Mineral Classification

Scientists group minerals based on their chemical compositions. The Dana Classification System originally listed nine main mineral classes: Native Elements, Sulfides, Sulfates, Halides, Oxides, Carbonates, Phosphates, Silicates, and Organic Minerals. Of these nine, only Silicates had subgroups: Nesosilicates, Sorosilicates, Cyclosilicates, Inosilicates, Phyllosilicates, and Tectosilicates. Over the years, the classification system has grown as scientists’ understanding of minerals increases. Today, there are 78 recognized mineral classes. Here we will use the old Dana Classification terms for simplicity.

Native Elements

 

 

 

 

 

 

 

 

 

 

 

Sulfides (sulfur + other elements)

 

 

 

 

 

 

 

 

Sulfates (sulfur + oxygen + other elements)

 

 

 

 

 

 

 

 

 

Halides (chlorine or fluorine + other elements)

 

 

 

 

 

 

 

 

 

 

 

 

Oxides (oxygen + other elements)

 

 

 

 

 

 

 

 

Carbonates (carbon + oxygen + other elements)

 

 

 

 

 

 

 

 

 

 

 

Phosphates (phosphorus + oxygen + other elements)

Phosphate is a chemical compound made of one atom of phosphorus and four atoms of oxygen (PO₄).

 

 

 

 

 

 

 

 

 

Silicates (silicon + oxygen + other elements) 

These are rock-forming minerals and the most common minerals on Earth.

 

 

 

 

 

 

 

 

 

*The elements that fill the X and Y positions in the formula vary depending on each garnet species (andradite, grossular, almandine, pyrope, spessartine, and uvarovite). Examples: Pyrope composition: Here Mg₃Al₂Si₃O₁₂. Andradite composition: Ca₃Fe₂Si₃O₁₂

 

Organic Minerals

Imagine you’re outside and you find a mineral. You want to identify it but you aren’t sure how. That’s okay! Mineralogists use a variety of physical and optical properties to help identify minerals without the help of special equipment. These properties include the mineral’s color, crystal shape, hardness, cleavage (the way a mineral breaks), streak, luster, magnetism, ability to transmit light, and specific gravity.

 

Physical Properties

Crystal shape

Crystal form refers to the common or characteristic shape of a mineral’s crystal or aggregate of crystals that are bounded by a set of flat faces that are related to one another by symmetry. Some minerals show recognizable shapes like cubes or octagons that are helpful in mineral identification. For example, garnets often form dodecahedrons (12-sides). Most minerals have only one common form, but there are a few that can develop into multiple forms. Galena is commonly found as a cube but it can also form an octahedral habit as well.

 

 

 

 

 

 

 

 

The following table lists some of the most common crystal shapes.

Pyramidals (5 sides)	sulfur
Cube (6 sides)	Galena
Rhombohedron (6 sides)	Rhodochrosite – Specimen is from the National Mineral Collection at the National Museum of Natural History, Smithsonian Institute – Rhodochrosite-NMNH_147520 https://geogallery.si.edu
Octahedron (8 sides)	fluorite
Dodecahedron (12 sides)	Garnet

 

Crystal Habit

The tendency for a mineral to repeatedly grow into characteristic shapes is called a crystal habit. Unlike crystal forms, crystal habits are not bound by crystal faces or symmetry. The crystal habit of a mineral can be used to differentiate minerals. The conditions and chemistry of the environment a mineral formed within can influence which habit develops. Some minerals, such as pyrite, can form in multiple crystal habits including cubic (a form and a habit) or radiating.

