1.3: Elements, Minerals, and Rocks

Last updated
Save as PDF

Page ID: 17525

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

1.28 Elements, minerals, and rocks

This figure (1.28) shows the relationships between elements (bottom), minerals (center), and rocks (top). Elements, singly or in combination, make up minerals. For example, some of the most common elements in Earth’s crust make up the minerals quartz, alkali-feldspar, and biotite. Minerals, singly or in combination, make up rocks. For example, subequal amounts of quartz and alkali-feldspar, sometimes with biotite and plagioclase, make up granite, a common crustal igneous rock (triangular diagram at the top of the figure).

All minerals, like all materials, consist of one or more elements, the building blocks of all matter. Some minerals, diamond for example, contain a single element (carbon). Others contain many elements. Some minerals have compositions that vary little in nature. Quartz for example is always close to 100% silicon and oxygen in the atomic ratio 1:2. Other minerals incorporate elemental substitutions, so their compositions may vary a great deal from sample to sample. Biotite, for example, always contains potassium, magnesium, iron, aluminum, silicon, and oxygen. It generally also contains lesser amounts of manganese, sodium, titanium, and many other elements, so natural biotite compositions are quite variable.

Rocks are aggregates of one or more minerals, mineraloids, and organic components. Rocks may form when minerals grow (crystallize) together, forming a crystalline rock, such as the granite shown above. They also can form when loose grains are cemented together, forming a clastic rock, such as sandstone. Crystalline rocks may form from a magma (e.g., granite), may form by metamorphism (e.g., gneiss), or may form by precipitation from water (e.g., gypsum). Most clastic rocks form from consolidated sediments, but some form by volcanic processes.

1.29 Dunite

Some rocks contain only one kind of mineral. Limestone (rock), for example, is often pure calcite (mineral). Anorthosite (rock) is made mostly or entirely of plagioclase (mineral). Quartzite (rock) may be made only of quartz (mineral). Dunite (Figure 1.29), an igneous rock that crystallizes from magma, is often nearly 100% olivine (mineral). The dunite shown in the photo also contains a few dark grains of chromite (mineral).

Limestone, anorthosite, quartzite, dunite, and most other rocks contain minerals. Some, less common rocks, however contain virtually no minerals. Examples include as pumice, which is almost entirely volcanic glass, and coal, which is mostly organic materials.