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FREQUENTLY ASKED QUESTIONS  MINERALOGY - GENERAL

 

      What is a mineral?

      How many minerals are there?

      How is a new mineral determined?

      Is water a mineral?

      How do crystals form?

      How fast do crystals grow?

 

What is a mineral?

     A mineral substance is defined as a naturally occurring, homogeneous solid, inorganically formed, with a well defined chemical composition (or range of compositions), and an ordered atomic arrangement, that has been formed by geological processes, either on earth or in extraterrestrial bodies.

How many minerals are there?

There are approximately 3800 known minerals. About 30 to 50 new minerals are described and one or two minerals are discredited each year. The most complete listing of minerals is J. Mandarino Fleischer's Glossary of Mineral Species 1999 published by the Mineralogical Record. He alphabetically lists mineral name, formula, crystal system, and reference. More complete descriptions of minerals are found in Handbook of Mineralogy (5 volumes have been published - The silicate section is online on this site) and Dana's New Mineralogy 8th ed ( numerous typographical errors). Discredited mineral names (35,000 to 40,000) are listed in Peter Bayliss Glossary of Obsolete Mineral Names published by the Mineralogical Record and the Jeffrey de Fourestier Glossary of Mineral Synonyms published by the Mineralogical Association of Canada ( Special Publication #2 ). A. M. Clark Hey's Mineral index: Mineral species, varieties and synonyms 3rd ed. lists both valid species and discredited minerals. Two online listings of mineral species are at David Barthelmy's or Jolyon Ralph's web site. In the late 1800's and early 1900's, many scientists named minerals that were only a different color or habit, or had a minor variation in chemistry. The development of X-ray crystallography in the 1930's and better chemical analyses has led to a clearer understanding of the difference between a mineral and variety of a mineral. When the IMA took on the responsibility for determining the validity of new minerals in 1959, the task was too large to "officially" determine the validity of previously described  minerals.

 

How is a new mineral determined?

For each new proposed mineral, data on the chemical composition, crystallography, and physical properties is submitted to the Commission on New Minerals and Mineral Names of the International Mineralogical Association (CNMMN of IMA). After it is determined that the description is sufficient to justify a new mineral, the describer is and they have two years to publish the information. The type material is deposited in a repository (usually a museum) if a comparison is ever needed of the originally described material ( the type specimen). see Nickel & Grice.

Are water and ice minerals?

       A mineral is defined as a naturally occurring, homogeneous solid, inorganically formed, with a definite chemical composition( or range of compositions), and an ordered atomic arrangement. Water does not pass the test of being a solid so it is not considered a mineral although ice; which is solid, is classified as a mineral as long as it is naturally occurring. Thus ice in a snow bank is a mineral, but ice in an ice cube from a refrigerator is not. The only exception to this rule ( there always seem to be one ) is that mercury is considered a mineral (more a result of history - mercury was an important alchemical substance). 

     The derivation of the definition of mineral has much more to do with the history of mineralogy than semantics. In the 17th and 18th centuries, there were collections of items of natural history being put together. For instance there were probably twenty different varieties of quartz ( rock crystal, amethyst, agate, chert, etc) that were classified as different things and some things that were considered the same ( jade - jadeite and nephrite), but were in actuality a number of different minerals. Humans have always tried to understand things by grouping the items together and determining generalizations about their properties. The two major points of the definition of minerals is that they have a long range order ( the crystallography ) and a definite chemical composition. With these two factors, it is possible to split the mineral world into different objects (minerals) that have similar properties.
       If one looks at the same chemistry (say carbon), then differences in crystallography will  effect the mineral's properties. Thus one can distinguish diamond, graphite, and fullerenes by their crystallography even though they have the same chemistry. If one takes the same crystallography with different chemistry, one can distinguish gold and silver. The work of HaĆ¼y in crystallography and by Berzelius in chemistry in the 18th and 19th century allowed the minerals to be systematically determined by the combined use of crystallography and chemistry (i.e. Dana's System of Mineralogy). There had been an attempt to use the Linnaean system ( the genus and species of biology) to classify minerals, but it was not successful. ( A good overview of the history of mineralogy can be found at the Euromin  Web Site )


     One of the problems with liquids was that until recently, there was no good way to study them. The modeling in computers and the development of the atomic force microscopes has finally opened up new methods to study fluids. One of the topics now being discussed is how much of a substance is needed before it can be considered a mineral? With the new scientific tools available, their are regions in noncrystalline materials (glasses) that do exhibit order.

     One of the unique properties of water is that it can exist in solid, liquid and gaseous states at surface conditions of the earth and the solid is less dense than the liquid. Mineralogists had to leave a few things for the hydrologists and meteorologists to study. If a mineral is melted, the liquid is not considered as being a mineral. 

