Rock-Forming Minerals – Source of Chlorite (Green, Micaceous Mineral) Chlorite, a common green phyllosilicate in low-grade metamorphic and altered igneous rocks, is mainly produced by the decomposition/alteration of which ferromagnesian minerals?

Introduction:
Chlorite is a diagnostic green mineral commonly seen in low-grade metamorphic terrains and hydrothermally altered igneous rocks. The question evaluates understanding of mineral alteration pathways of common mafic silicates into chlorite assemblages.

Given Data / Assumptions:

• Parent minerals considered: augite, hornblende, and biotite.
• Geologic processes: hydrothermal alteration, low-grade metamorphism, and weathering.
• Observation: production of secondary chlorite as a replacement mineral.

Concept / Approach:

Ferromagnesian (Fe–Mg) silicates are prone to alteration. Under suitable conditions of temperature, pressure, and fluid chemistry, pyroxenes, amphiboles, and micas can all retrogress to fine-grained chlorite, often with associated epidote, calcite, or sericite depending on bulk composition.

Step-by-Step Solution:

Verification / Alternative check:

Petrology handbooks document chloritization of pyroxenes, amphiboles, and biotite as widespread, producing characteristic green coloration in altered basalts and gneisses.

Why Other Options Are Wrong:

Limiting the source to any one mineral ignores the breadth of chloritization pathways. Orthoclase is a K-feldspar and not a typical precursor of chlorite.

Common Pitfalls:

Assuming only biotite alters to chlorite due to similar layer structures; overlooking chloritization of augite and hornblende in mafic rocks.

Final Answer:

All of the above ferromagnesian minerals can alter to chlorite