I wanted to offer hearty congratulations for the Fall 2015 issue. Robert Weldon’s cover is stunning, and the lead article on Colombian trapiche emeralds is a thought-provoking, important addition to crystallography and gemology.
I was interested in the authors’ trapiche nomenclature discussion in box A (p. 224). I understand the desire to distinguish sectoral zoning formed by inclusions or texture as “trapiche-like”; the trapiche pezzottaite reported in the Micro-World section of that same issue probably falls under that category (Skalwold and Koivula, 2015). I would argue that the cordierite-indialite intergrowths known as “cherry blossom stones,” described by the authors as “trapiche-like,” actually qualify as true trapiche growth on the basis that each sector is the result of individual crystal growth. These unique trapiches are the result of a very complex indialite-cordierite intergrowth phenomenon in which cordierite crystals grow epitaxially on the six prism faces of an indialite crystal. The latter’s remaining faces continue to grow and later alter to cordierite while grains of this intergrowth’s host accumulate along the sectors, forming the trapiche appearance. The indialite later alters to cordierite and eventually the entire intergrowth is replaced by muscovite, while retaining the outward appearance of the original mineral intergrowth (see Rakovan et al., 2006; Kitamura and Yamada, 1987).
Also, in Nassau and Jackson’s defense, I would add that later in 1970 they corrected the trapiche emerald locality information from their original American Mineralogist paper:
Based on information supplied by E.J. Tripp and L.H. Hernandez and E.J. Tripp (private communications) the actual origin of the trapiche emeralds previously attributed … to the Chivor mine is the Peña [sic] Blanca mine, near Muzo, Colombia. These have the clear center and are distinct from specimens from Muzo itself, which have a dark center. Most Peña Blanca trapiche emeralds were purchased and distributed by Mr. W.F. Bronkie, then manager of the Chivor mine, and were accordingly attributed to the Chivor locality (Nassau and Jackson, 1970). This erratum by the authors themselves is generally overlooked in articles that reference the original study.
Once again, congratulations on a superb paper, which is sure to be a classic reference across geological disciplines.
Elise A. Skalwold
Ithaca, New York
We are glad you appreciated our work. Our judgment on “trapiche pezzottaite” can be based only on your account and not on a direct analysis of the sample. Looking at the accompanying figures, above all figure 7 on p. 327, we think that this pezzottaite sample can be classified as “trapiche-like” because the texture seems related to the zoning (probably sector zoning). There is another sample from Myanmar with similar features (http://www.mindat.org/photo-718381.html); thus, the zoning is likely due to the growth condition in this specific locality. Moreover, there are two other reasons to classify this sample as “trapiche-like”: The first is that the inclusions do not form sharp boundaries separating the equivalent sectors. The second is the absence of organic matter observed in all trapiche minerals.
We disagree with your assessment, because in cherry blossom stones the trapiche-like aspect is due to the first formation of indialite, a mineral with the same point group as beryl. As a result, they have the same growth sectors. But the formation mechanism of cherry blossom stones is completely different, and even more complex, than that of trapiche minerals. In fact, the formation of cherry blossom stones requires intergrowth, phase transition, and pseudotwinning. On the contrary, a trapiche is a single crystal where the growth sectors are separated by inclusions.
Thank you for the complementary information about trapiche emerald origin from Nassau and Jackson. We were not aware of their correction, which is never cited in the literature. At the October 2015 International Emerald Symposium in Bogotá, researchers consistently referred to Chivor trapiche emerald. We are pleased to see that our field investigations in the Chivor-Macanal-Gachala mining districts were in keeping with the correction, and that in fact trapiche emerald is only present in the western emerald zone. Your letter will help to definitively correct this error of location.
University of California, Los Angeles
Institute of Research for Development, Toulouse, France
French National Center of Research, Vandœuvre