Mineral

Imagine a world where every rock you touch tells a story. Each mineral, with its unique chemical composition and crystal structure, is like a tiny piece of Earth’s history waiting to be discovered.

The Basics of Minerals

A mineral is more than just a pretty stone; it’s the building block of our planet. Defined by the International Mineralogical Association (IMA), minerals are solid substances with well-defined chemical compositions and crystal structures that form naturally in pure form. Unlike rocks, which can be composed of one or multiple types of minerals, each mineral has its own distinct identity.

Classification and Diversity

The world of minerals is vast and diverse. They are classified by their key chemical constituents using systems like the Dana classification and the Strunz classification. Silicate minerals make up approximately 90% of the Earth’s crust, with other important groups including native elements, sulfides, oxides, halides, carbonates, and sulfates.

Controversies in Mineral Classification

A topic of contention among geologists and mineralogists is whether biogenic crystalline substances should be included as minerals. While the IMA defines a substance as a distinct mineral if it meets certain criteria, some argue that all solids are potential minerals, including those created by biological processes.

Formation and Abundance

The abundance and diversity of minerals are controlled directly by their chemistry, which in turn is dependent on elemental abundances in the Earth. Eight elements account for most of the key components: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium.

Chemical Substitutions and Coordination Polyhedra

The chemical composition of minerals can vary between end member species of a solid solution series. For example, plagioclase feldspars range from sodium-rich albite to calcium-rich anorthite. These substitutions occur due to the balance of charges, with coordination polyhedra representing how cations are surrounded by anions.

Physical Properties and Identification

The physical properties used for classification include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, cleavage, fracture, specific gravity, fluorescence, magnetism, radioactivity, and more. These properties help in identifying minerals through tests like the Mohs scale of hardness or the CIPW norm.

Cleavage and Fracture

Cleavage is the breakage of a mineral along planes of weakness due to its atomic arrangement. Cleavage angles vary between minerals, with different structures producing distinct angles. Parting, on the other hand, is similar but caused by structural defects rather than systematic weakness.

Fracture and Tenacity

Minerals may exhibit good cleavage, poor cleavage, parting, or various fracture types, influenced by their crystal structure, composition, and physical properties. Specific gravity describes the density of a mineral, with high specific gravities being diagnostic for certain minerals.

The Rock Cycle and Mineral Reactions

Mineralogy is closely related to the rock cycle, with examples of mineral reactions illustrated throughout geological processes. For instance, orthoclase feldspar can transform into kaolinite or pyrophyllite under specific conditions.

Classification Systems for Minerals

Classification systems like the Dana and Strunz provide a framework for understanding minerals based on their chemical and physical properties. The largest grouping is silicates, which comprise over 90% of the Earth’s crust, with subclasses including orthosilicates, disilicates, inosilicates, phyllosilicates, and framework silicates.

Non-Silicate Minerals

Non-silicate minerals are subdivided into several classes by their dominant chemistry. Native elements, sulfides, halides, oxides, hydroxides, carbonates, nitrates, borates, sulfates, phosphates, and organic compounds make up this diverse group.

The Future of Mineralogy

As our understanding of minerals evolves, so do the classification schemes. Astrobiology suggests that biominerals could be crucial indicators of extraterrestrial life, making them a key focus in the search for habitability on Mars and beyond.

So next time you pick up a rock, remember: it’s not just a rock; it’s a story waiting to be told. Each mineral is a piece of Earth’s history, a testament to the dynamic processes that shape our planet.