Crystals and metals are classified based on their chemical composition and placed into one of the main groups listed below.
Main Groups of Crystals and Metals
The Oxide Mineral Class
The oxide mineral class is made up of compounds with a structure containing an oxide anion (negatively charged atom) which is bonded with positively charged metal alloys. It is a diverse class of minerals that includes soft minerals such as psilomelane or hard minerals like corundum made from aluminum oxide.
It is due to the large abundance of oxygen in the Earth’s crust that makes the oxide mineral class so large and diverse. In fact, other mineral groups like sulfates, carbonates, and silicates can all be considered to be part of the oxide class but are classified separately due to the structure of complex anion groups.
The oxide minerals and metals can vary in color and style. The hard aluminum oxide, corundum, includes what we know as rubies and sapphires depending on the color. It also includes chrysoberyl, which is often available in golden yellow.
The oxide mineral class also includes the hydroxide class, which holds any chemical compound that has one atom of hydrogen and oxygen bonded together – which can be in one or more groups.
Native Elements include the core group of pure metals: gold, silver, platinum, carbon (as diamond), copper, and titanium. They make up a chemical element group that can be available in the earth uncombined with other elements.
Only 19 of the chemical elements found in the earth are known as minerals and they are divided into three groups.
The metals group is often structured as a close-packed cubic or hexagonal shape while semimetals and nonmetal groups are more complex in structure and can depend on the conditions of formation.
This class includes chemical compounds containing the element sulfur and is made of three subclasses: organic sulfides, inorganic sulfides, and phosphine sulfides. This class includes a composition of metals with sulfur. The well-known fool’s gold (pyrite) is a sulfide and is the chemical composition of iron and sulfur.
Other well-known metals like copper, silver, mercury, zinc, and cadmium also occur in the earth as sulfides.
Silicates are the common igneous rocks that makeup approximately 90% of the Earth’s crust. They contain about 25% of all known minerals and they are fundamentally made with the silicon-oxygen tetrahedron. While silicates make up the largest mineral class, they can be quite complicated in nature due to the different ways that the silicate tetrahedrons combine.
The subclasses of silicates are grouped based on their structures – not their chemical composition.
Nesosilicate Subclass (Single Tetrahedrons)
This subclass is similar to other mineral classes like phosphates and sulfates and includes silicates that have silicon-oxygen (SiO4) tetrahedrons that are unbonded with other tetrahedrons. Often called orthosilicates, nesosilicates have a structure of closely packed ions and produce stronger bonds – resulting in a higher density. Therefore, there are more gemstones in this subclass than any other silicate subclass. Popular gems like Topaz and Garnets are included in the nesosilicate subclass.
Sorosilicate Subclass (Double Tetrahedrons)
The smallest of the silicate subclasses, sorosilicates have two tetrahedrons that are grouped by a single oxygen ion. Many of the compounds found in this subclass are rare with the exception of epidote, which can be more commonly found in metamorphic environments.
Inosilicate Subclass (Chains)
This subclass is made of single-chain and double-chain silicates. It includes crystals like Pectolite and Neptunite.
Cyclosilicate Subclass (Rings)
This subclass is similar to the inosilicate subclass where it forms chains but the chains link back around to form rings. Both the inosilicate and cyclosilicate subclasses share similar silicon to oxygen ratio of 1:3. The symmetry found in the rings usually translates to the symmetrical structure of the mineral. Three tetrahedrons can form triangular rings, four tetrahedrons can form a square structure, six tetrahedrons can form a hexagonal shape. Many gemstones in this subclass have high hardness, making them durable for jewelry making.
Phyllosilicate Subclass (Sheets)
This subclass is distinguished by the sheet-like structure, which is obtained when rings of tetrahedrons are linked to other rings in a two-dimensional plane. The ratio of silicon to oxygen for minerals in this class is approximately 1:2.5. The minerals in this subclass are often a soft hardness since the sheets are usually held together by a weak bond and can trap water molecules and other atoms in between them.
Also known as Framework Silicates, tectosilicates have a structure of tetrahedrons that point outward in all directions. The silicon to oxygen ratio is generally 1:2. The near-perfect state of only oxygen and silicon is what we know as quartz.
This class contains naturally occurring elements that do not present any crystallization. The solids in this class often lack the standard chemical composition that is required to be labeled as a mineral. Mineraloids are classified as amorphous, meaning they do not contain an ordered internal atomic structure. Well-known stones that are mineraloids include obsidian, opal, and pumice.
When talking about crystals, using the word organic means that it originates from an animal or plant. Fine examples of organic minerals include pearls (formed with grains of sand irritating the pearl oyster), amber (fossilized resin of a pine tree), and jet (a highly compressed form of coal).