Crystal habit	Description	Image of Mineral
Massive	mineral lacks crystal faces	Sulfur
Granular	crystal grains are approximately equal in size; grains range in size from about 2 to 10 mm	Pallasite meteorite
Lamellar	made up of layers	Molybdenite
Micaceous	also known as foliated; crystals that form a sheet-like or layered structure; often can be split into thin sheets	Mica
Bladed	elongate crystals that are longer than they are wide and their width is greater than their depth; resemble a straight sword or knife	Stibnite
Fibrous	occur as very fine fiber-like crystals	Actinolite
Radiating	aggregates of crystals grow outwards from a central point	Thomsonite
Oolitic	crystalline aggregates that are rounded and less than four millimeters in size	Ooids
Banded	minerals having narrow layers or bands of different color and/or texture	Agate Banded
Botryoidal	also called globular or mammillary; crystal aggregates that have a globular or rounded shape	Hematite
Columnar	long prisms with enough width that the name acicular (needle-like) does not apply	Gypsum
Geodic	clusters of mineral form a rounded mass by crystalization on the inside walls of a cavity	Amethyst geode (cathedral)
Rosette	clusters of tabular crystals in a radial arrangement that resembles a rose or flower	Barite Roses

 

Hardness

The Mohs scale of mineral hardness is based on the difficulty to scratch a mineral’s surface. First created in 1812 by the German geologist and mineralogist, Friedrich Mohs, the scale has 10 levels and is used to compare the hardness of different materials or minerals to see which scratches the other. For example, talc is most commonly used for Mohs Hardness 1, and diamond, being the hardness mineral on Earth, is used for Mohs Hardness 10. The table below illustrates a mineral at each hardness level and some common materials that can also be used to test the hardness of minerals.

Scale Number	Mineral Name	Mineral Photo	Common Object
10	Diamond
9	Corundum
8	Topaz		Masonry Drill Bit (8.5)
7	Quartz
6	Orthoclase		Steel Nail (6.5)
5	Apatite		Knife/Glass Plate (5.5)
4	Fluorite
3	Calcite		Copper penny (3.5)
2	Gypsum		Fingernail (2.5)
1	Talc

Table showing the Mohs Scale of Mineral Hardness. The specimen images for diamond, corundum, topaz, and orthoclase are from the National Mineral Collection at the National Museum of Natural History, Smithsonian Institute. https://geogallery.si.edu

 

Cleavage

When minerals break, they tend to break in a certain way. This is called cleavage. Cleavage occurs on planes that depend on the mineral’s crystal structure and where the mineral has weak bonds holding the atoms together. Minerals tend to break at these points of weakness. A mineral can have multiple cleavage planes. When you look at a mineral, usually the shape is defined by the cleavage planes. Sometimes individual crystals break or do not form well-defined crystals making it difficult to see a mineral’s cleavage planes.

Mica has one cleavage plane, which is referred to as basal cleavage. When split, the mineral’s cleavage planes can be “peeled” apart, like the pages of a book. Galena most commonly forms cubic cleavage. If you were to break a cube of galena, it would break into smaller and smaller cubes. Much like cubic cleavage, minerals like calcite that have rhombohedral cleavage can break into smaller rhombohedron crystals, which, like the name suggests, look like a rhombus.

 

Fracture

Some minerals have chemical bonds that are approximately the same in all directions and doesn’t have a predictable point of weakness. When a mineral does not break along a cleavage plane, it is called a fracture. When a mineral fractures, most result in uneven surfaces that are described as an irregular fracture. 

Some minerals, such as quartz, break into smooth, curved surfaces resembling broken glass. This fracture pattern is called a conchoidal fracture.

Minerals can fracture in other patterns as well, including fibrous, splintery, or hackly. Fibrous and splintery fracture looks similar to the way wood breaks. Hackly fracture looks like jagged fractures with sharp edges.

 

 

 

 

 

 

 

 

 

 

 

Streak

When dentifying a mineral, you can use its “streak” to aid identification. A streak is the color of the powdered mineral that is left behind on an unglazed porcelain plate. This plate is also called a streak plate. The streak plate has a hardness of 7 on the Moh’s hardness scale, so any mineral with a hardness higher than seven will not leave a streak, such as corundum with a hardness of 9. Instead, it will leave a white powder form where it scratched the plate. When the mineral has a lower hardness, the streak can be used to identify it. The color of the mineral is not always the color of the streak. Therefore, for minerals like quartz that can be a variety of colors, the color of the streak remains the same.

