Which of the following is not true for minerals?

Concept: A mineral is a naturally occurring, inorganic substance formed through geological processes; by definition it has a fixed (definite) chemical composition and, usually, a characteristic crystalline structure. This definition covers minerals broadly — it does not confine them to any single physical character such as being metallic or non-metallic.

Application: Checking each statement against this definition: minerals do form naturally, they do have a fixed chemical composition, and they do form across different geological environments (igneous, sedimentary, and metamorphic). Minerals are, however, classified into two broad groups — metallic minerals (such as hematite, galena, and chalcopyrite, which yield iron, lead, and copper) and non-metallic minerals (such as mica, gypsum, and quartz).

• Naturally occurring substances — true; every mineral forms through a natural geological process.

• Definite chemical composition — true; each mineral has a fixed formula, e.g., quartz is always SiO2.

• Different geological environments — true; minerals crystallize during igneous cooling, sedimentary deposition, and metamorphic transformation.

• All minerals are non-metallic — false; metallic minerals such as hematite and galena exist, so this blanket claim does not hold.

Result: Because metallic minerals clearly exist alongside non-metallic ones, the statement that all minerals are non-metallic is the one that is not true.