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Euhedral sinhalite crystals from Sri Lanka.

Euhedral sinhalite crystals, reported here from Sri Lanka for the first time, have yielded morphological data in agreement with that previously published for a crystal from Burma. Thirty-two faces were observed, representing 13 forms for one crystal, and 31 faces and 9 forms for the other. The pale brown color and weak pleochroism are attributed to ferric iron.


Sinhalite, MgAlB[O.sub.4], is among the rarest of the 30 or so gem minerals found in the alluvial gem deposits of Sri Lanka. It was first recognized and described as a new mineral species based on faceted gemstones which were long thought to be brown forsterite (Claringbull and Hey, 1952). Gubelin (1961) noted sinhalite from Eheliyagoda in Sabaragamuwa Province. Gunawardene and Rupasinghe (1986) included a brief description of sinhalite in their report on the gem minerals from the Elahera gem field in Central Province. Henn (1985) characterized a distinctive olive-green sinhalite found at Elahera in 1981. In a systematic of Sri Lanka's gem deposits, Dissanayake and Rupasinghe (1993) reported finding sinhalite in the Ambalantota, Avissawella, Elahera, and Ratnapura quadrangles.

Four additional occurrences of this species have been reported in the literature. It was observed in thin section with serendibite near Johnsburg, Warren County, New York (Schaller and Hildebrand, 1955; Grew et al., 1991a). The first morphological data for the species were determined on a water-worn crystal from Mogok, Burma (Payne, 1958). A pale pink to brownish pink, chromian variety of sinhalite has been reported in millimeter-size grains from a skam occurrence in the Kwakonje area of the Handeni district in northeast Tanzania (von Knorring, 1967; Bowden et al., 1969). It has also been found in drill cores as a mantle about ludwigite grains from the Aldan Shield in eastern Siberia (Grew et al., 1991b).

Although commonly represented in collections of rare gems in sizes up to 240 carats (Arem, 1977; Koivula et al., 1993), sinhalite is not often seen in mineral collections. Recently George Crevoshay, a dealer in Far Eastern gemstones, supplied the Harvard Mineralogical Museum with three sinhalite specimens from Sri Lanka suitable for display. A transparent, pale yellow pebble (#131010) measuring 2.6 x 1.6 x 1.2 cm and weighing 36 carats from Nivitigala, about 25 km southeast of Ratnapura, is intended for a "rough and cut" gem exhibit. The others are transparent, pale brown, rounded crystals from Balangoda, about 25 km east of Ratnapura, intended for the systematic mineral collection. Crystal #1 (#133407) measures 7 x 6 x 5 mm and weighs 0.673 g. It has a damaged region from which a small fragment was removed for optical, chemical, and X-ray study. Crystal #2 (#133408) measures 11 x 7 x 5 mm, weighs 0.717 g and is completely euhedral. As these are the only euhedral crystals of sinhalite known to us, save the one from Burma, a description and comparison with the British Museum crystal seem appropriate. The results are reported below.


The gemstones of Sri Lanka are found in ancient alluvial deposits eroded from Precambrian rocks of the Highland and Southwest groups. These rocks underlie the topographic central highland of Sri Lanka and represent an orogenic belt that runs from northeast to southwest across the island between the eastern and western blocks of the Visayan complex. The Highland group consists of a wide variety of rock types which were metamorphosed to pyroxene granulite grade during the Precambrian. The Southwest group was metamorphosed to the lower pressure cordierite granulite grade. Because the gem minerals are not found in situ, it is difficult to be specific about their mode of formation. Numerous authors have investigated these deposits and have offered various interpretations of the source lithologies. The review by Katz (1971) provides an overview of the relevant geology and sons out the unfamiliar petrologle terminology. Wadia and Fernando (1945) suggest that the gem minerals were derived from pegmatites. Katz (1972) emphasizes the cordierite gneisses and associated rocks, along with some contributions from marbles and pegmatites, as the source of Ratnapura-type deposits. Dahanayake et al. (1980) attribute the gem minerals to garnetiferous gneisses and skarns. Dahanayake (1980) determined the sources to be garnetiferous gneisses, cordierite gneisses, marbles and pegmatites. Specific study of the Weddagala area near Ratnapura led Dahanayake and Ranasinghe (1981) to identify garnetiferous gneisses and granulites as the source lithologies. Munasinghe and Dissanayake (1981) and Rupasinghe and Dissanayake (1985) point to the intrusion of charnockites as forming the source rocks by contact metamorphism (desilication) of aluminous sediments and the intrusion of pegmatites derived from a charnockitic parent. A geologic map showing the distribution of charnockites, garnetiferous gneisses, undifferentiated metasediments, and zircon granite in the Ratnapura-Balangoda area is given in Rupasinghe et al. (1984).

Synthetic sinhalite (Werding et al., 1981) is orthorhombic and shows {100} and {011} as dominant forms with {101} and {102} present less frequently. Twins and trillings on {103} also occur often. Of these forms only {102} was not observed in this study. A quantitative analysis of sinhalite morphology using computed surface energies for an ionic point charge model was made by 't Hart (1979). The computed equilibrium forms are {100}, {101}, {110}, {010}, {1112}, and {021}. All of these forms were observed in this study.


