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The most common kind of glass, which is used to make bottles or window panes, is soda-lime-silica glass. Glass manufacturers make glass by melting ingredients and cooling the melt so quickly that crystals cannot form.

Box 4-1 Making Glass from Minerals 4.6 Pouring molten glass.

This is one reason why manufactured glass makes a better window than transparent minerals. The process produces a noncrystalline isotropic glass, so light can pass through it equally well in all directions. For example, window glass is made by melting mixtures containing mostly quartz sand, and allowing the melt to solidify quickly so atoms cannot arrange themselves in a crystalline structure. This gives the materials properties that are sometimes very useful. The patches consist of cristobalite, a variety of SiO 2 that is a polymorph of quartz.Īmorphous solids are generally isotropic, so they have the same properties in all directions. Obsidian is noncrystalline, but over time it will sometimes begin to crystallize, creating the white patches seen here. It contains black volcanic glass (obsidian) with white patches. This photo (Figure 4.5) shows a specimen of snowflake obsidian. Natural volcanic glass, obsidian, is an example.

Some mineral-like substances are noncrystalline, also described as amorphous, which means they have a random atomic structure. 4.5 “Snowflake” obsidian (volcanic glass) Olivine, topaz, some pyroxenes, and topaz all sometimes have crystal shapes similar to barite’s. Still, when compared with many minerals, barite crystals are quite symmetrical. The crystals have lots of symmetry, but it is different from the symmetry of fluorite. The shape, termed orthorhombic, is similar to the shape of a shoebox.īarite is an anisotropic mineral. For example, the blue-gray barite (BaSO 4) crystals seen in this photo are tabular and have a long, an intermediate, and a short dimension. But, it may not be cubic symmetry like fluorite – other shapes are possible. So, well-formed mineral crystals often exhibit symmetry. Yet, crystals may possess other symmetry, even if they do not resemble a cube. Consequently, their crystals are not as symmetrical as fluorite’s. This means that they have different properties in different directions. Still, their crystals, when euhedral, are equant and generally have many identical faces. All these minerals, have a cubic arrangement of atoms, although their crystals may not be cubes. Other isotropic minerals, besides fluorite, include diamond (C), almandine (Fe 3Al 2Si 3O 12), gold (Au), pyrite (FeS), silver (Ag), spinel (MgAl 2O 4), and sodalite (Na 3Al 3Si 3O 12 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.