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Because high symmetry implies that a certain number of directions are equivalent, the positions of the dendrites can be predicted taking into account the crystal system.
The proposed formation model incorporates the structural geology of the deposits with the formation of trapiche and non-trapiche emeralds.Without a link between field observation and mineralogical studies, there is still no consensus on the causes of the growth mechanism and texture acquisition, or on the geological conditions necessary for the formation of trapiche emerald. The trapiche texture is characterized by dendrites and arms, though the specimen lacks a central core. Gems that belong to this group include emeralds from Colombia, corundum (e.g., Müllenmeister and Zang, 1995; Schmetzer et al., 1996, 1998; Sunagawa, 1999; Garnier et al., 2002a, 2002b), tourmalines (Hainschwang et al., 2007; Schmetzer et al., 2011), chiastolites (Rice and Mitchell, 1991; Rice, 1993), and garnets (Harker, 1950; Atherton and Brenchley, 1972; Wilbur and Ague, 2006).Win (2005) distinguished three types of trapiche minerals as a function of their appearance due to coloring elements, other mineral inclusions, or intergrowth of the same mineral. (2011) improved this distinction, defining only two groups: “trapiche” minerals and “trapiche-like” minerals (figure A-1). We also mention two examples of non-Colombian trapiche emerald crystals, one from the Brazilian state of Goiás (Del Re, 1994) and the other from the Mananjary area in Madagascar (Johnson and Koivula, 1998).The only exception is chiastolite, which belongs to the orthorhombic system but is pseudotetragonal ().The symmetry affects the number of arms and dendrites in the trapiche texture, as well as the directions where the dendrites develop. Looking down the crystal axis of a backlit 58.83 ct trapiche emerald from Peñas Blancas (also featured on the cover of this issue).
Photo by Robert Weldon/GIA, courtesyof Jose Guillermo Ortiz, Colombian Emerald Co.
The fluid accumulation at the faults’ tip in the black shales leads to maximum fluid overpressure and sudden decompression and formation of the emerald-bearing vein system.
The authors show that trapiche emerald growth starts at the beginning of the decompression that is responsible for local supersaturation of the fluid.
The hydrothermal fluid comes in contact with the black shale matrix, favoring the formation of emerald seed crystals.
During the growth of these seeds, textural sector zoning occurs, sometimes associated with chemical sector zoning, along with displacement of the matrix.
Two other features observed in trapiche crystals should be mentioned: symmetry and inclusions of organic matter.