Mineralogical, chemical, and spectroscopic properties of chrysoberyl crystals recovered from sapphire placer deposits, related to Tertiary volcanic rocks, in the New England gem fields in New South Wales (NSW), Australia, are presented. The samples appeared yellow, yellowish brown, or brown in transmitted light, and some crystals revealed a distinct sectorial zoning between brown i (011) and yellow o (111) growth areas. In reflected light, the i sectors showed a whitish appearance, and cabochon-cut samples with larger whitish i sectors displayed chatoyancy. On the basis of morphological properties, trace-element contents, and absorption spectra, the chrysoberyl samples were subdivided into four different groups, possibly originating from different host rocks. The largest such group, comprised of samples with distinct sectorial color zoning, also revealed a pronounced variation in trace-element levels of titanium, niobium, and tantalum between the different growth sectors. Smaller variations were found for boron, magnesium, and iron, and almost no variation was observed for gallium.
Chrysoberyl is formed through various magmatic and metamorphic processes. Two broad categories of deposits are widely known: those related to pegmatitic activity and those related to high-grade metamorphism. In particular, chrysoberyl is frequently crystallized directly from a pegmatite melt or in a reaction zone between a pegmatitic melt and aluminum-rich host rocks. With respect specifically to the chromium-bearing color-change chrysoberyl variety alexandrite, formation often occurs by reaction of a pegmatite intruding mafic or ultramafic rocks. Chrysoberyl is also found in high-grade (amphibolite-facies or granulite-facies) metamorphic rocks. Augmenting these primary occurrences, placer deposits may be derived from any of the foregoing types (Okrusch, 1971; Soman and Druzhinin, 1987; Franz and Morteani, 2002; Černý, 2002; Barton and Young, 2002; Beurlen et al., 2013). Such secondary deposits of gem-quality chrysoberyl related to high-grade metamorphic rocks are found, for example, in Sri Lanka, India, Tanzania, and Madagascar (Menon et al., 1994; Gunaratne and Dissanayake, 1995; Henn and Milisenda, 1997; Dissanayake et al., 2000; Milisenda et al., 2001; Manimaran et al., 2007). Likewise, secondary deposits related to pegmatites or pegmatites intruding aluminum-rich rocks have been discovered, for instance, in various Brazilian states (Proctor, 1988; Cassedanne and Roditi, 1993; Pedrosa-Soares et al., 2009).
Chrysoberyl recovered with sapphires related to volcanic host rocks, in contrast, is extremely rare. Among the limited discoveries, this type of chrysoberyl has been mentioned in connection with secondary deposits in Australia, related to Tertiary volcanic host rocks, including in Anakie, Queensland (Brightman, 1984); in the New England gem fields of New South Wales (NSW; figure 1) (Coenraads, 1990, 1991, 1995); and in northeastern Tasmania (Sweeney, 1995; Bottrill, 1996). Further mineralogical or gemological information, however, is minimal. For instance, although chrysoberyl was recognized in the late 19th century as occurring in association with gem-quality sapphires in the secondary New England gem fields (Liversidge, 1876, 1888), no comprehensive description of the material is available. Thus, the present study was undertaken to examine a collection of samples from these New England placer deposits and thereby to determine mineralogical and chemical properties for this type of chrysoberyl
MATERIALS AND METHODS
Because chrysoberyl associated with Australian sapphires shedding from Tertiary basalts and pyroclastics occurs only very rarely, miners often do not recognize the crystals. Rather, the stones are mistaken for corundum and sold within parcels of yellow and parti-colored rough sapphires.
