
This interpretation was made by Chappell and White in their 1974 paper based on their observations in the Lachlan Fold belt of southeastern Australia.
The I-S line is interpreted to be the location of a paleo-structure in the subsurface that separated the generation zones of the two different melts.
This interpretation comes from the plotting of different element concentrations against the level of evolution of the granite, usually as percent silica or its magnesium to iron ratio.
Granites traced to the same source region can often have very variable mineralogy; color index for example can vary greatly within the same batholith.
More recent studies have shown that the source regions of I-type and S-type magmas cannot be homogeneously igneous or sedimentary, respectively.