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FREQUENTLY ASKED QUESTIONS MINERAL PROPERTIES

 

     What is fluorescence?

     What is tenebrescence?

     What is luster?

     Where can I find information on stereographic projections?

     What is the difference between schiller and chatoyancy?

  

What is fluorescence?

     Probably the best place to start looking at fluorescent minerals is the web page of the Fluorescent Mineral Society.
http://www.uvminerals.org/
     There are a number of books that have been written about fluorescent minerals such as The Collector's Book of Fluorescent Minerals by Manuel Robbins. He also wrote a column on fluorescent minerals in the magazine Rocks and Minerals for a number of years. The Mineralogical Record had an issue ( Jan/Feb 1996) that had a number of articles on fluorescent minerals ( it was the theme mineral for the Tucson show that year - probably the best display of fluorescent minerals ever mounted). Rocks and Minerals also published a number of articles in their issue (Jan/Feb 1996), but unfortunately that issue is out of print ( it should be available at a library). You can find links to these magazines in the MSA's links page. 

What is tenebrescence?

Dr George Rossman replied.

     Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength.
     A common example is spodumene (the Mn-containing variety, kunzite). {American Mineralogist 38, p 919 (1953).}  Exposure to x-rays will turn it green and exposure to visible light will return it to pink. The energy of the x-rays (or gamma rays) will strip an electron from Mn3+ (pink) and effectively oxidize it to Mn4+ (green). The electron will be trapped somewhere in the crystal. The energy of visible light will free the trapped electron and allow it to diffuse back to the Mn4+, thereby reducing it back to Mn3+.

Cohen AJ, Janezic GG (1983) The crystal-field spectra of the 3d3 ions, Cr3+ and Mn4+ in green spodumenes. In: The Significance of Trace Elements in Solving Petrogenetic Problems & Controversies. Theophrastus Pubs. S.A., Athens.


     Tugtupite  is another mineralogical example. In this minerals, the color changes (colorless to pink to colorless) is brought about by light of different wavelengths (ultraviolet and visible) and represents changes in the polymerization of small sulphur molecules.

Povarennykh AS, Platonov AN, Tarashchan AN, Belichenko VP (1971) The color and luminescence of tugtupite (beryllosodalite) from Ilimaussaq, South Greenland. Meddelelsev om Gronland 181:1-12


     X-rays will turn corundum yellow, but that fades in room light to colorless. I don't think the detailed mechanism is known.

     In fact, many minerals show changes on exposure to x-rays and fade on exposure to visible light; usually we don't know the exact cause of the changes.

     Two books, both rather dated, discuss irradiation colors. I know of no better recent book on the subject.

Przibram, K. (1956) Irradiation Colours and Luminescence, a contribution to mineral physics. Pergamon Press, London.

Marfunin, A.S. (1979) Spectroscopy, Luminescence and Radiation Centers in Minerals. Springer Verlag, NY.

What is luster?

      Luster is an optical property of minerals. There are two main types of luster, metallic and nonmetallic, with an intermediate luster of submetallic. The intensity of the luster depends upon the amount of light reflected from the surface, which is generally related to the refractive index of the mineral. Color usually doesn't have an effect on the luster.

     Metallic luster is produced by minerals with a refractive index of greater than 3. The minerals are opaque and usually are from the native element and sulfide groups.

     Submetallic luster is formed by minerals with refractive indexes between 2.6 and 3.0. Minerals in this group include some sulfides and oxides.

     Nonmetallic lusters are further divided into the following types.

     Adamantine luster is the brilliant luster produced by minerals such as diamond. The refractive indexes for this luster range from 1.9 to 2.6. Minerals with this type of luster are cassiterite, zircon, and diamond. If these minerals have a brown color, the luster is often called resinous.

     Vitreous luster is the luster of glass. The refractive indexes range from 1.3 to 1.9. This luster is found in approximately 70% of the minerals such as silicates, carbonates, phosphates, etc.

     Surface effects of minerals can also affect the luster of a mineral. Diamonds have a greasy luster which is caused by slight roughness of the surface. A waxy luster is produced in cryptocrystalline (extremely small crystal aggregates) or amorphous minerals such as chalcedony and opal. A dull or earthy luster is produced when the mineral grains are small and porous such as in clay minerals. Silky luster is produced by parallel aggregates of mineral fibers such as asbestos or gypsum ( the variety "satin spar"). Pearly luster is produced in transparent minerals with lamellar cleavage such as talc, mica, and gypsum.

     The refractive index is related to the electronic structure of the mineral. If the electrons orbit the atoms at an energy that overlaps or is close to the energy of the electrons in excited states ( created by the absorption of light), the mineral will be opaque and have a high refractive index. If the electron levels (ground and excited) are widely spaced ( light has a small chance of being absorbed and raising the electrons to an excited state), most light will pass through the mineral and the refractive index will be low.

Where can I find information on stereographic projections?

     There are a fair number of resources on the web for the understanding of crystallography.
     One extremely useful site for finding info on the web about links to any mineralogical question is www.uni-wuerzburg.de/mineralogie/know1b.html
     A site that lists a tutorial on stereographic projection sponsored by the international union of crystallography is in their pamphlet 11.
www.lmcp.jussieu.fr/iucr-top/comm/cteach/pamphlets/11/index.html
      A short site with crystallographic notation and stereograms is at www.hpc.susx.ac.uk/~venables/proj/crystal1.html
Another good site is http://ruby.colorado.edu/~smyth/G30102.html
     There are several commercial programs that display crystals and their projections. You can find these by searching ( I use GOOGLE) for Wulff Net and mineralogy.        They have a few screen shots on their web sites that may be useful.

Gordon Nord also replied
     I especially like Steven Weber's JAVA page for stereographic projection

     You can enter in any cell and see how the planes and directions appear on the projection.

What is the difference between schiller and chatoyancy?

Roland Bounds replied

      Chatoyancy refers to the "cat's eye" effect in minerals which are translucent and have internal, parallel, fibrous structures (e g. rutile fibers) which can scatter light. Schiller or Schiller Flash as it is sometimes known, is the bronze or gold internal luster caused by reflections from inclusions and cavities along crystal planes. This is typically a product of solution etching on crystal planes (e g. internal cleavage planes).
      Chatoyancy is used generally to refer to the eye effect in minerals such as corundum, or the 'tigereye' variety of quartz, while Schiller generally refers to the golden flash seen when looking at some of the feldspars such as labradorite. 



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