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     What is the definition and method of formation of a "gwindel crystal"? 

     What is "faden" quartz?  

     What causes the purple color of amethyst? 

     What causes the blue color of quartz? 

     What is picture jasper?

     What is Ouro Verde?

     What minerals are found as inclusions in quartz?


What is the definition and method of formation of a "gwindel crystal"? I have seen the term applied only to quartz. Can a similar process result in a gwindel crystal of another mineral such as celestite? 

     Gwindel is a term used by the Swiss Strahlers ( mineral collectors/climbers) in the Alps of Europe for groups of stacked, twisted quartz crystals. These crystals are usually lying flat to the vug walls ( parallel to the "c" axis).
     There are several types of gwindels; regular, half-opened, or closed ( very rare habit - aka as sucre (sugar)). In France, gwindels are known as peigne (comb). The main localities are the Alps and Russia.
      Probably the best source of pictures of gwindels is the January/February 2000 issue of Rocks & Minerals ( Alpine Minerals - the Tucson show theme).
There are a number of different minerals which can be twisted. These include copper, silver, millerite, stibnite, phosgenite, celestite, and quartz. In quartz, crystals have been found twisted along the c-axis. Quartz fibers in chalcedony have also been found to have a twist.
     There are a number of minerals that consist of stacked crystals with a slight twist. These include spiral growths of aragonite from the Southwest mine, Bisbee, Arizona, which looks like screws, and coils of calcite. (Mineralogical Record September/October 1981)
      Rings have been formed from minerals such as pyrite, millerite, malachite, jamesonite, and boulangerite.
      There is an interesting article in the September/October 1998 issue of Rocks & Minerals on curly malachite from Austria. The author goes through the theory behind the crystal growth and considers that the strain introduced by substitution of zinc for copper in the malachite is the root cause of the curly crystals.

What is "faden" quartz?

     Faden quartz is a descriptive term for a usually tabular group of quartz crystals with a white thread-like or string-like zone running through the interior. The term faden is a German word for "thread". They are found in the Alpine region of Europe, Pakistan, Russia and Arkansas.
     They are found in areas of low grade metamorphism in which cavities in the rock are growing. When these zones grow, the quartz crystals in them are broken (repeatedly), and the healing and regrowth incorporates fluid and gaseous inclusions ( the white threads). The edges of the crystals grow faster and incorporate the liquid inclusions in the center of the crystals.
     An excellent discussion of fadens is R. Peter Richards (1990) "The Origin of Faden Quartz" Mineralogical Record  v21 pp 191-201.

For examples of pictures of faden see:



What causes the purple color of amethyst?

      The color in amethyst comes from color centers in the quartz. These are created when trace amounts of iron are irradiated ( from the natural radiation in the rocks).
The purple color in ghost town glass comes from small amounts of manganese in the glass when it has been exposed to ultraviolet light. The manganese was used as a clarifying ingredient in glass from 1860 to 1915. Prior to that, lead was used, and subsequently, selenium is used.
       Two books that have sections on the creation of color in quartz are the third volume on silica in Dana's System of Mineralogy ( the seventh edition in three volumes). The MSA Reviews in Mineralogy had a chapter on color formation in volume 29 Silica ( an excellent explanation of the various color causes in quartz). 

      Quartz will commonly contain trace amounts of iron ( in the range of 10's to 100's parts per million of iron). Some of this iron sits in sites normally occupied by silicon and some is interstitial ( in sites where there is normally not an atom). The iron is usually in the +3 valence state. Gamma ray radiation ( from nuclear decay in the surrounding rocks ) can knock an electron from an iron lattice site and deposit the electron in an interstitial iron. This +4 iron absorbs certain wavelengths (357 and 545 nanometers) of light causing the amethyst color. You need to have quartz that contains the right amounts of iron and then is subjected to enough natural radiation to cause the color centers to form. 

What causes the blue color in quartz?

     The blue color in quartz results from Rayleigh scattering in the quartz ( similar to why the sky is blue). If you have a thin slab of the material, it will be blue in reflected light, while it will be orangeish-brown in transmitted light.
     In studies of blue quartz from the Llano uplift in Texas, inclusions of rounded ilmenite with a diameter of 0.06 mm at a concentration of 125 per cubic micrometer were found by TEM. A similar type of quartz was also present in the Roseland district of Virginia. ( see Zolensky et al 1988 American Mineralogist pp313-323)
      There have been some synthetic blue quartz colored by cobalt or iron, but these are not known to occur naturally.

Gordon Nord  wrote:
     Rayleigh scattering requires a refractive difference between the host mineral (quartz) and the inclusion (ilmenite). Also the inclusions must be 1/4 the wavelength of light to backscatter blue visible wavelengths. This means the ilmenite precipitates in quartz must be on the order of 100 nm or 1/4 of 4000 nm wavelength. In a Transmission electron Microscopy study of Roseland Blue Quartz, bluer than Llano Uplift quartz, the ilmenite precipitates were disk-shaped, 100 nm in diameter and widely separated. Amazingly you could see the precipitates using a light optical microscope. At 1000x they appear as fine dots dusting the volume of the quartz. You can focus up and down through the quartz and see the precipitates go in and out of focus. The coarser rutile needles do not contribute to the blue scattering.

     If the precipitates are in the same orientation star effects can be generated. 

What is picture jasper?

     Jasper is an impure form of cryptocrystalline quartz (a fine grained variety). It is related to agate, chalcedony, flint, and chert. It commonly will contain a significant amount of iron oxide and hydroxide minerals. Open veins in the rock will often have water solutions percolating through faults. Under certain conditions, the dissolved ion content of the water becomes supersaturated and minerals will precipitate out( similar to "hard" water). There can be changes in the solution over time such as the amount of oxygen in solution which will give rise to differences in the oxidation state of iron ( red - highly oxidized, green - a more reduced state). The different colors in the picture jasper can arise either during the initial deposition or more oxidizing solutions seeping through the more permeable layers and fractures within the jasper masses.
      The picture part of the jasper is that the patterns in the rock look like scenes from the western US. There are many localities worldwide that produce picture jaspers, but some of the best come from eastern Oregon and Idaho.
A  website that has pictures is  here

What is Ouro Verde?

     Ouro Verde is a quartz from Brazil that has been artificially irradiated to produce a yellow green color. ( i e. it is put in a nuclear reactor and the neutrons change the color of the quartz) 

What minerals are found as inclusions in quartz?
      Tourmalinated quartz are crystals of quartz that have inclusions of tourmaline in the quartz. When the quartz crystals grow, there are often inclusions of other minerals and the solutions from which the crystals grew ( fluid inclusions can be used to tell the conditions under which the crystal grew).
      Common minerals which occur with quartz include rutile, hematite, micas, actinolite, and riebeckite. A number of varieties are named for specific inclusions such as rutilated quartz ( rutile), Venus hair stone ( rutile), Thetis hair stone ( actinolite), and Cupid's darts ( tourmaline).
      Inclusions can also show a younger stage of growth inside a crystal as a phantom within the crystal. See also article on varieties of quartz.


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