Pyroxene — identify the correct characteristic from the list below. Select the single best statement.

Introduction / Context:
Pyroxenes are a major rock-forming group of single-chain inosilicate minerals found in igneous and metamorphic rocks. The question probes mineralogical properties to identify the single correct attribute among distractors with subtle errors.

Given Data / Assumptions:

• We evaluate crystal habit, alteration behavior, density, and hardness.
• Only one statement is fully correct as written.
• Units and mineral names must be accurate.

Concept / Approach:
Typical pyroxene properties include two cleavages at ~90 degrees, prismatic habit, hardness around 5–6, and density generally near 3.2–3.6 g/cm³. Alteration may produce amphibole, chlorite, or serpentine, but not “chlorine.” Wording and units matter.

Step-by-Step Solution:

Verification / Alternative check:
Standard mineralogy references list pyroxene hardness approximately 5 to 6 and specific gravity roughly 3.2–3.6, confirming our evaluation.

Why Other Options Are Wrong:

• Octagonal crystals: Mischaracterizes the common prismatic habit and cleavage.
• Converts to chlorine: Confuses chlorite with chlorine; incorrect process and product.
• Density 2.3–3.6 g/cm³: Lower bound too low and original units often misstated.
• All of the above: Fails because multiple statements are incorrect.

Common Pitfalls:
Overlooking unit accuracy and mistaking chlorite (mineral) for chlorine (elemental gas).

Final Answer:
Hardness on Mohs scale is 5 to 6