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First draft: November 20^th, 2013 | Last update: April 30^th, 2016 | References: 89

True Trapiche Quartz from Colombia?

A New Find from Boyacá and Santander Departments, Colombia.

While visiting a mineral and gem fair in 2013, I came across a well-known Colombian emerald dealer based in Idar-Oberstein, Germany, who was offering sliced and polished quartz crystals (Figure 1). These slices were labelled as "trapiche quartz" and were said to originate from the Boyacá and Santander Departments in Colombia.

The dealer explained that he wasn't sure these stones actually were trapiche, but that he still had acquired the entire parcel because he was intrigued by their appearance (to be true, I would have done the same). He also said that when the slices were observed through the polariscope (Figure 3) they revealed similarities with the pattern of the trapiche emeralds he had in his inventory.

Figure 1.- The samples of "trapiche quartz" acquired. The smallest slice has a cross section of about 18.5mm and weighs 20.17 carats. The largest slice measures (21.9 x 25.0)mm and weighs 26.95ct. Thickness of the slices varies between 4 and 5 millimeters.

The designation "trapiche" refers to a growth phenomenon that has been defined by the skeletal and sectoral growth of a crystal, yielding a fixed star when observed in sections perpendicular to the c- and optic axis of the crystals (Schmetzer et al., 2011). From a historical point of view, it was first identified in emerald (Bertrand, 1879) and because of its resemblance of the spoke wheel once used to crush cane sugar, it was baptised "trapiche" (McKague, 1964).

After its discovery in emerald in 1879 trapiche has been studied intensively to explain the growth process responsible for the structure (Bernauer, 1926; McKague, 1964; Leiper, 1967; Schiffmann, 1968; Chaudhari, 1969; Nassau & Jackson, 1970; Tripp & Hernandez, 1970; O'Donoghue, 1971; Nassau, 1978; Sinkankas, 1981; Themelis, 1987; DelRe, 1994; Overton, 2005).

For long the phenomenon was thought to occur solely in emerald, but it also occurs in the chiastolite variety of andalusite. Then came the discovery of trapiche rubies in Mong Hsu in Myanmar (Burma) (Müllenmeister & Zang, 1995; Henn & Bank, 1996; Schmetzer, et al., 1996; Schmetzer et al., 1998; Sunagawa et al., 1999; Garnier et al., 2002; Win, 2005).

Trapiche sapphires followed quite soon (Koivula et al., 1994; Beaton, 2008; Kotchanin et al., 2009; Kiefert, 2012), to be followed by beryl not long after that (Koivula, 2008). However, a similar phenomenon involving unequal growth rates of beryl crystals seems to have been described earlier on (Sahama, 1966). Cordierite may display the phenomenon as well (Rakovan et al., 2006; Rakovan, 2007).

Recently, trapiche tourmaline originating from Zambia (Hainschwang et al., 2007; Schmetzer et al., 2011) was added to this list. Not much later, a "trapiche garnet" was shown to the world on GemologyOnline.com, clearly showing the characteristic pattern.

And also trapiche quartz is found. According to Mindat.org, it is rare and has been found in the Dorogawa-Goyomatsu mine, Honshu Island, Japan. No slices of this material are known so far. Recently, some cabochons appeared on the market which were said to originate from the Mogok Stone Tract in Myanmar (Burma). Some very odd slices were also produced from a find in Canada.

Figure 2.- A trapiche emerald showing fine growthy sectors found on a gem fair (courtesy of D. Sanchez). Trapiche gems are defined by their sector growth structures, colour zoning, and the presence of hallmark inclusions separating the sectors.

The structure observed in the crystal slices acquired, that is, a fixed star separating different sectors, does show analogies with other trapiche gems but the phenomenon only becomes fully apparent when the slices are examined through the polariscope (Figure 3). The core and arms seem to be very clean which is the absolute contrary of trapiche growth known so far in the mineral and gem world.

Three phenomena seem to appear together in this material: (1) sector zoning following the negative and positive rhombohedron and probably also following the prism, (2) Brazil law twinning in each of the sectors, and (3) different concentrations of thread- or fiber-like inclusions in each of the sectors instead (Schmetzer, Hyrsl, 2015). However, in trapiche gems, inclusions define the fixed star and in this case clear zones ensure contrast.

Due to this feature which does not correspond with the earlier definition as issued by Win (2005), and later refinements made by Schmetzer et al. (2011) concerning symmetry, colour-zoning, and inclusions, the reverse situation of the fibrous inclusions being present in the sectors and slightly overlapping some rather than in the separations between them, suggests that the crystals grew in a completely different manner.

Krzemincki & Laurs (2014) examined and described the material and state that "[this] phenomenon is not the result of growth dynamics responsible for the trapiche pattern". Therefore the designation trapiche is not appropriate. As recent as April 2016, Jaroslav Hyrsl showed me slices of exactly the same material but originating from Dalnegorsk in Russia's Far East.

Photomicrographs

Legend: FoV: Field-of-View; DoF: Total focusing depth; DF: darkfield; Obl: oblique illumination (fiber optics); Pol: polarising filter before objective; Sh: shadowing; TM: transmitted illumination, UVL: long-wave ultra-violet radiation; UVS: short-wave ultra-violet radiation; X-Pol: crossed polarising filters

A general impression of one of the slices and its striking characteristics
6x (FoV ± 25mm) DF, Obl, Pol

Clear areas overlap the boundaries between growth sectors - the opposite pattern observed in "real" trapiche gems

Detail of the thread-like inclusions observed in the cores of the sectors
18x (FoV ± 3mm) Obl, Pol

9x (FoV ± 6mm) DF

Twinning highlighted by interference colours in the section shown on the left
9x (FoV ± 6mm) X-Pol

6x

6x

9x

9x

Airy's Spiral observed through the conoscope clearly demonstrates Brazil Law twinning in one of this slice's growth sectors

Higher-order interference colours are observed when the section is tilted
6x X-Pol

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