H.F.Shurvella L. Rintoul and P.M. Fredericks Centre for Instrumental and Developmental Chemistry, a Present address: Department of Chemistry,
Introduction Jade (Jadeite & Nephrite) The American Geological Institute defines "jade" as a hard, extremely tough, compact rock consisting of either the pyroxene mineral jadeite, or amphibole group minerals typically tremolite and actinolite (known as nephrite). Early humans were aware that the hardness and toughness of nephrite jade made it suitable for axe-heads and other tools. Jade has an unevenly distributed colour ranging from dark green to greenish white. Other colours such as pink, brown and black may also be found. The appearance varies from translucent to nearly opaque. The mineral takes a high polish and has long been used for carved articles, jewelry and various ornamental objects. Jadeite is by far the rarer of the two minerals and its occurrence has been somewhat of a mystery since it was known for a long time only as stream-worn boulders from Burma, or carved objects from Mayan ruins in Mexico and Guatemala. "Archaic jade" objects from China are made from nephrite, since jadeite did not come into common use in China until the Ching Dynasty (mid 17th century) [1]. Properties of Jade Minerals Pyroxenes Pyroxenes are a group of minerals of the inosilicate class that have single chains of silicate tetrahedra aligned along the c-axis on the unit cell. They crystallize in two different systems, orthorhombic (orthopyroxene) and monoclinic (clinopyroxene) [2]. Jadeite is a clinopyroxene and the chemical composition is NaAlSi2O6 (see Fig. 1). Diopside has a similar composition to jadeite but with Na and Al replaced by Ca and Mg, respectively. The name jadeite may be applied to members of the jadeite-diopside compositional space where substitution of Na and Al is limited to approximately 20%. The hardness of jadeite is 6.5-7.0 and the density is 3.3-3.5.
Amphiboles Amphiboles are a related group of inosilicate minerals, but contain hydroxyl (OH) groups. Amphiboles have double chains of silicate tetrahedra aligned along the c-axis of the unit cell. They crystallize in two different systems, orthorhombic (orthoamphibole) and monoclinic (clinoamphibole). Tremolite is a clinoamphibole and has the chemical formula, Ca2Mg5Si8O22(OH)2. The structure of tremolite is shown in Fig. 2. Actinolite has a similar composition, with much of the Mg replaced by Fe, and occasionally by other metals, such that its composition may be represented by the chemical formula, Ca2(Mg,Fe)5Si8O22(OH)2. Nephrite is a compact, microcrystalline rock composed of tremolite-actinolite solid solutions. Nephrite is not quite as hard as jadeite, but before the discovery of metals it was used extensively for stone tools [2]. The hardness is 5.5-6.0 and the density is 3.0-3.4.
Figure 2. The crystal structure of tremolite viewed down the c axis Previous Vibrational Spectroscopy
Pyroxenes and Jadeite Raman spectra The vibrational frequencies of the monoclinic pyroxene mineral, diopside (CaMgSi2O6) have been calculated and compared with experimentally determined frequencies [5]. Assignments were made to Si-O stretching modes near 1000 cm-1, Si-O-Si stretching modes near 650 cm-1 and O-Si-O deformation modes near 500 cm-1. External modes of the monoclinic unit cell were calculated to lie below 300 cm-1. A Raman spectroscopic study of pyroxene structures containing varying amounts of iron and manganese was reported at a recent conference [6]. Raman bands characteristic of pyroxenes were observed in the 980-1028, 650-700 and 300-400 cm-1 regions. These bands were attributed to vibrations of the SiO4 tetrahedra. Clinopyroxenes show only one intense Raman band in the 650-700 cm-1 region, while orthopyroxenes have two intense bands in this region. Observed wavenumbers in the Raman spectra of the three pyroxene minerals enstatite, clinoenstatite and diopside have been reported [7]. The spectra of all three minerals are similar, but enstatite and clinoenstatite have strong doublets near 670 cm-1, while diopside has only a strong single peak in this region. The Raman spectrum of a sample of a confirmed jadeite jade sample from Burma has been recorded [8] and a spectrum of Burma jadeite is available from an instrument manufacturer [9]. In these spectra, a very strong band at 699 cm-1 and two strong bands near 1000 cm-1 are characteristic of jadeite. Amphiboles and Nephrite Infrared