Illustrate with neat sketches internal structure of pyroxene group of minerals. Give a generalized classification of pyroxene group based on composition.

Internal Structure of Pyroxene Group Minerals

The fundamental building block of pyroxenes is the silicate tetrahedron (SiO₄). These tetrahedra link together to form chains. In pyroxenes, the chains are single chains, meaning each tetrahedron shares two oxygen atoms with adjacent tetrahedra. This chain structure is the defining characteristic of the inosilicate class.

Chain Structure and Coordination Polyhedra

The single-chain structure is illustrated below:

(Image source: Wikimedia Commons - Illustrative example, actual sketch should be hand-drawn for exam purposes)

Within the chain, cations occupy two distinct sites:

• M1 Site (X): This site is typically occupied by larger cations like Ca²⁺, Mg²⁺, and Fe²⁺. It is coordinated by six oxygen atoms, forming an octahedral coordination.
• M2 Site (Y): This site is occupied by smaller cations like Mg²⁺, Fe²⁺, Al³⁺, and Ti⁴⁺. It is also octahedrally coordinated, but the octahedron is slightly distorted.

The arrangement of these cations and the silicate chains gives pyroxenes their characteristic prismatic crystal habit.

Generalized Classification of Pyroxene Group Based on Composition

The pyroxene group is broadly classified based on the dominant cations present. Several end-member compositions are recognized, and natural pyroxenes typically represent solid solutions between these end-members.

Major End-Members and Solid Solution Series

The following table summarizes the major pyroxene end-members and the solid solution series they form:

End-Member	Chemical Formula	Dominant Cations	Solid Solution Series
Enstatite	MgSiO₃	Mg²⁺	Enstatite – Ferrosilite (MgSiO₃ – FeSiO₃)
Ferrosilite	FeSiO₃	Fe²⁺	Enstatite – Ferrosilite (MgSiO₃ – FeSiO₃)
Diopside	CaMgSi₂O₆	Ca²⁺, Mg²⁺	Diopside – Hedenbergite (CaMgSi₂O₆ – CaFeSi₂O₆)
Hedenbergite	CaFeSi₂O₆	Ca²⁺, Fe²⁺	Diopside – Hedenbergite (CaMgSi₂O₆ – CaFeSi₂O₆)
Augite	(Ca,Mg,Fe)₂(Si,Al)₂O₆	Ca²⁺, Mg²⁺, Fe²⁺, Al³⁺	Complex solid solution involving Diopside, Hedenbergite, and other components
Jadeite	NaAlSi₂O₆	Na⁺, Al³⁺	Jadeite – Albite (NaAlSi₂O₆ – NaAlSi₃O₈)
Aegirine	NaFeSi₂O₆	Na⁺, Fe²⁺	Aegirine – Omphacite (NaFeSi₂O₆ – NaMgFeSi₂O₆)

Augite is the most common pyroxene found in many igneous rocks. It represents a complex solid solution series, often containing significant amounts of iron, magnesium, and calcium. The presence of aluminum in the tetrahedral site is also common in augite.

The classification is further refined based on the relative proportions of these end-members, determined through chemical analysis (e.g., using electron microprobe analysis). The resulting compositions are often plotted on triangular diagrams to visualize the solid solution relationships.