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The vast majority of Earth's crust is composed of a small subset of minerals called rock-forming minerals, dominated by silicates—minerals built around silicon-oxygen tetrahedra (SiO₄). The silicate framework structure (isolated, chain, sheet, and framework silicates) controls a mineral's melting temperature and resistance to weathering. Feldspars, quartz, micas, pyroxenes, amphiboles, and olivine together constitute over 90% of crustal rocks. Understanding which minerals form under which pressure-temperature conditions is the foundation for interpreting rock history.
Learning to identify the major rock-forming minerals by their diagnostic properties (cleavage angles for feldspars vs. pyroxenes, lack of cleavage in quartz) is more durable than memorizing chemical formulas. Classifying minerals as felsic (quartz, feldspar) vs. mafic (olivine, pyroxene) provides a quick framework for predicting rock composition.
From your understanding of mineral crystal structure — how atoms arrange themselves in repeating three-dimensional patterns held together by ionic and covalent bonds — you are ready to focus on the specific minerals that make up nearly all of Earth's crust. Despite thousands of known mineral species, fewer than a dozen rock-forming minerals account for over 90% of crustal rocks. They are almost all silicates, built around the same fundamental unit: the silicon-oxygen tetrahedron (SiO₄⁴⁻), in which one silicon atom sits at the center of four oxygen atoms arranged at the corners of a tetrahedron. The way these tetrahedra connect to each other — or don't — creates the major silicate structural classes and determines each mineral's physical properties.
In isolated (island) silicates like olivine, individual tetrahedra are not bonded to each other; they are linked instead through metal cations (Mg²⁺, Fe²⁺) between them. This produces a compact, dense structure with no cleavage planes — olivine fractures rather than splitting along flat surfaces. In single-chain silicates (pyroxenes), tetrahedra share oxygen atoms to form continuous chains, producing two cleavage planes at roughly 90°. Double-chain silicates (amphiboles like hornblende) link pairs of chains side by side, yielding two cleavage planes at about 60° and 120°. Sheet silicates (micas, clay minerals) share three of four oxygens to form continuous flat sheets, producing the perfect single-plane cleavage that lets you peel mica into paper-thin flakes. Finally, framework silicates (quartz, feldspars) share all four oxygens between adjacent tetrahedra, creating a fully three-dimensional network. Quartz, being pure SiO₂ with every oxygen shared, has no weak planes and therefore no cleavage — it fractures conchoidally like glass.
The practical classification that matters most in field geology divides these minerals into felsic and mafic groups. Felsic minerals — quartz, potassium feldspar (orthoclase), sodium-rich plagioclase, and muscovite mica — are light-colored, relatively low-density (~2.6–2.7 g/cm³), and silica-rich. They dominate continental crust and granitic rocks. Mafic minerals — olivine, pyroxene, amphibole, and biotite mica — are dark-colored, denser (~3.0–3.5 g/cm³), and rich in iron and magnesium. They dominate oceanic crust and basaltic rocks. This felsic-mafic spectrum is not arbitrary; it maps directly onto melting temperature (mafic minerals crystallize at higher temperatures), weathering resistance (quartz is nearly indestructible at the surface while olivine weathers rapidly), and tectonic setting (mafic rocks form at mid-ocean ridges, felsic rocks concentrate at convergent margins).
Knowing the rock-forming minerals gives you a decoder ring for reading Earth's history. When you see a rock made of olivine and calcium-rich plagioclase, you know it formed from high-temperature, silica-poor magma — probably from the upper mantle. A rock dominated by quartz and potassium feldspar formed from cooler, silica-rich magma typical of continental settings. A sandstone made entirely of quartz grains tells you the sediment was intensely weathered, because every less-resistant mineral was destroyed during transport — only quartz survived. Each mineral's presence, absence, or relative abundance constrains the conditions under which the rock formed and the journey it has taken since.