The present study began with 39 crystals or crystal fragments and one chatoyant cabochon previously cut from such material. The research material was selected by one of the authors (TSC) from approximately 1 kg of rough, which had been purchased from the late Tom Nunan, one of the larger sapphire miners operating in the New England sapphire fields (Coldham, 2014). The rough parcel was comprised of a collection of atypical-appearing stones that were set aside by sapphire sorters over many years from the production of several mines in the New England region, including Swanbrook Creek, Reddestone Creek, and Kings Plains (for a general overview of the New England sapphire fields, see Coenraads, 1990, 1991, 1994; Abduriyim et al., 2012 a,b). These unusual stones were essentially anything that had caught the eyes of the sorters by virtue of being different from the blue, yellow, green, and low-quality sapphire commonly seen (figure 2). The collection included multiple types of material rare to the area, such as pink, purple, red, and orange corundum; unusually shaped stones; and those with strange color banding. Most stones within this kilogram of rough material were quite small, under 2 ct in weight.
As might be expected from the process of visual selection just described, the possibility remained that the initial 40 research samples (in total weighing about 65 carats) might still contain some corundum crystals. For this reason, all rough samples were first examined by traditional gemological methods, especially in the immersion microscope to facilitate observation of specific growth structures and sectorial zoning. Because some of the smaller crystal fragments did not show any microscopic properties of diagnostic value (i.e., neither characteristic growth structures nor mineral inclusions), these smaller samples were tested by micro-Raman spectroscopy using a Horiba XploRA confocal Raman microscope facility with a 532 nm laser. Micro-Raman spectroscopy was also employed to confirm the identity of several larger crystals. In total, 20 samples were examined by Raman spectroscopy; ultimately, six smaller crystal fragments were identified as yellow corundum. The present study is thus based on the remaining 34 chrysoberyl samples originating from the secondary New England sapphire deposits. To have found and selected only this small quantity of chrysoberyl associated with many thousands of kilograms of rough sapphire related to Tertiary volcanics mined over many years reiterated the rarity of the material.
The weight and size of the research material ranged from 11.71 ct (17.4 × 8.9 mm) to 0.45 ct (5.3 × 2.9 mm) for the rough samples. Coenraads (1995) reported similar sizes in the 10 to 5 mm range for chrysoberyl from the New England gem fields.
To better observe the structures without interference from the rough, heavily corroded and/or mechanically abraded or otherwise contaminated surfaces, certain samples were “windowed” with one or two polished faces, and a small group of transparent stones was completely faceted (figure 3, left). A few samples with larger whitish areas were cut as cabochons showing chatoyancy (figure 3, right).
In the present paper, the term “sectorial zoning” or “sectorial color zoning” is used to describe a different coloration between adjacent growth sectors, while “color zoning” refers to different colors within a specific growth sector. For all the chrysoberyls studied (samples in the as-received state, windowed crystals, or cut samples), growth structures, sectorial zoning, and color zoning were determined by immersion microscopy in methylene iodide using the methods described by Schmetzer (2011). Four cabochon-cut samples showing chatoyancy were examined in reflected light using a Leitz Ortholux II Pol-BK polarization microscope at high magnification (up to 1000×).
To obtain an overview of the qualitative chemical composition, 10 chrysoberyls were tested by energy-dispersive X-ray fluorescence spectroscopy (EDXRF) using a Bruker Tracer III-SD handheld unit.
Quantitative trace-element composition of 12 chrysoberyls was determined by means of laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS), employing a Quantel Brilliant 266 nm Nd:YAG laser coupled to a PerkinElmer DRCe quadrupole ICP-MS. NIST SRM 610 glass was used as the external calibration standard, and Al served as the internal standard. The spot size was set to approximately 50 µm and the frequency to 10 Hz. To determine chemical zoning within the samples, traverses consisting of four to twelve single analysis points were recorded for all 12 chrysoberyls examined by LA-ICP-MS. The analysis was carried out on 55 elements; only those elements with concentrations higher than the detection limits are reported (see table 3).
Absorption spectra were obtained for six of the chemically analyzed samples with a CCD-type Czerny-Turner spectrometer in combination with an integrating sphere (for further details, see Schmetzer et al., 2013a). Only non-polarized spectra were recorded.