spectra A review of the infrared spectra of amphiboles has been published [10]. In the review, the emphasis is on the hydroxyl stretching region, but the vibrational spectra of the silicate anions are also discussed. An infrared spectrum of tremolite from 2-15m (5000-667 cm-1) is included in a paper on infrared absorption studies of some silicate structures [11]. Wavenumbers of bands observed in the infrared spectra of actinolite, tremolite and nephrite are included in an early collection of tables of wavenumbers of minerals and related inorganic compounds [12]. The spectra of the three minerals all contain absorption bands between 3700 and 3400 cm-1 due to stretching vibrations of the OH groups. There is also a very strong broad absorption band between 1150 and 900 cm-1. This band has several shoulders and peaks near 1100, 1060, 1000 and 950 cm-1. These absorptions are attributed to Si-O stretches and Si-O-Si stretching. Other absorption bands are observed between 700 and 400 cm-1. These absorptions are assigned to O-Si-O deformation modes. Raman spectra A Raman spectrum of actinolite was included in a recent study of the cation distribution in amphiboles [15]. Most amphiboles belong to the monoclinic system with space group C2/m. A factor-group analysis yields the number and activity of the vibrational modes of the unit cell as: 30Ag(R) + 30Bg(R) + 28Au(IR) + 35Bu(IR). The internal vibrations of the two Si4O116- ions of the unit cell contribute the following modes: 20Ag(R) + 19Bg(R) + 19Au(IR) + 20Bu(IR). A very strong line near 670 cm-1 in the Raman spectrum of actinolite was assigned as an Ag mode. Wang et al. [15] also assigned weaker bands near 400 and 200 cm-1 to Ag modes and eight weak features between 1100 and 150 cm-1 to Bg modes. A group of three lines (strong, medium, weak) near 3680 cm-1 was also shown in the article. An earlier report of the Raman spectra of a group of magnesium silicate minerals included a listing of the observed lines in the spectrum of tremolite [16]. Raman spectra of actinolite and tremolite from 3800-3600 and 1050-100 cm-1 were shown in a recent article describing a Raman microprobe study of archaic jades [1]. Spectra of tremolite and actinolite were shown to be similar in the 1050-100 cm-1 region, but different in the 3800-3600 cm-1 region. This paper included a table of wavenumbers of bands observed in the Raman spectra of nephrites from Canada and Taiwan. The Caltech collection of Raman spectra of minerals [17], includes spectra of tremolite and actinolite between 1200 and 100 cm-1. Previous Vibrational Spectroscopy A recent article [1] describes a Raman spectroscopic study of archaic nephritic jades using a Raman microscope. Raman spectra (1050-100 cm-1) of numerous minerals that might be associated with archaic jades are shown. Minerals such as brucite, chromite and rutile were detected in archaic jades. Opal and hematite were also detected in some artifacts. In this study we examine a variety of jade minerals and objects to evaluate the effectiveness of IR and Raman techniques for the identification and characterisation of jade artifacts Experimental Methods Jadeite samples Several jadeite samples were examined in this study. Dr. Grahame Brown, of the Gemmological Association of Australia supplied Blue/green, blue/white and brown polished jadeite cabochons and a sample of Burma jadeite. Mr.Bernard Peckover, a Brisbane collector/dealer in jade objects, supplied a small grey-green carved jade elephant and two large green cabochons. Nephrite and nephrite mineral samples Three nephrite jade minerals, tremolite, actinolite and nephrite were examined in this study. Dr. T. Kloprogge of the Centre for Instrumental and Developmental Chemistry, Queensland University of Technology supplied the samples of tremolite from Campo Longo, Italy and actinolite from Froland, Norway. Sawn nephrite tablets were supplied by Mr. Bernard Peckover, who also submitted several carved archaic nephrite jade objects for spectroscopic examination. These were: three carved objects (a dancing woman, a person wearing a skirt, and a carved blade) believed to be from the Han Dynasty (200BC-200AD), a simple carved bird believed to be from the Lonshan Culture (~3500BC) and several other carved objects of unknown origin. A greenstone pendant from Greymouth, New Zealand and various items of jade jewelry were also examined. A selection of jade objects is shown in Figure 3.