The faces of the sinhalite crystals are finely pitted, which we attribute to etching rather than abrasion. This roughness precluded generation of usable signals during initial trials on a Goldschmidt two-circle goniometer. The crystals were then treated by spraying twice with a clear high-gloss acrylic resin solution, with drying between coats. Similar treatment of a microscope cover glass allowed determination of the coating thickness as approximately 3 [[micro]meter]. Goniometer signals sufficiently sharp to allow measurements with a repeatability of better than 22 minutes of arc were then attained. The crystals were measured in two settings: with (001) polar and with (001) polar. A total of 32 faces representing 13 forms were observed on crystal #1. The forms are given in Table 1, with observed and calculated angles. Three forms ({1011} {140}, and {121}) not observed in the earlier morphological study (Payne, 1958) were found in this work, while the form {132}, observed earlier, was not seen here. The assignment of letters to forms follows that of Payne (1958), except for forms not observed by him. In these cases, the assignments of Dana (1892) for forsterite ("chrysolite") are used. A scanning electron microscope photograph with indexed faces is provided in Figure 1. The idealized crystal drawing, Figure 2, shows 28 faces out of a possible total of 58. The crystallographic orientation used here, a [less than] c [less than] b, is that used by both Claringbull and Hey (1952) and Payne (1958), and also follows the generally accepted orientation for minerals with the olivine structure. This convention differs from current practice for orthorhombic crystals, which is c [less than] a [less than] b, and from that used in the structural study of sinhalite by Fang and Newnham (1965) where c [less than] b [less than] a was employed. Crystal #2 was measured in the same way, with 31 faces representing 9 forms being observed. No new forms were seen on this crystal.


A small fragment, measuring 45 x 58 x 178 [[micro]meter], was removed from the damaged region of sinhalite crystal #1. An optical study of this fragment conducted on a spindle stage showed it to be biaxial negative, with 2V = 55.6(8) [degrees] (calc.), and no dispersion of the optic axes. The indices of refraction are [Alpha] = 1.669(2), [Beta] = 1.698(2), and [Gamma] = 1.706(2), in close accord with those of Claringbull and Hey (1952) and Bank (1977). The optical orientation, identical to that of olivine, is a = Z, b = X, and c = Y, which agrees with Payne's observation that the optic plane parallels the base and that the acute bisectrix is parallel to b. Weak pleochroism was observed, with X = pale yellow, Y = pale blue-gray, and Z = pale pink-gray.

The density of the entire crystal measured with a Bennan balance is 3.47(2) g/[cm.sup.3]. This result compares favorably with 3.47 to 3.50 g/[cm.sup.3] measured by Claringbull and Hey (1952), and 3.49 g/[cm.sup.3] calculated by Fang and Newnham (1965). Crystal #2 yielded an identical result.

Fluid inclusions were observed in both crystals. A daughter crystal was noted in one fluid inclusion in crystal #1. In addition, solid inclusions were seen in crystal #2.


Optical absorption spectra in the range 420 to 1,000 nm were obtained from both crystals utilizing a Sequoia-Tumer Model 340 spectrophotometer with an 8 nm bandwidth. The measurements were made with the crystals immersed in symtetrabromoethane and with the light beam parallel to [010]. Both crystals exhibited a smooth decrease in absorption from 1,000 nm downwards. This behavior contrasts with the results of Henn (1985) on olive-green crystals containing 1.9 wt. % FeO. Henn observed a strong absorption band at 870 nm which, on the basis of the more detailed study by Farrell and Newnham (1965), he attributed to divalent iron. The lack of this band in the present spectra suggests the iron present in the crystals used in this study is trivalent.


The same fragment used for the optical study was mounted in a 114.6-mm Gandolfi camera and exposed to Cu K[Alpha] radiation through an Ni filter. The pattern obtained contained 19 measurable reflections which are consistent with literature values for natural and synthetic [TABULAR DATA FOR TABLE 1 OMITTED] sinhalite (Claringbull and Hey, 1952; Schaller and Hildebrand, 1955; and Capponi et al. 1973). Least squares refinement gave a = 4.327(2), b = 9.887(10), c = 5.678(3) [[Angstrom].sup.3], with a Smith-Snyder figure of merit of 6(0.033,101). These values are very close to those observed by Fang and Newnham (1965) in their refinement of the crystal structure.


The same fragment used for the optical and X-ray studies was mounted, polished, and analyzed with a Cameca MBX electron microprobe. The standards used were Marjalahti olivine for Mg, kyanite for Al, synthetic fayalite for Fe, and synthetic tephroite for Mn. The results were [Al.sub.2][O.sub.3] = 41.41, MgO = 28.83, [Fe.sub.2][O.sub.3] = 1.61, and MnO = 0.03 for a total of 71.88 wt. %. Boron, an essential element in sinhalite, is beyond the range of conventional electron microprobes, but should theoretically be represented by 27.62% [B.sub.2][O.sub.3], bringing the total to 99.5 wt. %. As anticipated from the optical absorption study, qualitative wet chemical analysis showed that all the iron was in the ferric state, thus accounting for the pale color and the weak pleochroism.

A redetermination of sinhalite structure and a crystal chemical study have been made by Hayward et al. (1994). Their results, in contrast to ours, suggest that the iron in sinhalite is divalent, at least in the presence of excess aluminum. The microprobe analysis presented here, however, shows no excess of aluminum.


The authors thank George Crevoshay for supplying the Museum with the sinhalite specimens. The electron microprobe analysis was done by D. E. Lange of the Department of Earth and Planetary Sciences, and the SEM photograph was graciously provided by Yuan Lu of the Division of Applied Sciences. Several obscure references were brought to our attention by Dr. Pete J. Dunn. The manuscript benefited from reviews by Richard C. Erd and Dr. Anthony R. Kampf.


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RUPASINGHE, M. S., BANERJEE, A., PENSE, J., and DISSANYAKE, C. B. (1984) The geochemistry of beryllium and fluorine in the gem fields of Sri Lanka. Mineralium Deposita, 19, 86-93.

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Author:Pitman, Lawrence C.; Hurlbut, Cornelius S., Jr.; Francis, Carl A.
Publication:The Mineralogical Record
Date:Mar 1, 1995
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