The green color was lost by heating to 500☌ and could not be restored by subsequent irradiation, instead the crystals turned violet. Upon heating to 350☌ the violet color was lost, while the green color was preserved. He found that upon irradiation the crystals assumed a violet color in addition to the green color already present. Nassau, 1977, studied green quartz crystals collected by E.S. The color spectrum of these crystals is similar to that of artificially grown green quartz crystals with Fe 2+iron. The heat-induced color changes of quartz crystals have been summarized by Lehmann and Bambauer, 1973, and they report occasional color changes from violet to pale yellow (at about 350☌) to green (at about 500☌) in certain amethysts. The color and the dichroic behavior of these artificially grown crystals is very much like that of the crystal shown in the image. Green crystals with a color very similar to that of prasiolite have been grown in solutions rich in potassium, K, and bivalent iron, Fe 2+iron. Normally the trivalent iron, Fe 3+, will cause a golden color. It is possible to artificially grow green quartz crystals by adding iron salts to the watery solution. It is, however, more likely that the color is not caused by an embedded mineral, but by color centers similar to the ones found in amethyst and smoky quartz. To cause dichroism, the colorizing agent must at least be embedded into the crystal epitactically, that is, in some fixed orientation relative to the crystallographic axes, and must be dichroic by itself. The dichroism is an indication that the color is not caused by randomly oriented and finely distributed inclusions of a mineral (like in eisenkiesel). Like in other dichroic quartz varieties, the effect is quite weak and can only be observed in polarized light. Crystals can show zones of amethyst color.Īt least the specimen I got is dichroic, with the green color varying from more turquoise to more yellow tones. The color varies from a pale gray green to a deep grass green. Prasiolite is a transparent to translucent quartz variety, although clear gem quality material is usually produced artificially from amethyst. This crystal is dichroic the significance of this finding is discussed under " Specific Properties". Some cracks that contain small amounts of a red and brown substance (very likely iron oxides) run through the crystal and its hard to tell if the olive tones are caused by brown inclusions or a true local variation of the crystal's color. The patchy color is more obvious when the crystal is put before a neutral background. Mine carried the label "Minas Gerais" but that label is virtually put on anything from Brazil, so I'm not certain about its origin, and it could just as well be from Bahia. This crystal resembles a prasiolite crystal from Southern Bahia shown on a web page of the Caltech Mineralogy Dept. This is the only specimen I got and the only I ever had a close look at, so what I say about that crystal is not meant to be a general description of prasiolite properties. The color varies from deep green to olive green. The image shows a supposedly natural prasiolite crystal from Brazil. Currently, I have no further information on the methods (trace elements, kind of radiation, heat treatment) for producing green quartz. Recently quartz has appeared on the marked that has been artificially turned green by irradiation. It has been suggested that reducing environments (as opposed to oxidizing ones) lead either to the formation of amethyst that turns green upon heating or to the direct formation of prasiolite (only the latter seems to be true - more on this under "Specific Properties"). Iron compounds with Fe 3+ ions are often yellow or orange, whereas Fe 2+ compounds are green. Usually amethyst turns yellow, orange, or brown when heated, in part due to the formation of iron oxides. Accordingly, it is sometimes called green amethyst, amegreen or vermarine. Most prasiolite is artificially produced by heating amethyst of certain locations, e.g. Many authors had doubts that prasiolite is a natural quartz variety, but there is an increasing number of reports of natural occurrences. Prasiolite is a transparent green quartz.
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