Numerous common amphiboles can be represented within the Mg 7 Si 8 O 22 (OH) 2 (anthophyllite), Fe 7 Si 8 O 22 (OH) 2 (grunerite) and Ca 7 Si 8 O 22 (OH) 2 (hypothetical pure calcium amphibole) compositional field (Fig.1). This diagram is commonly referred to as the amphibole quadrilateral.

Fig.1: Ternary Composition Diagram for Pyroxenes. Complete substitution exists between enstatite (Mg 2 Si 2 O 6 ) and ferrosilite (Fe 2 Si 2 O 6 ), and complete solid solution of iron for magnesium exists between diopside (CaMgSi 2 O 6 ) and hedenbergite (CaFeSi 2 O 6 ).

Peridotites can also form as cumulates in layered intrusions, peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Fig.1: Ternary system Ca 2 SiO 4 - Mg 2 SiO 4 - Fe 2 SiO 4 for olivine minerals. The name forsterite is restricted to those species with no more than 10 percent iron substituting for magnesium; fayalite (from Fayal Island in the Azores) is restricted to species with no more than 10 percent magnesium substituting for iron.

Amphibolite is a gneissose or granofelsic metamorphic rock mainly consisting of green, brown or black amphibole and plagioclase (including albite), which combined form ≥75% of the rock and both of which are present as major constituents; the amphibole constitutes ≥50% of the total mafic constituents and is present in an amount of ≥30%.

This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.

Gabbro may also contain small amounts of olivine (olivine gabbro) amphibole (hornblende gabbro) and biotite. The quartz content in gabbro is less than 5% of total volume, but quartz gabbros or monzogabbros are also known to occur, and are probably derived from magma that was oversaturated with silica.

Corona texture Corona texture: several concentric layers of one or more minerals completely encircling an older phase. The layers (which range from one to five in number) represent a sequence of reactions that have taken place (none to completion) to replace the mineral in the core or center of the corona.

• Alteration: Commonly alter to uralite (a light coloured amphibole), or may alter to serpentine, chlorite, biotite, carbonates and /or other silicates. • Relief: High positive. • Distinguishing Features: Inclined extinction, higher birefringence, 2nd order interference colours, low 2V, optically positive in comparison with opx.

This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.