Inosilicates (Chain Silicates)

Scientific Definition

The term “Inos-” is derived from the Greek word for “thread” or “fiber.” In inosilicates, silica tetrahedra ($[SiO_4]^{4-}$) share oxygen atoms to form elongated chains. This structural arrangement is categorized into two primary types:

• Single Chains: $Si:O$ ratio of $1:3$ (Pyroxene Group).


• Double Chains: $Si:O$ ratio of $4:11$ (Amphibole Group).

Crystallographic Structure

These chains align parallel to one another and are bonded together by metallic cations (e.g., $Ca, Mg, Fe, Na$). While the bonds within the chains are strong, the bonds between the chains are relatively weaker. This differential bond strength creates the distinct prismatic cleavage characteristic of this mineral group.

Key Distinction: Pyroxene vs. Amphibole

• Pyroxenes: Characterized by single chains. They exhibit cleavage angles of approximately 90° (specifically 87° and 93°).


• Amphiboles: Characterized by double chains and the presence of hydroxyl ions ($OH^-$) in their structure. They exhibit distinct cleavage angles of approximately 60° and 120°. This cleavage difference is a primary diagnostic tool for geologists in the field.

Physical & Chemical Properties

• Hardness: Moderate, typically 5 to 6 on the Mohs scale.


• Crystal Habit: Frequently prismatic, columnar, or acicular (needle-like).


• Color: Highly variable, ranging from dark green and black to brown, primarily due to their diverse chemical composition (especially iron and magnesium content).

Key Minerals

• Pyroxene Group: Includes minerals such as augite and enstatite; these are fundamental components of mafic rocks like basalt and gabbro.


• Amphibole Group: Includes minerals such as hornblende and actinolite; these are critical components in many metamorphic and intermediate igneous rocks.


• Note: Certain acicular amphiboles (specifically certain varieties of asbestos) also possess this double-chain structure.

Geological & Economic Significance

• Geothermometry: The composition of pyroxenes serves as a vital indicator of crystallization temperatures within igneous rocks.


• Environmental Indicators: The presence of amphiboles (due to their $OH^-$ content) signifies hydrated formation environments, whereas pyroxenes typically form in drier conditions.


• Economic Geology: Certain varieties, such as jadeite, have been among the most prized gemstones throughout human history.