• Minerals with a metallic luster tend to have a dark streak. Minerals with a nonmetallic luster tend to have a light-colored streak.
• Hematite has a red streak

 

Magnetism

Most minerals are not attracted by a magnet. Therefore, magnetism is a useful property for identifying minerals because there are few magnetic minerals. Minerals that are not magnetic are referred to as diamagnetic minerals. Alternatively, the few minerals that are magnetic are called paramagnetic minerals. The most magnetically active minerals are ferromagnetic, such as magnetite (composed of iron and oxygen; Fe₃O₄.)

 

 

 

 

 

 

 

Ferromagnetic minerals are important in understanding the magnetic field of the Earth. These minerals record the direction of the Earth’s magnetic field and therefore help geophysicists to reconstruct the motion of the Earth’s tectonic plates (pieces of the crust and mantle). Geochronology, which uses ferromagnetic minerals to measure how the Earth’s magnetic field has changed through time, is the study of the age of rock and geologic events.

Effervescence

When dilute hydrochloric acid is applied to the surface of some minerals, the mineral will bubble, or effervesce. This reaction is characteristic of minerals containing carbonate (CO₃). The amount of effervescence depends on how soluble the minerals are. For example, calcite (CaCO₃) effervesces more than dolomite (CaMg(CO₃)₂).

The chemical reaction that occurs:

• • CaCO₃ + 2HCl → Ca²⁺ + H₂O + 2Cl + CO₂ (gas)
  • When calcium carbonate and hydrochloric acid react, water, and carbon dioxide (gas) are produced. As the carbon dioxide releases, it bubbles through the water and remaining hydrochloric acid on the mineral.

Video demonstrating the acid test for calcite and dolomite.

 

Optical properties

Color

When identifying minerals, it is important not to relying solely on color because it is often variable. Color can be misleading for minerals such as quartz and calcite. Impurities in quartz can give it a variety of tints including purple (amethyst), yellow (citrine), and black (smoky quartz). Gold has a characteristic color, however pyrite, also known as “Fool’s Gold”, shares a similar color. To identify between the two, other optical and physical properties of the minerals are needed.

Native Sulfur – For native elements, the color of the mineral is the color of the element.

Luster

Luster is the appearance of light reflected from the surface of a mineral. There are two types of luster: metallic and nonmetallic. 

Metallic is the luster of polished metal – for example, the appearance of steel, copper, and gold. This luster reflects light like metals and is opaque to transmitted light.

Nonmetallic luster is shown by many minerals that transmit light. The appearance of nonmetallic luster varies from a highly polished glass surface to a dull earthy-like appearance. For example, feldspar has a nonmetallic luster that is dull and earthy. Most minerals have a nonmetallic luster and are commonly described with adjectives such as vitreous, glassy, dull, earthy, pearly, or silky. In nonmetallic minerals, luster is often caused by the breaking of chemical bonds along cleavage planes.

 

• •  Vitreous luster

• • Glassy luster
    

    Obsidian (a variety of quartz) with conchoidal fracture – a diagnostic physical property of quartz. Source: https://www.sandatlas.org/conchoidal-fracture/

  • Dull luster
    

    Orthoclase

  • Pearly luster
    

    Mica

  • Silky luster
    

    Gypsum

Ability to Transmit Light

Hold a clear glass cup and, notice how the light passes through it. The light passing through the glass can be described as opaque, transparent, or translucent. The ability of a mineral to transmit light is commonly used in the identification process of minerals like quartz. Opaque minerals do not transmit light. Translucent minerals allow some light but not a clear image. When both light and an image can be transmitted through the mineral, it is described as transparent. A large sample of muscovite, at first glance, looks opaque However, when the layers are separated along the cleavage planes, the individual layers are transparent.

Transparent:

 

Translucent:

 

Opaque:

Double Refraction

When light passes through a transparent mineral, it doesn’t always go through as a single ray. Certain minerals, like calcite, will split plain, unpolarized light into two rays. When a piece of calcite is placed overprinted text, these split light rays will cause the text to appear twice. Watch the video below for a demonstration of optical calcite and what happens when the light is polarized by a filter.

