Mineral Formation
Minerals are formed by solutions, melts, and vapors. As water evaporates, solid minerals precipitate out by solution (e.g., halite and gypsum). As molten magma crystallizes or water freezes to ice, minerals are formed by a melt (e.g., ice and feldspar). As a vapor begins to cool, the disassociated atoms or molecules lock into a crystalline solid (e.g., topaz and sulfur). In general when minerals grow very slowly large single crystals form. It has been estimated that quartz crystals can take up to 200,000 to 300,000 years to form. Some large minerals can be formed in very short periods of time though, for example, gypsum crystals half a meter in length have grown in a matter of hours in a desert lagoon environment and can disappear or go back into solution, with rainfall!
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Mineral Names and Classification
Minerals are named for the site in which they are found. For example, franklinite, www.galleries.com/minerals/fablocal/franklin.htm or in German -www.mineral.org/egeler/franklin.htm- is named for Franklin, New Jersey, www.ohwy.com/nj/f/franklin.htm. In addition to location of the find, minerals can be named for a special property, such as magnetite that is magnetic. Minerals can be named in honor of the person and in the past, in honor of the one who found the specimen. For example, smithsonite, www.sil.si.edu/Exhibitions/Smithson-to-Smithsonian/who_04.html, which is named for James Smithson, www.sil.si.edu/Exhibitions/Smithson-to-Smithsonian/who_01.html, a mineralogist, geologist, and founder of the Smithsonian Institution, www.si.edu.
Another opportunity! If you are enrolled in this course, please email me at saber@emporia.edu, place GO 324 mineral test points in the subject line; provide me with the URL for the international commission responsible for naming minerals. I must receive this by September 21. In the email, remind me to add one point to your first test for participating and following instructions!
Minerals are classified or grouped by their chemical composition and internal crystalline structure, which together represent the essence of a mineral and determine its properties. Classification by chemical composition is attributed to Berzelius, a Swedish chemist, sometime between 1779 and 1848. The classification shown below is credited to Hugo Strunz, a German mineralogist, and is from 1938. According to this scheme, the minerals are divided by the dominant anion or anionic group because this provides greater commonalities than minerals containing the same dominant cation. A second reason for this classification is that these minerals, related by the same anion, tend to occur together in the same or similar geologic environments.
| Native Element | Mineral consists of single element | Copper, Cu or Gold, Au
|
| Sulfides | Metallic cation + sulfur anion, S-
| Galena, PbS or Pyrite, FeS2
|
| Halides | Anion of halide, fluoride, etc.
| Halite, NaCl or Fluorite, CaF2
|
| Oxides and Hydroxides
| Anion of oxygen, O2- or hydroxyl, OH-
| Hematite, Fe2O3 or Brucite, Mg(OH)2
|
| Carbonates
| Anion of CO3 2- | Calcite, CaCO3 or Malachite, Cu2(CO3)(OH)2
|
| Sulfates
| Anion of SO4 2- | Gypsum, CaSO4 2H2O or Barite, BaSO4
|
| Phosphates
| Anion of PO4 2- | Apatite, Ca5(PO4)3(F, Cl, OH)- |
| Silicates
| Anion of SiO4 4- in various combinations | Olivine, (Fe,Mg)2 SiO4 or Quartz, SiO2
|
| Organics
| Not true minerals
| Amber or Kidney Stones- Whewellite, CaC2O4 H2O
|
Chemistry alone does not classify a mineral. For example, quartz, SiO2, would be classed as an oxide rather than a silicate, which it is in some countries. However, the importance of internal crystalline structure was recognized in 1669 by Steno, a Danish scientist named Niels Stensen, and called the law of constancy of interfacial angles. In 1784, René J. Haüy furthered this law with his concept of the unit cell, or the basic building blocks of minerals. Internal crystalline structure or the regular and ordered arrangement of atoms that characterize minerals was not positively demonstrated until the 20th century with the advent of X-ray diffraction. This technology was suggested by Max von Laue, a German, but it was Friedrich and Knipping who actually demonstrated that crystals could diffract X-rays. W.L. Bragg and V.M. Goldschmidt were the first to apply crystallochemical principles to silicate minerals. They subdivided this group based on chemical composition and internal structure into the categories shown below, all variations of the basic linkage involving SiO4 tetrahedra.
Silicate Subdivisions
Nesosilicate
| isolated silica tetrahedra, e.g., olivine
Sorosilicate
| paired silica tetrahedra, e.g., epidote
|
Cyclosilicate
| ring silica structure, e.g., beryl
|
Inosilicate
| single or double chain, eg. pyroxene & amphibole
|
Phyllosilicate
| sheet silica structure, eg. mica & clays
|
Tectosilicate
| three-dimensional structures, eg. feldspar & quartz
| | | | | | |
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Mineral Properties Useful in Identification
Observation and descriptive skills are utilized in the identification of minerals. Properties of minerals may be seen, felt, tasted, or smelled. Hardness, luster, color, streak, cleavage, fracture, and form or habit are unique properties that will assist in the identification of unknown specimens.
- Hardness is the resistance a mineral has to scratch and abrasion. Simply stated, a harder mineral will scratch a softer one and minerals of equal hardness will scratch each other. Mohs Hardness Scale was developed to provide comparison minerals (see Table 1-1.). A list of common minerals were assigned numbers from 1 to 10. Useful comparison testing items to arrive at a relative figure include glass and fingernail. If your fingernail scratches the mineral, the mineral hardness is equal to or less than 2.5. If the unknown mineral does not scratch glass, then the mineral hardness is less than 5.5. If the unknown does scratch glass, the mineral hardness is greater than or equal to 5.5. The specimen is 3, 4, or 5 in hardness if it cannot be scratched with a fingernail and does not scratch glass.
