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Mineral and Gem Characteristics Help

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By — McGraw-Hill Professional
Updated on Apr 25, 2014

Crystalline Structure

Most minerals can be found in crystalline form. Whether they are rounded, have shear faces, or have no set form, most minerals have specific internal geometric structures. Sometimes the structures are the same between different minerals, but their chemical compositions are different.

As individual as people, what goes on inside a mineral determines its physical and optical properties, shape, hardness, cleavage, fracture lines, specific gravity, refractive index, and optical axes. The regularly occurring arrangement of minerals, atoms, and molecules in space determines its form.

The lattice structure of a mineral is based on its arrangement of atoms, ions, and molecules within an individual sample.

There are four different types of bonding that occur in crystalline solids. These determine what type of solid it is. The four types of crystalline solids are molecular , metallic , ionic , and covalent.

Bonding

These types of crystalline solids have molecules at the corners of the lattice instead of individual ions. They are softer, less reactive, have weaker non-polar ion attractions, and lower melting points.

A molecular solid is held together by intermolecular forces. The bonding of hydrogen and oxygen in frozen water shows how hydrogen forms bonds between different water molecules.

Another type of crystalline solid is made up of metals. All metals, except mercury, are solid at room temperature. The temperature needed to break the bonds between positive metal ions in specific lattice positions, like iron disulfide (FeS 2 ), and the electrons around them is fairly high. This strong bonding gives stable molecules flexibility. It allows metals to be formed into sheets (malleable) and be pulled into strands (ductile) without breaking.

A metallic solid like silver is held together by a positively charged “central core” of atoms surrounded by a general pool of negatively charged electrons. This is known as metallic bonding . This arrangement of (+) ions and electrons (–) make metals good conductors of electricity.

Ionic solids form a lattice with the outside positions filled by ions instead of larger molecules. These are the “opposites attract” solids. The contrasting forces give these hard, ionic solids (like magnetite and malachite ) high-melting points and cause them to be brittle. Hardness is not the same as brittleness . Brittleness, a measure of mineral strength, is dependent on a mineral’s overall structure. Think of it like building a house without the proper internal supports. Brittle minerals fracture easily. Figure 9-2 shows the way crystalline solids can be arranged.

Minerals and Gems Bonding

 

Fig. 9-2. Crystalline solids have different configurations.

Ionic bonding in a solid occurs when anions (–) and cations (+) are held together by the electrical pull of opposite charges. This electrical magnetism is found in a lot of salts like potassium chloride (KCl), calcium chloride (CaCl), and zinc sulfide (ZnS). Ionic crystals, which contain ions of two or more elements, form three-dimensional crystal structures held together by the strong ionic bonds. Figure 9-3 illustrates the cubic arrangement of table salt (halite, NaCl).

 

Minerals and Gems Bonding

Fig. 9-3. Sodium chloride has a cubic crystalline structure.

Covalent bonding holds hard solids together. Assembled together in large nets or chains, covalent multilayered solids are extremely hard and stable in this type of configuration. Diamond atoms use this type of structure when arranged into three-dimensional solids. One carbon atom is covalently bonded to four other carbons. This strong crystalline structure makes diamond the hardest known organic solid.

Covalent crystals are all held together by single covalent bonds. This type of stable bonding produces high melting and boiling points.

Allotropes are different structural forms of the same element. Graphite, diamond, and buckminsterfullerene are all allotropes of carbon.

The different bonding and forms of carbon in a diamond (pyramid shaped), graphite (flat-layered sheets), or buckminsterfullerene (C 60 and C 70 , shaped like a soccer ball) illustrate the variety and stability of covalent molecules. Nets, chains, and balls of carbon bonded into stable molecules make these solids hard and stable.

Minerals also have well-studied properties, such as color, hardness, crystalline structure, specific gravity, luster (shine or luminescence), cleavage, and tensile strength (resistance to being pulled apart). Many of these properties can vary slightly within a single mineral. Some minerals have very specialized properties like fluorescence and radioactivity.

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