Prase

 

last modified: Thursday, 25-Oct-2012 01:29:19 CEST

Document status: complete

"Prase" is a light to dark green macrocrystalline quartz variety. Originally, the term prase was more often used for massive macrocrystalline, sometimes microcrystalline, but rarely for well crystalline forms, but the recent availability of crystals from certain locations sold under that name has led to a shift in terminology. Crystals can be shiny, but usually are dull. I would consider naming a green chalcedony "prase" at least confusing, as there already is a term for chalcedony of similar color: "plasma". There is, however, no strict definition of what prase is. Here I will only describe the crystalline forms.

 

Specific Properties

The color of prase is caused by various green inclusions, more or less evenly distributed in the crystals. Quartz with chlorite inclusions, however green it may be, does usually not count as prase. The most common inclusion seem to be fibrous inclusions of amphibole minerals like actinolite, Ca2(Mg,Fe)5[(OH,F)|Si4O11]2. The pyroxene mineral hedenbergite CaFe[Si2O6] is frequently mentioned, but so far I have not found literature in which hedenbergite is confirmed as inclusions in quartz. In a personal communication Gerhard Niedermayr told me that, to his knowledge, so far all prase specimen studied (from localities including China, Greece, Russia, Italy) had actinolite or ferro-actinolite inclusions, and no hedenbergite.

Since the fibrous inclusions interfere with crystal growth, prase crystals often have peculiar and characteristic shapes that may help to determine the provenance of the specimen.

 

Occurrence

Prase is typically found in so called skarns, rocks that formed when magma of igneous rocks intruded into carbonate rocks. The hot silica-rich fluids that escape from the cooling magma enter the neighboring carbonate rocks and cause chemical alterations and the formation of new minerals. Skarns are often associated with iron ore deposits. Common companions of quartz in these environments are pyroxenes (like hedenbergite), amphiboles (like actinolite), epidote, iron oxide minerals like hematite, and garnets.


 

Locations and Specimen

Prase is only found at very few locations. The most famous ones are the Island of Elba, Italy, and the Island of Serifos, Greece. Other locations include Dalnegorsk, Russia, and recently Morocco.


Greece


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Prase  crystals from Serifos, an island of the Kyklades, have a peculiar shape. They are elongated and often do not show individual rhombohedral faces at their ends. Instead, the ends look like pointed bundles of many small crystals. The overall shape can be cigar-like or hourglass-like, but in both cases smaller crystals sprout from the prism faces, as can be seen in the large central crystal in the image to the right.

The crystals sit on a crumbly grey-green matrix of irregular grains that contains soft aggregates of fibrous mineral fibers. These are often said to be hedenbergite, but I am not sure if they are not made of actinolite that has also been found inside the prase crystals (Hyrsl and Niedermayr, 2003). They are sometimes accompanied by iron roses (hematite), one of them can be seen as a small aggregate of grey metallic crystals at the lower left corner.



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This  is a more typical example of Serifos prase, cigar-shaped translucent green crystals on a crumbly matrix. Again there is a dark gray iron rose sitting between the crystals in the center. As in the former specimen, smaller crystals sprout from the base and the prism of larger ones.



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Dark green quartz with steep - albeit imperfect - rhombohedral faces. The crystal looks as if it was green through and through, but in fact the green inclusions are concentrated in blurred phantoms a few millimeters under the crystals faces. By contrast, the yellow and brown inclusions of iron minerals lie immediately under the surface. The green inclusions of an unknown mineral seem to be dust-like, and not fibrous like in all the other specimen presented on this page. The reason for presenting it here is that the crystal is from a skarn environment and has been found together with garnet and epidote. From the village of Kato Vrondou at the Kato Nevrokopi Basin, Drama Prefecture, Northern Greece. Donated by Anastasios Tsinidis.


Italy

Prase from the island Elba often shows well developed crystal faces and prisms. The nicest pieces of prase I have seen so far are on exhibit at the Museo Minerali Elbani in Rio Marina, which is only a few kilometers away from the classic location Porticciolo.


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I  bought the specimen to the right on Elba, which was not as easy as one might guess, as most shops offer stuff from Brazil, and not the local finds. It was not really cheap either. Strange enough, I've never seen any prase from here outside Elba on a mineral show or at a museum, and neither had a dealer I was talking to recently. At least some specimen are on display on the net.

Locals on Elba call prase "prasio", of course, the Italian name for prase. One could easily confuse prasio with prasiolite, which is very different.

My attempts to find some prase myself were rather unsuccessful, but I've only searched for 2 hours. At least I found a very small green crystal embedded in an asbestos matrix, so I must have been at the right spot. It is at the eastern coast of Elba, south of Rio Marina.

On the first picture you can see light brown needles of the asbestos the crystals are usually found in. The second picture just shows the flip side of the same specimen.


Morocco


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This  is a very dark specimen from Morocco, according to an Internet source possibly Djebel Ajnat, High Atlas. The crystals are arranged in groups of individuals oriented in parallel. The prism faces show a parquet like structure. Fracture is - unlike the conchoidal fracture of pure quartz - uneven. The crystals are translucent and have well developed faces, but the inclusions themselves are invisable. However, on some fractures one can see a very fine fibrous structure apparently caused by enclosed parallel fibers of an undetermined mineral.

Different from the specimen from Elba, there are no signs of fibrous or needle-like minerals in the matrix the crystals grew on. But in a small gap between the crystals I found very fine parallel fibers of gray-green color, possibly asbestos (second picture).

The third picture is a detailed view of a crystal group from the same specimen.


Russia


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Small  crystals of dark prase from Sinerechenskoe, Dalnegorsk, sit on a matrix that contains dark green actinolite needles and brown andradite garnet, both typical minerals of skarn rocks. Actinolite inclusions cause the green color of these prase crystals. Dalnegorsk prase typically shows a sprouting quartz-like pattern on its prism faces.



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A single prase crystal from Russia, very likely from Dalnegorsk, as it shows the typical small sprouting quartz crystals all over the prism faces. One can see individual green actinolite needles at the tip of the crystal, and a dense fibrous fabric of actinolite at its base.


 

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