Table 1-1. Mohs Hardness Scale
| Hardness | Index Mineral
| Useful Comparison
| 1 | Talc | Candle Wax (1) | 2 | Gypsum | Fingernail (2.5) | 3 | Calcite | Penny (3) | 4 | Fluorite | | | 5 | Apatite | Glass (5.5) | 6 | Orthoclase | | | 7 | Quartz | Unglazed Tile (7) | 8 | Topaz | | | 9 | Corundum | | | 10 | Diamond |
| | | | | |
- Luster refers to what the mineral looks like. It is the quantity and quality of light that is reflected from the surface of a mineral and is divided into two categories of metallic and nonmetallic. Metallic luster is the look of metal and depending on the intensity of the reflection, can be referred to as bright or dull metallic. Nonmetallic luster can be divided into several categories that can be both seen and felt, such as: soapy, earthy, greasy, and vitreous.
- Color or the lack of color is helpful, but should not be used alone in the identification of minerals. Sulfur is always yellow and malachite is always green, but quartz may be yellow and green, as well as orange, brown, pink, reddish, purple, and white. Quartz is colorless in a pure state. Streak, on the other hand, is very reliable.
- Streak is the true color of the mineral and found by rubbing the specimen across an unglazed porcelain tile. The color that results is referred to as mineral streak, which is the color of a mineral in a powdered form. Streak is often most helpful in distinguishing metallic minerals, such as hematite from magnetite.
- Fracture or cleavage refers to the way a mineral reacts to stress and demonstrated with a strike from a hammer. If the break is very smooth and flat, the mineral cleaves or separates along planes of atomic weakness. Cleavage results in these smooth, plane surface that reflect light back as if one rotated a mirror in the light. It can occur in one, two, three, four, or six different directions. Fracture is the break that results when atoms in minerals are connected or bonded so that strength is uniform in all directions. With no particular atomic plane of weakness, the mineral will break in an irregular manner. Two common fractures are uneven (rough surface results) and conchoidal (smooth, curved, shell-like surface). Metallic lustered minerals may have a hackly or splintery fracture.
- Form or habit of a mineral refers to the external shape. If a mineral has free space when it is growing, smooth faces and geometric shapes will result and are a reflection of the orderly internal arrangement of atoms. Quartz commonly occurs with hexagonal cross-section or a hexagonal prism form, while halite is often a cubic form. A common form of calcite is scalenohedron, which is also called dog-tooth spar. Habit terms are not symmetrical but describe the shape of minerals. Common habits include stalagtitic or tapering cone shape; bladed or an elongate, broad, or flat shape; acicular or needle-like; equant or equidimensional; globular or rounded masses that can resemble a bunch of grapes.
- Other properties specific to certain minerals can aid in identification. Magnetite is magnetic and a magnet will cling to the surface of the mineral. Halite tastes salty. Sulfur and sphalerite have pungent smells after powdering a specimen on a streak plate. All carbonate minerals will fizz in a dilute hydrochloric acid or in vinegar or dilute muriatic acid. Mineral fluorescence is also useful in identification, but not as practical in field identification.
Another opportunity! View gigantic gypsum crystals in a former silver mine located in Spain, visit http://news.bbc.co.uk/1/hi/sci/tech/787776.stm. Send a one-page summary, with citations and references, to saber@emporia.edu. Place GO 324 gypsum geode-add a point to test 1 in the subject line and I must receive it by March 17.
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Recommended References and Links
- Atlas of Rocks and Minerals from the University of North Carolina, www.geolab.unc.edu/Petunia/IgMetAtlas/mainmenu.html
- Introduction to Crystallography and Crystal Systems, by Mike and Darcy Howard, www.rockhounds.com/rockshop/xtal/
- Amethyst Galleries, Inc. on franklinite, www.galleries.com/minerals/fablocal/franklin.htm
- Stéphane Gigandet´s Mineral Stamps - -www.mineralstamps.com/ -
collection of photos of minerals, crystals, gems, rocks, fossils, and meteorites from around the world!
- Mineralien von Franklin, NJ, USA, www.mineral.org/egeler/franklin.htm
- Franklin, New Jersey, www.ohwy.com/nj/f/franklin.htm
- The Smithsonian pages, www.si.edu/, and specifically:
- World of Amber, www.emporia.edu/earthsci/amber
- Cal Poly at Pomona, mineral identification, geology.csupomona.edu/alert/mineral/minerals.htm
- DLESE, A Digital Library of Earth System Education, http://www.dlese.org/library/query.do?q=minerals&s=0. See different mineral activities.
- Rocks and Minerals of Kentucky, www.uky.edu/KGS/rocksmn/
- John Betts on Mineral Cleaning, www.johnbetts-fineminerals.com/jhbnyc/articles/minclean.htm
- Minerals at Wikipedia, en.wikipedia.org/wiki/Mineral.
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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. The curriculum is © by the author, 2001-2011. Creation and last update February 26, 2011. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu.
To understand copyright, visit www.copyright.gov/. All rights reserved. Susan Ward Aber.
Crystal Systems
Formation
Names and Classification
Properties
References 
Sedimentary Rock
Course Syllabus