A Gemme is known as trapiche when one sees figures of stronger concentration in chemical elements in the stone. If it is about a emerald (hexagonal crystalline system) one observes a Hexagone, if it is about a Corindon or of a Tourmaline (trigonal) one observes Triangle S.

Example of the emerald :

This phenomenon is due to a temporary enrichment of the medium in elements, such as for example the Chrome for emerald.

In spite of its pace the emerald trapiche (Spanish word indicating a toothed wheel used in the crushing of the ores) is not a macle, but a monocrystal. It is presented in the form of made up of a central core surrounded by 6 " rayons" who developed parallel to the faces {10.0} prism. The interstices are filled with a cream-coloured mixture, accentuating contrast.

The most outstanding samples come from the Gachalá mine close to Chivor, located at the center of Colombia. The area of Chivor with Gachalá forms the Eastern district of the emerald mines. The very folded zone separating Gachalá de Chivor is the seat of the fault San Fernando. This fault separates the paleozoic metamorphic rocks (530 - 245 my) from the deposits limestone alternating with deposits of black schists rich in organic matters (Cretaceous inferior 130 - 95 my). Indeed, the Be++ ions find their origin in these schists where they were captive manganese and iron oxyhydroxydes. Many faults parallel with the plans of sedimentation support the movement of the fluids minéralisateurs. For the images of this emerald to consult: http://www.agab.be/mineralogie/album/juil-aout2002.html

Crystallization

• Phase 1: The crystallization of the central prism occurs then grows rich out of white feldspar which crystallizes in the shape of fine crystals mixed with beryl; it is formed an eutectic which continues as much as there exists an arrival of fluids. An interruption creates a crust on the faces of the initial prism.

• Phase 2: It begins again at the time of a new contribution of fluid and is done the faces of the initial prism parallel to. The eutectic chooses the edges of the prism, only sites of growth available. It is necessary that the water-mothers are saturated with white feldspar for the continuation of the growth.

Except Colombia, the emerald trapiche of greenish grey color is known only in Madagascar.

See too