A Cobalt-Rich Spinel Inclusion in a Sapphire from Bo Ploi, Thailand
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A cobalt-rich spinel inclusion in a sapphire from Bo Ploi, Thailand JINGFENG Guo*, W. L. GRIFFIN+ AND S. Y. O'REILLY* * School of Earth Sciences, Macquade University, NSW 2109, Australia + CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, NSW 2113, Australia Abstract Cobalt-rich spinel is found as a ~ 200 ixm inclusion, together with a glassy phase, in a gem-quality blue sapphire from Bo Ploi, Thailand. This is the first reported natural occurrence of such a spinel. Its composition is directly analogous with that of cochromite, a previously reported rare cobalt-rich chromite. The compositional ranges for the cobalt-rich spinel, obtained using electron microprobe and proton microprobe (methods described below), are A1203 48.18-61.27 %, CoO 19.7-22.84 %, Cr203 0-12.28 %, FeO 8.64-9.67 %, MgO 6.04-6.89, TiO2 0.49-0.73 %, Ni 2251-2532 p.p.m., Zn 335-371 p.p.m., Mn < 177-849 p.p.m., Ga 113-153 p.p.m., Nb 24-1252 p.p.m., Zr <4-167 p.p.m., Sn 22-428 p.p.m., As <4-56 p.p.m., Sr <4-59 p.p.m., Ag 13-64 p.p.m. Transition elements decrease in abundance from core to rim of the spinel while the other elements increase. Crystal chemical considerations suggest that a vacancy-creating substitution mechanism may be operative in the cobalt- rich spinel despite the small scale, i.e. 3Co 2+ = 2Al 3+ + [411--].The glassy phase coexisting with the spinel is likely to be the product of heating by the host basaltic magma. The mode of occurrence of the cobalt-rich spinel prevents further physical investigation. This unusual spinel is considered to be the result of a complex magma mixing process in the lower crust. KEYWORDS: cobalt, spinel, corundum, sapphire, megacryst, alkali basalt, Thailand Introdnction paper reports the occurrence of a spinel inclusion with a unique composition -- a cobalt-rich spinel. INCLUSIONS of one mineral in another reveal valuable information about the physical and Geological background chemical environment of the host mineral at the time of its formation. This is particularly Bo Ploi is located approximately 40 km north of important for the study of gemstones, which are Kanchanaburi, about 130 km to the west of often found in geological contexts that tell little Bangkok, Thailand (Fig. 1). Sapphires are about their origin, e.g. in alluvial deposits or as recovered nearly exclusively from the alluvial/ xenocrysts in volcanic rocks. Earlier investigations eluvial deposits near basaltic outcrops in the of gem-quality corundum (sapphire and ruby) vicinity of Bo Ploi and the mining is concentrated have shown that a wide spectrum of mineral on the so-called 'gem-bearing gravels' which lie a phases (over 30 species) are present as inclusions few m below the surface (Vichit et al., 1978). Some but each gem-field tends to give different inclusion sapphires were reportedly obtained from residual combinations (Giibelin and Koivula, 1986). The soils overlying basaltic rocks (Bunopas and mineral paragenesis and the chronological rela- Bunjitradulya, 1975). At present the Bo Ploi tionship between individual minerals in principle mines are Thailand's major producer of sapphire; could be used to identify the source characteristics this is mainly blue, but occasionally yellow and and the processes involved in the generation of the pink. host sapphires. As part of a project on the genesis Due to the tropical climate of the region, most of sapphire, a recent study of the mineral rocks have been heavily weathered, forming an inclusions within sapphires from various fields extensive coverage of soils and vegetation. has yielded important new information. This However, detailed studies on the alkalic basaltic Mineralogical Magazine, June 1994, Vol. 58, pp. 247-258 Copyright the Mineralogical Society 248 J. GUO ET AL. & Te~l~ ~on~ ~ basal Me~zolc @range p~aeozoe Pr~ambr~n Flo. 1. The geological map of Bo Ploi area, Thailand (Simplified after Vichit et al., 1978). The attached map shows the distribution of Cenozoic basaltic rocks throughout the Southeast Asian region. rocks of Thailand have been carried out and 1975). The Precambrian complexes are inter- yielded significant amounts of information (e.g. bedded metasedimentary sequences affected by Barr and MacDonald, 1978, 1979; Vichit et aL, an upper amphibolite facies metamorphism. The 1978). According to Barr and MacDonald (1978), mineral assemblages of the gneisses include nepheline hawaiite is the only type of basaltic rock microcline, tremofite, plagioclase, quartz, biotite, around Bo Ploi and the erosion in this area has muscovite, apatite and zircon. The massive been so severe that nepheline hawaiite remains marbles contain accessories such as muscovite only as a semicircular plug 300 m in diameter, and tremolite. The Palaeozoic sedimentary succes- representing the old volcanic eruption centre. The sion is composed of limestone, sandstone, phyllite, hawaiitic plug contains abundant spinel