How Does a Trapiche Emerald Form?

What is a Trapiche Emerald?

Trapiche minerals are characterized by crystallographically equivalent growth sectors that are separated by more or less sharp boundaries of inclusions.

A rare type of emerald known as a Trapiche Emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern.

Trapiche emerald is a rare variety of the gemstone emerald, characterized by a six-pointed radial pattern of ray-like spokes of dark impurities. It is one of several types of trapiche or trapiche-like minerals, which also include trapiche ruby, sapphire, garnet, chiastolite and tourmaline. The name comes from the Spanish term trapiche, a sugar mill, because of the resemblance of the pattern to the spokes of a grinding wheel.

How Does a Trapiche Emerald Form?
“Wilma Van Der Giessen Collection,” trapiche emeralds. Photo by Jeffery Bergman, © Primagem.

Trapiche emeralds were first described by Émile Bertrand in 1879. They are generally only (and rarely) found in the western part of the Eastern Cordillera basin, in the Muzo, Coscuez and Peñas Blancas mines of Colombia (but likely not in Chivor as reported in older literature). Singular finds in Brazil and Madagascar have also been reported.

Despite its starlike appearance, this unique “spoked” pattern isn’t a case of asterism. However, trapiche emeralds may reveal chatoyancy, a “cat’s eye” effect. Parallel growth tube inclusions can create a cat’s eye in the “pie-shaped” sections as well as, rarely, along the length of whole cabbed trapiche emeralds. Expert lapidaries can orient and cut these stones to bring out this effect.

How Does a Trapiche Emerald Form?
A trapiche emerald from Muzo Mine, Colombia. Photo: Luciana Barbosa

Structural Setting

Emerald is a gem variety of beryl, a cyclosilicate with the ideal formula Be3Al2Si6O18. Its structure is characterized by six-membered rings of silica tetrahedra lying in planes parallel to (0001).

During the formation of an emerald crystal, black carbon impurities may enter the gemstone mix. Because of emerald’s hexagonal crystal structure, these impurities may fill in at the crystal junctions, forming a six-point radial pattern. In some trapiche emeralds, inclusions of albite, quartz, carbonaceous materials, or lutite may outline the hexagonal emerald core. From there, they extend in spokes that divide the surrounding emerald material into six trapezoidal sectors.

The emeralds showed dark, fibrous inclusions between the prismatic edges, starting from the middle of the crystal and enlarging toward the prism’s corners. The inclusions, which seemed to be emphasized by the corrosion, were composed of quartz, muscovite, carbonates, pyrite, and a dark carbonaceous matter (probably with an organic or bituminous origin), sometimes with biotite and kaolin.

Multiphase inclusions with liquid, vapor, and solid phases were also observed. The core of the emeralds had the shape of two opposite hexagonal pyramids with their vertices located in the middle of the crystal. Sometimes these pyramids were so unevenly developed that the core resembled a column. The core was richer in inclusions, some of them more darkish to black, than the rest of the crystal.

Trapiche emerald was characterized by four morphological elements:

  • A central deep green hexagonal prism tapered toward one end and without inclusions
  • Six trapezoidal-shaped prisms extending from the {1010} faces of the central prism and containing opaque inclusions
  • A colorless fine-grained beryl occurring between and within the six trapezoidal prisms but also in the central prism, with opaque minerals (probably altered in limonite) observed on and between the beryl grains
  • An overgrowth separated from the six prisms by scattered patches of opaque inclusions
How Does a Trapiche Emerald Form?
Emerald Trapiche from Muzo Mine, Colombia

    Mode of Growth

    The evidence indicates that growth began with the clear, tapered, central beryl prism in a normal fashion. After a transition stage which produced the albite-containing outline of the central core, growth of the outer sectored region is interpreted as the basis of a eutectic type growth with both beryl and albite growing simultaneously. The beryl grew parallel to the faces of the original prism and a two phase beryl-albite structure grew at the corners.

    In the regions of clear beryl outside the central prism there are striations parallel to and perpendicular to the faces of the prisms. These markings indicate that the lateral faces of the trapiche emeralds grew with a plane interface. This plane interface moved out parallel to the original faces of the prism.

    Because of these markings we have discarded the possibility that the two-phase region of growth from the corners of the prism occurred first, followed by a filling-in of the material at the prism faces. If this had occurred, there would be growth markings out from the corners, rather than parallel to the original prism faces.

    The growth markings indicate clearly that the two-phase growth at the corners did not precede the single phase beryl growth of the flat faces but that the two occurred simultaneously.

    During two-phase growth the two phases usually, but not always, originate from a single pair of nuclei. In the case where a crystal of one of the phases was pre-existing, one would expect, as indeed happened in this case, that the growth of the beryl phase maintains the orientation of the original crystal. This kind of structure commonly occurs in metal eutectic systems where it has been studied extensively.

    One of the phases of the eutetic will grow from a single crystal of that material. The other phase forms either from a single nucleus which subdivides extensively or from many nuclei, and grows in an intimately mixed fashion with the starting phase. Thus the observation that all of the beryl present in a single trapiche emerald has the same orientation can be readily accounted for in terms of the eutectic growth. Multiple nucleation of the ablite probably occurred since it does not appear to be all one single crystal.

    One of the problems in the morphology of these emeralds is to understand why the two phase growth occurred at the corners of the prism' During the growth of a polyhedron, the corners and edges are in a more favorable position for diffusion than the centers of the faces. The concentration of rejected species should therefore be highest near the center of the faces.

    This effect leads to the well-known hopper growth of crystals where the corners and edges of the crystal grow more rapidly, the centers of the faces becoming hollow and depressed. This diffusion effect also Leads to dendritic types of growth. In the present instance, however, the precipitation of the second phase occurred preferentially at the edges of the crystal.

    A similar phenomenon is observed, infrequently, in snow flakes.

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