Infrared Spectra Infrared reflectance and micro-ATR spectra of various jade objects were obtained using a Perkin-Elmer System 2000 FT-IR spectrometer equipped with a Perkin-Elmer i-Series infrared microscope, which had both standard and ATR objectives. The ATR objective was a Si internal reflection element. Kramers-Krönig transformations were carried out on the external reflectance spectra. Raman Spectra Raman spectra were recorded using a Renishaw Raman microscope or, when intense fluorescence was encountered, on a Perkin-Elmer System 2000 FT-Raman spectrometer. For Raman microscopy a 50X objective was usually used and the spectra were excited by the 633 nm line of a He/Ne laser or by the 785 line of a diode laser, operating at 8 mW at the sample. The excitation laser for the FT-Raman spectra was a Nd:YAG laser of 1064nm wavelength, which could be operated at powers up to 500mW, but lower powers were usually used to minimize heating of the sample. Results and Discussion Figure 4 compares the infrared spectra of nephrite and jadeite minerals in the 1200 to 400 cm-1 region. There is a superficial similarity between the infrared spectra of jadeite and nephrite, as might be expected since both minerals are silicates. However, there are clear differences that enable the two types of jade to be identified. While both spectra show very strong broad absorption in the 1000 cm-1 region due to various silicon-oxygen stretching modes, the spectrum of nephrite has a larger number of resolved peaks compared to that of jadeite. Also, nephrite has two sharp peaks near 756 and 685 cm-1, which are not present in the spectrum of jadeite. Another distinguishing feature is the absence of absorption in the OH stretching region of the spectrum of jadeite.
Infrared spectra of the minerals tremolite and actinolite are shown between 3700-3600 and 1200-400 cm-1 in Figure 5. In the 1200-400 cm-1 region the spectra are almost identical. The only major difference between the two spectra appears to be in the OH stretching region, where the spectrum of tremolite has one band at 3673 cm-1, while actinolite has three (3673, 3659 and 3644 cm-1). The infrared spectrum of nephrite (Figure 4) is almost identical to that of actinolite. Table 1 contains the observed wavenumbers in the infrared spectra of tremolite, actinolite, nephrite and New Zealand greenstone. Comparison of columns 3 and 4 of Table 1 shows that New Zealand greenstone is in fact nephrite. The first column of Table 2 gives the wavenumbers obtained from the infrared spectrum of a sample of jadeite mineral.
Figure 5. Infrared spectra of tremolite and actinolite
Table 1. Infrared spectra (wavenumbers,
cm-1)a of tremolite,
Table 2. Infrared spectra (wavenumbers, cm-1)a
Figure 6 shows the Raman spectra of samples of the jade minerals nephrite and jadeite between 1200 and 200 cm-1. The observed wavenumbers for nephrite are found in column 3 of Table 3, which also lists the wavenumbers for the Raman spectra of tremolite, actinolite and greenstone. The first column of Table 4 contains the wavenumbers reported in the literature from a Raman spectrum of Burma jadeite [8].
Figure 6. Raman spectra of (A) nephrite and Again, there is a superficial similarity between the Raman spectra of jadeite and nephrite. Both spectra contain a very strong band near 700 cm-1, which is attributed to symmetric Si-O-Si stretching. There are also two bands near 1000 cm-1 and a group of bands near 400 cm-1. In nephrite spectra the symmetric Si-O-Si stretching band is observed near 675 cm-1, while in jadeite spectra the band is observed ~25 cm-1 higher near 700 cm-1. The prominent feature of a jadeite Raman spectrum is the very strong sharp band near 700 cm-1. Other characteristic features are a doublet consisting of a strong band near 1040 cm-1, with a band of lower intensity near 990 cm-1 and a strong band near 375 cm-1. This band usually has a shoulder on the high wavenumber side. The Raman spectrum of nephrite, like that of jadeite, is dominated by a very strong sharp band near 700 cm-1. However, in this case the peak maximum occurs at 25 cm-1 lower near 675 cm-1. In jadeite-diopside solid solutions, it has been shown that the peak decreases from 700 cm-1 for pure jadeite to 667 cm-1 for diopside as the jadeite content decreases (5). Therefore a peak at 675 cm-1 is also consistent with a low jadeite, jadeite-diopside solid solution. Thus for positive identification of nephrite other characteristic features of the spectrum should be confirmed. For nephrite these are a doublet comprising a strong band near 1060 cm-1 and a band of lower intensity near 1030 cm-1, and a group of four peaks centred on a band of medium intensity near 395 cm-1.
Table 3. Raman spectra (wavenumbers, cm-1)a
of tremolite,
Table 4. Raman spectra (wavenumbers, cm-1)a
Infrared Spectra of Some Figure 7 shows the infrared spectra in the 1400 to 400 cm-1 region of samples scraped from a piece of jadeite mineral (A), a small carved jade elephant (B), and a green/white polished jade cabochon (C). The observed wavenumbers are given in Table 2. The spectra are very similar. All have a very strong broad absorption band with maximum intensity near 1100 cm-1. There is a second very strong peak near 1000 cm-1 and a peak of medium intensity near 925 cm-1. These features are assigned to various silicon-oxygen stretching modes. Two other prominent bands are observed near 600 and 460 cm-1. These are attributed to O-Si-O angle deformation modes. Other constant features in the infrared spectra are the weak absorptions near 855 and 660 cm-1.