Fluorescence and Phosphorescence

Minerals, such as gypsum,when illuminated with ultraviolet (UV) light, X-rays, and/or electron beams appear to glow in vibrant colors that are not present when the mineral is viewed with regular light.  For example, calcite that looks white in visible light can fluoresce in a variety of colors such as red, blue, pink, green, and orange. The fluorescence color is affected by trace elements within the mineral.

 

 

 

 

 

 

 

 

Minerals, such as fluorite, can continue to glow after the initial activating UV light is removed. The occurrence of light emitted from a mineral after the UV light is removed is called phosphorescence. Check out the Sternberg Museum’s video to learn more about how and why minerals glow when illuminated with UV light.

 

Similar to how each of our bodies is made of cells, each mineral has a unit cell, the smallest reproducible pattern in a crystal structure, that builds up to form the crystals of a mineral. All minerals can be divided into one of seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, cubic, and rhombohedral. These systems are established at the atomic level of the mineral’s crystal structure. The mineral’s atoms will take on one of these systems and the external shape of the crystal often reflects the internal arrangement of the atoms. The primary differences among these systems are the lengths of the axes (how far apart are the atoms) and the angles between the atoms.

• Cubic – The cubic system is also called the isometric system. This system is defined by all three axes being equal lengths and the angles all set at 90 degrees. Unit Cell – smallest reproducible pattern in a crystal lattice. 

Example:

 

 

 

 

 

 

 

 

 

• Tetragonal – Similar to the cubic system, two of the axes are the same length, but the third axis is longer, making the shape look like a rectangular box. All of the angles are also 90 degrees. The crystals tend to be of prismatic habit.

Example:

 

 

 

 

 

Chalcopyrite (need to shoot – if you want this mineral)

 

 

 

• Orthorhombic – all three angles are at 90 degrees but all of the axes are different lengths. The crystals tend to be of prismatic, tabular, or acicular habit.

Example:

 

 

 

 

 

 

 

 

 

 

• Rhombohedral – also known as trigonal. similar to tetragonal, two of the axes are the same length and the third is longer. Two of the axes form at 90 degrees while the third is at 120 degrees. The crystals tend to form rhombohedra and triangular prism habits.

Example:

 

 

 

 

 

 

 

 

 

 

• Monoclinic – similar to tetragonal but the angles are different. The long axis is at an oblique angle to the two short axes, which are set 90 degrees to each other. The crystals tend to form as long prisms

Example:

 

 

 

 

 

 

 

 

 

• Triclinic – similar to orthorhombic but all three axes are at different angles to one another. Therefore, there are no set angles among the axes nor are the axis lengths equal. The crystals tend to be of tabular habit.

Example:

 

 

 

 

 

 

 

 

 

• Hexagonal – two hexagons (6-sided) stacked on rectangles. The same as the rhombohedral system, two of the axes form at 90 degrees while the third is at 120 degrees. The crystals tend to produce hexagonal prisms and pyramids. The hexagonal system and Rhombohedral system differ in their symmetries. Because these two systems are so similar, rhombohedral is often grouped within the hexagonal system, thus the list of crystal systems is sometimes on six systems long. When taking the symmetry of the crystal into consideration, rhombohedral becomes its own system, making it a seventh crystal system. 

Example:

 

 

 

 

 

 

 

 

 

 

 

 

The six Silicate subgroups are: 

• Nesosilicates–simplest subclass, characterized by a lone silica tetrahedron, which is bound together by positively charged ions. Many gemstones are nesosilicates due to their simple structure.

• Sorosilicates–two silicate tetrahedra share oxygen, creating a figure-eight pattern. 
  
  

  Image modified from Silicate_structure_Encyclopedia_Britannica https://kids.britannica.com/students/assembly/view/2492

• Cyclosilicates–this silicate group forms chains of interlocking tetrahedra that link back around to form ring patterns. The name Cyclosilicate comes from the fact that the atoms of these minerals are arranged in a ring shape – much like the wheels of a bicycle.

• Inosilicates–tetrahedrons forms in either single or double chains.

 

 

 

 

 

 

 

 

 

 

 

 

• Phyllosilicates–silicate tetrahedra arrange themselves in sheets. 

• Tectosilicates–these silicates are very stable and are known as framework minerals.