    At absolute zero ( theoretically possible, but unattainable in nature) there is no movement of any atoms in a solid. As the temperature increases, there is more vibration in the atoms, but the bonds between the atoms are strong enough that they are not broken. When one reaches the melting point, these vibrations become large enough that they break the bonds between the atoms and the substance melts. The thermal energy allows the atoms to move about enough so that there is no tendency to form any type of long range order in the substance. 

How do crystals form ?

     Crystals can form in many different ways. Almost all of the earth is formed from crystals ( except the parts that are molten). Most of the time, the crystals have grown in a way that they are crowded together and show no external faces (anhedral). Crystals grow when the solubility of elements in a liquid phase is exceeded and they need to transform into a solid or the energy needed to keep them liquid is not sufficient.
     A crystal can form from a vapor. Sulfur can condense from a vapor and form crystals in fumarole vents in volcanoes. A more common example for those of us in the North is the formation of frost on a windowpane. The ability of the air to contain water vapor is exceeded and crystals of ice grow.
     Crystals can also grow from solutions of ions in a fluid such as water. A large number of economic ores are concentrated into mineable deposits by this means. The quartz crystals from Arkansas are an example of this process. Some sedimentary rocks are formed entirely by this process (evaporites). Salt will also crystallize out of solutions when evaporation of the water saturates sodium and chloride ions in the water.
     When a magma ( molten rock) cools, crystals can form as the magma solidifies. Certain minerals will form at various temperatures and will drop out as the magma cools. This forms one of the major ores of chromium as chromite crystallizes as a magma solidifies.
     Crystals can also grow at the expense of earlier formed crystals. The silver sulfide argentite is stable at high temperatures. As the temperature is lowered, there is a transformation to the mineral acanthite. There is no change in the chemical composition of the material, but the arrangement of the atoms is changed. Pressure changes can also change the crystallography of the minerals without any changes in the chemistry. Diamond is a mineral that is not stable at the earth's surface, but the rate that it transforms into the mineral graphite is so slow that it will last for long periods of time (metastable).

     There are several good resources on the web for crystal growing. These include
the Rockhounding Arkansas site.
Another site with a lot of good links is here.

     Good crystals that show the external faces (euhedral) are relatively rare since they require the growth into an open space and not a lot of nucleation which would result in the growth of many numerous smaller crystals. There has been a relatively recent discovery of large gypsum crystals in Spain. They are about three feet long in a twenty foot cavity, but the location in the mine is very hot, so people can't stay in the area for more than a few minutes. Pegmatites often have very large crystals because the melt is very fluid rich. Beryl crystals to eighteen feet in length were found in the Bumpus Quarry at Albany, Maine. (also see article by J. Betts).

   A recent study by Gasser et al. (Science 292:258-262) on the "crystallization" of colloid particles indicated a critical size of 60 to 160 spheres to form a stable nucleus for the further growth of a crystal. The nucleus was ovoid in shape and the same packing as the bulk material.

How fast do crystals grow?


     Crystals grow at many different rates. The speed depends upon the supply of the elements, the degree of oversaturation present, and the mechanism of element transport.
     For instance, garnets can grow by solid state diffusion. It has been estimated that they can grow about one atomic layer per year ( a two centimeter crystal growing over a period of ten million years ). In mines, crystals can grow extremely fast. I have a pipe that was used to pump water out of the mine, and there are inch crystals of gypsum that developed in five or six years. The gypsum crystals in Jet, Oklahoma grow over a period of a few years. Salt crystals will grow in the period of a few months, when the water evaporates from playas in the West. The minerals that form in the slags of ores from Laurium, Greece will have formed over a period of about a couple of thousand years ( these are usually a couple of millimeters in size).
      Most other types of crystals will take periods of time between these extremes to grow into crystals. 

     One of the main problems with determining on how fast a crystal grows in nature is the difficulty in measuring when the crystal started growing and when it ended. The nice thing about the mines and the salt flats, is that we can determine maximum ages with relative certainty. The garnet growth was slow enough that Sm/Nd dating can be used to date the center and edge of the crystal and the whole crystal was not equilibrated during crystal growth ( see Vance & O'nions 1990 Earth & Planetary Science Letters pp 227-240). One problem with this method is that often the uncertainties are larger than the duration of crystal growth. Another article on garnet growth can be found on this site's materials research articles ( v1n1) by Spears.
The salt flats can be found at their web site.
     Another good source of information is volume 26 of the MSA's  Reviews in Mineralogy  Contact Metamorphism.
     Since a lot of minerals grow as the result of hydrothermal activity, the maximum time of formation will be how long it takes the systems to cool down. This is usually in the range of a few thousand years for near surface intrusions/extrusions to a few million years for deeper emplaced intrusions. Plumbing changes due to earthquakes could shorten the time for crystal growth. Growth from vapors in volcanic areas can be extremely rapid.

   For an article on hydrothermal crystal growth in New Zealand

A link to a web page that has mineral identification key.


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