I-type granites are a category of granites originating from igneous sources, first proposed by Chappell and White (1974). They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics that indicate, for example, magma hybridization in the deep crust. 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.
Petrographic characteristics
Primary minerals
Minerals that crystallized from the silicate melt are considered primary minerals. They are grouped into "Major", "Minor", and "Accessory" minerals based upon their modal percentages in the rock.
Major mineralogy
Primary minerals in I-type granites are plagioclase, potassium feldspar, and quartz as in S- and A-type granites. I-type granites have less quartz then their S-type granite color index equivalents. Plagioclase displays zonation and albite twinning. Potassium feldspar can show perthite textures, carlsbad twinning, and, in microcline, tartan twinning. Quartz and potassium feldspar scarcely show granophyric textures.
Minor minerals
Biotite is the most common minor mineral in I-type granites. The biotites in I-type granites are greener in general than those in S-type, both in hand sample and in plane polarized light. More mafic composition granites, those with a higher color index, contain more hornblende and biotite. Hornblende is a typical I-type granite mineral which never occurs in S-type granite. Hornblende crystals can be twinned and compositionally zoned.
Accessory minerals
Zircon and apatite can occur in both I- and S-type granites, whereas titanite (sphene) and allanite are considered diagnostic accessory minerals for I-type granites. Allanite is typically surrounded by radial fractures, caused by the subsolidus increase in volume of allanite as a result of metamict alteration due to radioactive decay. While apatite inclusions are common, they are not as abundant or large as those in S-Type granites. Primary muscovite can occur in weakly peraluminous fractionated I-type granites. Therefore, the presence of muscovite alone is not diagnostic of S-type granites.
Subsolidus and alteration minerals
Minerals that form in the rock as a result of chemical reactions that take place between primary minerals and hydrothermal fluids are classified as subsolidus minerals. They form below the temperature and pressure conditions of the solidus in the absence of a silicate melt. Other alteration minerals may form at surface conditions from interaction of the minerals present in the rock with groundwater and the atmosphere.
Alteration of biotites can produce fluorite, chlorite, and iron oxides such as magnetite and ilmenite. Sericitic alteration is seen within feldspars. In more evolved I-Type granites, calcite occurs as a late stage and/or a subsolidus mineral. Fluorite, like calcite, is rare and where observed it is associated with the more evolved I-type granites. It can form as a late stage product of crystallization. It is commonly observed as part of the subsolidus alteration of biotite along with chlorite and opaque oxides. Muscovite occurs as an alteration of feldspars and biotite. Epidote can be found, especially on the edges of allanite.
Color index
Color index, or the modal abundance of minerals other than quartz, plagioclase and alkali feldspar (e.g., mafic silicates, oxides, sulfides, phosphates, etc.), can be used to infer the maturity of a granite. Juvenile I-type granites have a higher color index. Amphibole, biotite, sphene, allanite, and oxides are typically more abundant. In contrast, more evolved (i.e. fractionated) I-type granites have a lower color index, and may contain minerals such as muscovite that are indicative of their fractionated nature.
Textures
I-type granites can have variable textures. I-type granites, like other granite types, can vary in crystal size from aphanitic to phaneritic; crystal size distributions include porphyritic, seriate, and rarely equigranular textures. Like other granites, phenocrysts in I-type granites are commonly feldspars, but can also be hornblende. Amphibole is a diagnostic feature on the hand sample scale between S-type and I-type granites.