Chemistry

How do Crystals and Minerals Grow



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Crystals, or minerals are everywhere around us. How did they come into existence?  How did it happen?  How fast do they grow?

Are crystals like plants?   Many plants you can grow from seed, like corn, and watch them mature  until  harvest time.  Some plants, like geraniums, you can grow from cuttings. Can you do this with crystals?   Yes, sometimes.   Plants are alive whereas crystals are not.   What’s the difference?  

The amateur crystal grower or school student can learn a lot by  growing copper sulfate crystals (or alum) in a jam jar at home.  Saturate a hot water solution of copper  sulfate and let it cool to room temperature.  After a few hours beautiful blue crystals of  copper sulfate hydrate of 1 or 2 mm size will form on the base and walls of the jar. Pick out the biggest crystal and redo the experiment with this crystal suspended by a thread.   It will grow even bigger and will become the preferred place for deposition.  It can grow quite fast,  maybe a millimeter in a  few hours.

Such crystals are found in Nature and their mineral name is Chalcanthite.   Often they crystallize from the ground water in copper mines.   They form at ambient temperature, perhaps 10 degrees C,  and with very slow growth perfect crystals are formed..

Another crystal that grows at ambient temperature in Nature is Gypsum,  (aka Selenite),  which is the hydrated calcium sulfate.    In dried up salt lakes you may find clear transparent gypsum crystals a foot long, or as a rosette bunch of platy crystals, called “the desert rose”.  When there is water in the lake, maybe once a year,  it will contain dissolved salts.  On evaporation in times of drought,  the salts  will crystallize out when the water becomes saturated,  first gypsum and then salt, or halite cubes, and other salts, perhaps even gold nuggets.

In some metalliferous veins, gypsum may grow to form enormous crystals.  In the El Teniente copper mine in Chile, near Santiago,  there is preserved a  crystal cavern  having a transparent prismatic gypsum crystal measuring  1 meter across and 4  meters long.  Also, the Naica Mine, near Chihuahua, Mexico,  has a crystal cavern with gypsum crystals  up to 11 meters long  and weighing 50,000 kg.  They are thought to have grown from prolonged stable groundwater conditions at  about  58 degrees C.  

Quartz is the most common mineral you can find.   It forms a wide range of single crystals (e.g., rock crystal and  amethyst), crystal aggregates and ornamental micro crystalline  material, such as  agate,  carnelian and jasper.  Quartz  is found in massive crystalline rocks like quartzite and granite.   Calcite is another common mineral found in modest  to tiny and microscopic crystals in marbles,  limestones and as shells and fossils, and as white veins traversing sedimentary rocks.

Some different processes of crystal growth found in Nature, with common examples are as follows:

(1) Slow cooling from a melt, such as a silicate magma,  e.g.,  a lava flow from a volcano,  starting temperature say 1200 degrees C and rapid cooling (days or weeks) produces a volcanic rock with very tiny crystals of feldspar, with quartz or olivine etc.,  a fraction of a millimeter size.   In contrast, is the slow cooling of a granite magma from about 700 degrees C, far beneath the surface of the earth,  producing coarse crystals of  1 to 10 mm size,  of quartz,  feldspar and mica,  taking possibly thousands of years.

(2) Crystal growth from a saturated aqueous solution, either by slow cooling or evaporation of the solvent, which is water, or by the transfer of components along a temperature gradient.  Temperatures may range from ambient,  up to 600 degrees C,  with the crystallization of pegmatite veins associated with granite.   

(3) Crystal growth from the vapor,  or a vapor to solid reaction.  Very common with water vapor to form ice crystals,  such as Hoar Frost,  and icy car wind screens.   At volcanic vents issuing smelly gases,  the surroundings may develop a  yellow encrustation and larger crystals of sulfur, which have grown directly from sulfur vapor. Also formation of  “dry ice” (solid carbon dioxide micro crystals),  on the surface of Mars,  also the vapor process used  to grow man-made diamonds.

(4) Biological crystal growth:   Many living things are able grow microscopic crystals that form an essential part of their body.   Animals and fish have bones and teeth,  shellfish have shells of calcite or its polymorph aragonite,   tiny forams use silica,  all accomplished at  constant  ambient temperature.  Life forms are very clever!

(5) Recrystallization, from a lot of tiny crystals to fewer larger ones, often at constant temperature. The medium can be a melt (high temperature),  or aqueous solution, or vapor.   The driving force here is the difference in surface energy.   A given weight of micro crystals has a much larger surface area than an equivalent single crystal of the same weight.  A lower energy state of the system is achieved by spontaneous solution of the micro crystals and growth of  fewer (ideally one) large crystal.      Very common in Nature.   

For geological examples,  a limestone or chalk rock composed of tiny shells of  calcite or aragonite, may on deep burial in the earth’s crust recrystallize as a marble composed of  much larger crystals of calcite.   Clastic sediments composed of tiny clay and other particles may form alternative crystals more stable than the original ones,  therefore we have the metamorphic sequence of mud stone to shale or slate, to mica schist and gneiss, formed with increasing temperature and pressure.

(6)  Solid state reactions or crystal growth within the solid phase.   A good example is the devitrification of glass, either natural (obsidian lava, or artifacts) or man-made glasses.   Glasses are usually formed by rapid cooling of a  silicate melt so that crystals have not had time to nucleate and grow.   Glasses (even window glass) given time,  will eventually crystallize, maybe 1000 or a million  years  hence.  

In conclusion,  the phenomenon of crystal growth, and its reverse, of crystal solution, is going on around us all the time.  The driving force  is to achieve a lower energy state for a given system.   We live in a hetereogeny of overlapping systems each out to do its own thing.   The change  in Nature is extremely slow and rarely noticed by humans,  until you experiment  growing alum or copper sulfate crystals in your kitchen or workshop.   

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ARTICLE SOURCES AND CITATIONS
  • InfoBoxCallToAction ActionArrowhttp://chemistry.about.com/od/crystalrecipes/a/coppersulfate.htm
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Chalcanthite
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Gypsum
  • InfoBoxCallToAction ActionArrowhttp://www.janetsgoldnuggets.com/supergene_nuggets.htm
  • InfoBoxCallToAction ActionArrowhttp://www.bootsnall.com/articles/08-08/el-teniente-chile.html
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Sulfur
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Obsidian