RESULTS
The chrysoberyls from the New England placer deposits in New South Wales were free of mineral inclusions when examined using the magnification of the gemological microscope (up to 100×), but they showed variation in chemical composition, internal morphology (growth structures and sectorial zoning), color, and color zoning. Thus, trace-element contents, color, internal growth features, sectorial zoning, color zoning, and spectroscopic properties were used to subdivide the samples into four primary groups, designated as groups I through IV in this study (figure 4). The few samples from the original group of 40 that showed mineral inclusions in the gemological microscope (e.g., zircon crystals with tension cracks) were all identified by Raman spectroscopy as corundum.
Morphology and Growth Features. The different crystal forms present in the New England chrysoberyls are listed in table 1. The habit of the samples was formed by the combination of two pinacoids a and b; three prism faces i, s, and r; and three dipyramids o, w, and n.
When examined in the immersion microscope, there were three main directions of view presenting the major internal growth features: parallel to the a-axis, parallel to the c-axis, and intermediate between the b- and c-axes. A fourth direction intermediate between the a-, b-, and c-axes was of less importance (see also table 1). A characteristic crystal showing the main crystal forms is depicted in figure 5A. In a view parallel to the a-axis, growth features parallel to the prism i and occasionally parallel to the pinacoid b were observed (figures 5C and 6). In a view parallel to the c-axis, growth features and faces seen were the pinacoids a and b, the prism s, and less frequently the prism r (figures 5B and 7). The different growth sectors in this latter view could also show sectorial zoning and color zoning.
In a view between the b- and c-axes and parallel to the [011] direction (i.e., parallel to the prism i and the dipyramid o), the faces a, o, and i were observed, occasionally in combination with a small w dipyramid. This view also revealed the principal variation among the samples in crystal morphology (figure 5, D1, D2, and D3). In some samples, the size of the i prism faces was balanced with the size of the o dipyramids (figure 5, D1). In others, either the o or the i faces predominated. If the o faces were dominant, the i prism was small or not observed (figure 5, D2). If the i faces were dominant, the o dipyramid was smaller (figure 5, D3).
In an intermediate direction between the a-, b-, and c-axes, a combination of i and n faces was occasionally seen (figure 14).
Taking into account color and these just-described morphological features, the four groups were characterized as follows (see table 2 and figure 8, examples I through IV):
Group I: yellow color, o dipyramids dominant, no sectorial zoning, no color zoning
Group II: yellow color, size of o dipyramids and i prism faces balanced, weak to absent color zoning or sectorial zoning
Group III: yellow to brownish yellow or yellowish brown color, size of o dipyramids and i prism faces balanced, strong sectorial zoning in which o growth sectors were yellow and i growth sectors had a whitish appearance in reflected light but were yellowish brown to brown in transmitted light, color zoning in various zones but primarily in i growth sectors (see also figures 9 and 10)
Group IV: brownish yellow or yellowish brown color, i prism faces dominant, sectorial zoning in which small o growth sectors were yellow and i growth sectors were whitish in reflected light but yellowish brown to brown in transmitted light, color zoning mainly in i growth sectors. This group also contained the only two twinned crystals within the 34 chrysoberyls examined (figure 11).
The samples of groups I and II and the predominantly yellow zones of chrysoberyls from groups III and IV occasionally showed an additional slight greenish hue.
The second morphological feature that influenced the habit of the crystals was the relative size of the a and b pinacoids. Chrysoberyls in which the sizes of the a and b pinacoids were balanced showed prismatic habit (figure 12 A,B,D,E), while samples with larger a faces were platy or tabular (figure 12C) and could also be twinned (figure 12F).
In a few samples of groups II and III, subordinate w and n dipyramids were also apparent (figures 12D, 13, and 14).
In certain crystals it was possible to see an additional series of planes inconsistent with the typically observed growth pattern. An example in which such a series of parallel lines crossed the normal growth pattern of a, o, w, and i planes is depicted in figure 15. In the particular example presented here, this series of additional planes was identified according to its orientation to other common growth planes and runs parallel to the dipyramid (114), a face observed as a growth plane neither in chrysoberyls from New England nor in crystals from other locations. The system of planes was not parallel to the common twin plane of chrysoberyl (031) either.