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"Smaragdite"

Allalin-Gabbro is a meta-gabbro with omphazite and actinolite

Foto: K. Sieber, www.makrogalerie.de

 

The name "Smaragdite" was used in the past for a whole range of rocks which had in common that they contained one or more grass-green minerals. Therefore, it is not surprising that there are a variety of different definitions for this term in literature. Most aptly, "smaragdite" is defined in the book "Große Enzyklopädie der Steine" (Great Encyclopedia of Stones) as "a term for zoisite or for actinolite or for diopside or for green quartz (Prasem) or as a trade name for a green glass melt as an emerald imitation" [Börner & Hill, 2012]. It should be noted that a rock from Piedmont (Italy) that contains omphacite is currently also offered as "smaragdite".

Amongst elderly collectors, the term is usually understood as a rock with grass-green, long prismatic actinolite crystals. Interestingly, the term "smaragdite" is only used for rocks from the European Alps, which is why it is to be considered a "local term".

One of the rocks formerly described as "smaragdite" is the so-called "Allalin Gabbro" originating in the Swiss Saas Valley. It is mined at the Mattmark water reservoir, but boulders can also be found far down in the valley. In geology, they are of great importance as they are used as keystones to determine the flow paths of the glacial Saaser and Rhone glaciers.

Mineralogically speaking the rock is a metamorphically altered olivine gabbro. During the drawdown into great depths, minerals of the eclogite facies (omphacite, jadeite, pyrope, zoisite) were formed. During a subsequent heaving and decreasing metamorphous conditions actinolite, albite and quartz were formed locally. Therefore, the rock can also be considered as eclogitized gabbro. The petrographic appearance of "Allalin Gabbro" is very irregular. The green components can consist of omphacite and/or actinolite. Even magmatic olivine and augite have been preserved here and there. The white areas contain different amounts of jadeite, zoisite, albite and rarely quartz. In addition, garnet rich in pyrope are found in intermediate areas.

References:
Börner, K. & Hill, D. (2012): Große Enzyklopädie der Steine; 15. Aufl.; Abraxas Verlag; Hasede, ISBN 978-3-934219-17-5.
Bucher, K.; Grapes, R. (2009): The Eclogite-facies Allalin Gabbro of the Zermatt-Saas Ophiolite,Western Alps, J. of Petrology, 50, 1405 -1442