4.3 Fluorite crystals on top of scheelite and calcite Many spectacular specimens of cubic fluorite crystals like this one are found in museums. They are interpenetrating deformed cubes. The photo in Figure 4.3 shows light purple cubic fluorite crystals. The same atoms are encountered in any direction through the mineral and, consequently, mineral properties are the same all directions. Isotropic minerals have very symmetrical atomic arrangements and atoms are arranged in an overall cubic pattern. For example, this ball and stick model (Figure 4.2) shows the atomic arrangement in fluorite (CaF 2).įluorite is one of a small number of common minerals that are isotropic. This means that they are solids with an orderly repetitive atomic arrangement.

Mineral crystals may be twinned, containing domains with slightly different atomic orientations.Ĥ.1 Crystalline and Noncrystalline Solids 4.2 The arrangement of atoms in fluorite (CaF 2)īy definition (with just a few special exceptions) minerals must be crystalline.

Minerals may have defects involving misplaced or missing atoms.

Minerals may not form or be stable under all conditions.

Some minerals form during weathering or diagenesis.Metamorphic minerals form by solid-state reactions during metamorphism.Hydrothermal minerals precipitate from warm flowing waters.Aqueous minerals precipitate from water they include compounds of high solubility.Igneous minerals precipitate from a magma most of them are silicates.Crystals grow from small seeds and sometimes become very large.Crystalline substances have an orderly and repetitive atomic arrangement.With just a few exceptions, all minerals are crystalline.4.1 Light blue aquamarine (beryl), black tourmaline with light colored orthoclase.

Provably rare gems