There are small differences between the spectrum of the sample scraped from the jadeite elephant (B) and the other two spectra of Figure 7 (A and C). The position of the maximum of the very strong absorption band near 1100 cm-1 is lower in the spectrum of the jadeite elephant (1068 cm-1) than in the other two spectra (1090 and 1086 cm-1). Also, the relative intensities of the absorptions near 510 cm-1 (broad shoulders) and 460 cm-1 (strong sharp peaks) are reversed in the jadeite elephant spectrum, with the first becoming a strong broad band Raman Spectra of Some Jadeite Jade Objects
Raman Spectra of Some Jadeite Jade Objects Figure 9 compares the FT-Raman spectra of three carved nephrite jade objects. The spectra of the "dancing" woman (A) and the bird carving (C) were obtained using 200 mW of laser power and 400 scans were collected. The spectrum of the person wearing a skirt (B), required only 100 mW of laser power. The spectra are very similar. All have the characteristic very strong band near 675 cm-1, the doublet near 1060/1030 cm-1 and the group of weaker bands centred near 400 cm-1 characteristic of nephrite.
Figure 10 compares the spectra obtained from a New Zealand greenstone pendant, an archaic carved blade and a sawn nephrite tablet. These spectra show that New Zealand greenstone is in fact nephrite. Table 3 contains the observed wavenumbers between 1200 and 200 cm-1 for the sawn nephrite tablet and the New Zealand greenstone pendant, (columns 3 and 4). The close agreement between the wavenumbers again confirms that New Zealand greenstone is nephrite. The wavenumbers recorded from the Raman spectrum of a jade pendant are included in Table 3 (column 5). Very similar wavenumbers and intensities were observed for the archaic carved jade objects studied (Figures 9 and 10). Again the wavenumbers and relative intensities are very similar to those observed for nephrite and confirm that the jade pendant and the archaic jade objects are made from nephrite
Infrared Micro-ATR Spectra The ATR accessory on the infrared microscope enables non-destructive examination of jade objects to be made. Spectra can be recorded from various areas of an object. The useful wavenumber range is limited but reasonable spectra have been obtained in the 1400-700 cm-1 range. Figure 11 shows a micro-ATR spectrum of a light brown carved jade tablet believed to be from the Han Dynasty. This spectrum is compared with infrared transmission spectra of samples of nephrite and jadeite and clearly shows that the material of the tablet is nephrite. Various other carved archaic jade objects and items of jade jewelry were identified as nephrite using the micro-ATR technique.
Archaic Faking of Jade Objects The "street" value of modern carved nephrite jade objects is increased if they can be made to appear archaic. Apparently, this can be achieved by immersion in car battery acid (~5 molar H2SO4). It was hoped that detectable differences would be found between the Raman or infrared spectra of treated and untreated nephrite samples supplied by Mr. Bernard Peckover. Figure 12 shows the infrared spectra (in absorbance) in the 1200 - 800 cm-1 region of samples scraped from treated and untreated nephrite tablets. The spectra show some small differences in peak positions and relative intensities. The peak at 1064 cm-1 in the spectrum of the untreated tablet is shifted down to 1060 cm-1 in the treated sample, while the peak at 949 cm-1 shifts up to 953 cm-1. There is also a small variation in relative intensities between the peaks at 1102 and 1064 cm-1.
These differences may be due to sample variation rather than the acid treatment. No detectable differences were observed between the Raman spectra obtained from the surfaces of treated and untreated nephrite tablets. Unfortunately, it appears that in the present case, vibrational spectroscopy does not provide an unequivocal identification of faked archaic nephrite jade. Conclusions The IR and Raman spectra of nephrite and jadeite have several major differences, which enable the two forms of jade to be easily distinguished. Infrared transmission spectroscopy provides a simple method for the identification of jade minerals and objects. However the technique requires that a sample of material be removed from the object. This is not always desirable. The use of a micro ATR accessory in conjunction with an infrared microscope has been shown to be a viable alternative non-destructive method. Raman microscopy is a valuable technique for the study of jade objects. It is
non-destructive and permits different areas of the object to be examined to within a
spatial resolution of a few microns. However, intense fluorescence is often encountered,
in which case the Fourier transform Raman technique can be used, but sample heating may
still present a problem. References
Received 26th September 2001, accepted 14th November 2001.
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