lherzolite shale and locally quarzite, and is overlain by minor xenoliths and megacrysts; the most common Mesozoic sediments of similar lithology. Gravels megacrysts are black clinopyroxene, spinel, nephe- and sandy alluvium were widely developed since line, plagioclase, sanidine and anorthoclase. Tertiary time. Although the nepheline hawaiite of Bo Ploi was not dated, it was estimated to be younger than 3 Sample descriptions Ma, belonging to the large Cenozoic Southeast Asia volcanic province (Barr and MacDonald, In this study, more than 20 individual sapphire 1981). Direct observations of sapphire in situ are fragments were selected and optically examined generally rare, but the presence of sapphire in for inclusions. Sample BP8, in which an unusual basaltic rocks at Bo Ploi has been reported as inclusion was found, is a parti-coloured greenish 'quite usual' compared with other sapphire fields blue stone with no fractures. The inclusion is an in Thailand (Gunawardene and Chawla, 1984). elongated multiphase assemblage (composite The basement rocks in the area of Bo Ploi inclusion). The central portion is a dark Co- and range from Precambrian marble and gneiss Al-rich phase surrounded by a relatively softer complexes to the Palaeozoic sedimentary succes- yellowish brown material that appears to be sion intruded by granitic bodies of Cretaceous to homogeneous and glassy (Fig. 2). A number of Triassic age, and minor Mesozoic sedimentary small euhedral crystals of the Co,Al-rich phase are sequences (Fig. 1); (Bunopas and Bunjitadulya, distributed at both ends of the elongated inclusion SPINEL INCLUSIONS IN SAPPHIRE 249 FIG. 2. Backscattered electron images of cobalt-rich spinel in sapphire. (a) The entire composite inclusion with a length of 0.6 mm (Some material was rubbed off at both ends of the assemblage during the grinding- polishing process). (b) A section 20 gm deeper than (a). Numbers indicate the glass areas analysed by electron microprobe. (c) Close-up of the large cobalt-rich spinel grain. The gradual change of contrast from centre to margin reflects the compositional variation of spinel, principally Cr and A1. (d) Sketch map of (c) showing the actual positions of EMP analyses (see Table 1 for data). in contact with the host corundum. Under throughout the sample BP8, and the character- transmitted light, the Co- and Al-rich inclusions istic element zoning of the large crystal, we infer are dark but transparent and isotropic, exhibiting that there has been no heat treatment and that the a deep blue colour near the edge of the grain. The cobalt-rich spinel is a mineral formed by natural largest grain is approximately 250 ~tm x 150 ~tm. processes. Apart from this composite inclusion in grain BP8, albite was found in this study to be a common type Analytical methods of inclusion in Bo Ploi sapphires. Other investiga- tions on sapphires from Bo Ploi have identified After mounting in epoxy, the sapphires were inclusions of K-feldspar, Na-Mg-AI amphibole carefully cut by diamond wheel and repeatedly and pyrrhotite (Gunawardene and Chawla, 1984). ground on 15 lim corundum powder until the It must be mentioned that we cannot confirm mineral inclusions were exposed at the surface. whether the sapphire samples used in the present Subsequent polishing utilized 6 I~m, 3 gm and 1 investigation have been heat-treated. However, lma diamond pastes, which resulted in a good judging from the lack of obvious fractures quality polishing of the relatively soft inclusions. 250 J. GUO ET AL. The prepared samples were then carbon-coated for Proton microprobe (PMP). Minor and trace microprobe examination. element contents in cobalt-rich spinels were Electron microprobe (EMP). A LINK Energy analysed using the proton microprobe at the Dispersive Spectrometer system (EDS) was used to CSIRO Heavy Ion Analytical Facility identify and quantitatively analyse the unknown Laboratory. The facility and the data reduction minerals. The acceleration voltage was set at 15 kV methods have been outlined by Griffin et al. (1988) and the electron beam on the specimen was and described in detail by Ryan et al. (1990). focused down to a 4-5 ltm diameter spot. The Briefly, a beam of 3 MeV protons, produced by a beam current was calibrated on pure cobalt metal tandem electrostatic accelerator, was centred and with a specimen current of 50 nA being used for focused onto the target using an electrostatic routine quantitative analysis. Each spectrum was quadruplet lens. The beam spot on the sample collected during a 100 s counting time. Any beam surface was regulated to approximately 20-30 ttm drift was corrected by remeasuring the beam in diameter, with specimen currents around 10 nA. current for 5 s immediately after the spectrum The X-rays produced by the bombardment were acquisition. The built-in ZAF correction program screened by A1- and Be- filters and then collected was used for quantitative calculation of element by a Si(Li) energy dispersive detector, followed by concentrations.