Prasiolite

 

last modified: Sunday, 24-Jul-2011 17:05:31 CEST

Document status: complete

Prasiolite is a transparent green quartz. Many authors had doubts that prasiolite is a natural quartz variety, but there is an increasing number of reports of natural occurrences.

Most prasiolite is artificially produced by heating amethyst of certain locations, e.g. Montezuma, Brazil. Accordingly, it is sometimes called green amethyst, amegreen or vermarine. Usually amethyst turns yellow, orange, or brown when heated, in part due to the formation of iron oxides. Iron compounds with Fe3+ ions are often yellow or orange, whereas Fe2+ compounds are green. 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").

Recently quartz has appeared on the marked that has been artificially turned green by irradiation. Currently, I have no further information on the methods (trace elements, kind of radiation, heat treatment) for producing green quartz.


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The  image shows a supposedly natural prasiolite crystal from Brazil. The color varies from deep green to olive green. 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. This crystal resembles a prasiolite crystal from Southern Bahia shown on a web page of the Caltech Mineralogy Dept. (more links below). 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.



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The  patchy color is more obvious when the crystal is put before a neutral background. 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.

This crystal is dichroic; the significance of this finding is discussed under " Specific Properties".


 

Specific Properties

Prasiolite is a transparent to translucent quartz variety, although clear gem quality material is usually produced artificially from amethyst. The color varies from a pale gray green to a deep grass green. Crystals can show zones of amethyst color.

At least the specimen I got is dichroic, with the green color varying from more turquoise to more yellow tones. Like in other dichroic quartz varieties, the effect is quite weak and can only be observed in polarized light. The dichroism is an indication that the color is not caused by randomly oriented and finely distributed inclusions of a mineral (like in eisenkiesel). 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. 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.

It is possible to artificially grow green quartz crystals by adding iron salts to the watery solution. Normally the trivalent iron, Fe3+, will cause a golden color. Green crystals with a color very similar to that of prasiolite have been grown in solutions rich in potassium, K, and bivalent iron, Fe2+iron. The color and the dichroic behavior of these artificially grown crystals is very much like that of the crystal shown in the image.

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°C) to green (at about 500°C) in certain amethysts. Heating above 600°C pales these crystals. The color spectrum of these crystals is similar to that of artificially grown green quartz crystals with Fe2+iron.

K. Nassau, 1977, studied green quartz crystals collected by E.S. Dana in Brazil in the year 1884. He found that upon irradiation the crystals assumed a violet color in addition to the green color already present. Upon heating to 350°C the violet color was lost, while the green color was preserved. The green color was lost by heating to 500°C and could not be restored by subsequent irradiation, instead the crystals turned violet. Nassau considers the green and violet colors to be based on structurally independent causes and the crystals to be different[1] from "greened amethyst" as described in Lehmann and Bambauer, 1973.

Recently Hebert (2008) has studied the nature and formation of the color in green quartz from Thunder Bay and found evidence for a color center formation due to changes in the chemistry of the watery solution and subsequent irradiation. The color agent is probably iron that substitutes silicon. This would make prasiolite the fifth quartz variety with irradiation induced color centers (the others being smoky quartz, amethyst, aluminum-bearing citrine and pink quartz). Hebert also did heating experiments in which Thunder Bay prasiolite turned yellow, brown or colorless, just like amethyst, upon heating to more than 300°C.

 

Occurrence

Prasiolite is typically accompanied by amethyst. It has been found in metamorphic, igneous and volcanic rocks.

 

Locations and Specimen

Finds of natural prasiolite have been claimed in Brazil, Canada (Thunder Bay), Namibia, Poland, Tanzania, U.S.A. (New Hampshire and Nevada), and Zambia.

A well documented occurrence is that at Farm Rooisand in the Gamsberg region in Namibia (pictured report by G. Niedermayr in Jahn et al., 2006). Here prasiolite occurs together with amethyst in a quartz vein in quartzite. Some of the crystals show alternating zones of pale green and amethyst color. Niedermayr, 1998, reports that both, the violet and the green zones of the crystals seem to be polysynthetically twinned after the Brazil law, as it is typical for amethysts, which has been interpreted as an indication that amethyst is the precursor of prasiolite. However, as pointed out above, this conclusion is probably not correct (Hebert, 2008).



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The first natural prasiolite has been found in the early 19th century in the vicinity of Płóczki Górne, in Lower Silesia, Poland. Here olive-green prasiolite occasionally outlines geodes, usually with an initial layer of agate. The color of these prasiolites is caused by bivalent iron, Fe2+. The photo shows a particularly large specimen (12.5 cm wide) from the locality Wielość, Płóczki Górne. Collection Jerzy Cybulski, Poland.


 

Further Information, Literature, Links

The Caltech Mineralogy Department is currently investigating the cause of the color of green quartz from various locations. Links to images of specimen can also be found here: http://minerals.gps.caltech.edu/mineralogy/Current_Projects/Green_quartz/Index.html
The abstract of a paper in preparation by Laura J. Baker and George R. Rossman can be found at http://minerals.gps.caltech.edu/Manuscripts/in-prep/Greenish_Quartz/Index.htm

The Gemological Association of All Japan has put a research lab report on green quartz online at http://www.gaaj-zenhokyo.co.jp/researchroom/kanbetu/2006/2006_05-01en.html.
It names locations of natural prasiolite, briefly introduces methods for producing them artificially by means of heating and irradiation, and discusses methods for distinguishing natural and heated from irradiated specimen.

Gerhard Niedermayr has published a pictured report on the prasiolite location in Namibia in a ->book about Namibian mineral locations.


Footnotes

1 However, when comparing both publications, I do not quite understand why Nassau thinks there is a difference between both types.


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