Infrared and Raman spectra of jade and jade minerals

H.F.Shurvell^a L. Rintoul and P.M. Fredericks

Centre for Instrumental and Developmental Chemistry,
Queensland University of Technology,
Gardens Point,
Brisbane Qld 4001
Australia

a Present address: Department of Chemistry,
Queen’s University, Kingston,
ON K7L 3N6 Canada

Abstract
Infrared micro-ATR and infrared transmission spectroscopy, Raman microscopy, and Fourier-transform Raman spectroscopy, have been used to study various jade minerals and jade objects. Infrared and Raman spectra have been recorded of various samples of jadeite, actinolite, tremolite, greenstone and nephrite. Spectra have been obtained from several archaic Chinese jade objects and various coloured jade cabochons. The spectra provide an unequivocal identification of jadeite and nephrite types of jade. Spectroscopic methods for the detection of “faked” archaic nephritic jade objects have been explored. A review of previous vibrational spectroscopic work on jade and jade minerals is presented.

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.

Figure 1. The crystal structure of jadeite viewed down the c axis

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 of Jade Minerals

Pyroxenes and Jadeite

Infrared spectra

A chapter of the book edited by V.C. Farmer [3] contains a review of the infrared spectra of common chain, ribbon and ring silicates (including pyroxenes and amphiboles). Infrared spectra of a sample of jadeite from Tibet and samples of enstatite and clinoenstatite were included in a study of 18 silicate minerals published in 1952 [4]. The spectra were recorded linear in wavelength from 2 to 15 microns (µm) with a NaCl prism spectrometer. In the spectrum of jadeite, a strong broad absorption centred near 10 µm (1000 cm^-1) showed a shoulder at 8.85 µm (1130 cm^-1), two very strong features at 9.35 and 9.97 µm (1070 and 1003 cm^-1), a strong feature at 10.70 µm (935 cm^-1) and a weaker peak at 11.65 µm (858 cm^-1). A weak band was also observed at 13.40 µm (746 cm^-1).

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^-1and 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 micro-Raman study of natural amphibole crystals including actinolite has been reported [13]. The strongest band in the Raman spectrum of actinolite was observed at 672 cm^-1 and assigned to the symmetric Si-O-Si stretching vibration. In an earlier report, Smith and Boyer [14] discuss the Raman spectra of natural high-pressure amphiboles including tremolite. These authors note that the Raman spectra of amphiboles resemble those of some pyroxenes in their peak intensities in four main regions of interest: ~3650, ~1040, ~660 and 150-600 cm^-1.

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: 30A_g(R) + 30B_g(R) + 28A_u(IR) + 35B_u(IR). The internal vibrations of the two Si₄O₁₁^6- ions of the unit cell contribute the following modes: 20A_g(R) + 19B_g(R) + 19A_u(IR) + 20B_u(IR). A very strong line near 670 cm^-1 in the Raman spectrum of actinolite was assigned as an A_g mode. Wang et al. [15] also assigned weaker bands near 400 and 200 cm^-1 to A_g modes and eight weak features between 1100 and 150 cm^-1 to B_g 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 of Jade Objects

An extensive literature search reveals that very little has been published on the vibrational spectroscopy of jade objects. Some archaic “greenstone” axe-heads from Mesoamerica were recently examined with a Raman microprobe spectrometer [8,18]. One of these axe-heads (from Guatemala) consists of a bluish green coloured mineral, which was shown from the Raman spectrum to be nearly pure jadeite jade. Another axe-head from Mexico contained several materials including clinopyroxene and probably clinoamphibole minerals.

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.

Figure 3. Same jade minerals and artifacts, clockwise from top left, a carved bird, a carved horse, a rectangular jadeite cabochon, a carved person wearing a dress, a decorated blade, the dancing lady and an elliptical jadeite cabochon.

Infrared transmission spectra were recorded from KBr pellets using a Perkin-Elmer System 2000 FT-IR spectrometer. Typically, 8 scans were collected at a nominal resolution of 4 cm^-1. Samples were scraped from sawn tablets of nephrite, from a New Zealand greenstone pendant and from under a foot of a Burma jade carved elephant. A piece of raw sapphire was used to remove material from these very hard jade samples.

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

Infrared Spectra of Jade Minerals

Figure 4 compares the infrared spectra of nephrite and jadeite minerals in the 1200 to 400 cm^-1region. 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.

Figure 4. Infrared spectra of (A) nephrite,
and (B) jadeite from 1300 to 400 cm^-1

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
from 3700 to 3600 and 1200 to 400 cm^-1

Table 1. Infrared spectra (wavenumbers, cm^-1)^a of tremolite,
actinolite, nephrite and greenstone.
^a relative intensities are denoted by:
s=strong, m=medium, w=weak, v=very, sh = shoulder

Table 2. Infrared spectra (wavenumbers, cm^-1)^a
of jadeite and two jade objects.
^a relative intensities are denoted by:
s = strong, m = medium, w = weak,
v = very, sh = shoulder, br = broad
^b str = stretch, def = deformation, asym = antisymmetric

Raman Spectra of Jade Minerals

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
(B) jadeite minerals from1200 to 200 cm^-1

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^-1for 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,
actinolite, nephrite and two nephrite jade objects.
^a relative intensities are denoted by:
s=strong, m=medium, w=weak, v=very
^b sym = symmetric, str =stretch

Table 4. Raman spectra (wavenumbers, cm^-1)^a
of jadeite and some jadeite jade objects.
^a relative intensities are denoted by:
s=strong, m=medium, w=weak, v=very,
sh = shoulder ^b str =stretch, def=deformation, sym = symmetric

Infrared Spectra of Some Jadeite Jade Objects

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.

Figure 7. Infrared spectra of a sample of jadeite (A),
a carved jadeite elephant (B) and a green/white cabochon (C).

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

Figure 8 shows the Raman spectra of four jadeite jade objects. The upper two of these spectra (A and B) were obtained using the FT-Raman spectrometer. The lower two spectra (C and D) were recorded using the Raman microscope. In Table 4 the wavenumbers reported by Smith and Gendron [8] for Burma jadeite are compared with the wavenumbers observed in the Raman spectra of two cabochons and a grey carved elephant. The spectra clearly show that these objects are all made from the mineral jadeite. All spectra have a very strong peak near 700 cm^-1, the doublet near 1040/990 cm^-1 and the characteristic band near 375 cm^-1 with a shoulder on the high wavenumber side.

Figure 8. FT-Raman spectra of a small blue/white jadeite cabochon (A) and a large green cabochon (B) and micro-Raman spectra of a carved jadeite elephant (C) and a small brown cabochon (D)

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 9. FT-Raman spectra of three archaic nephrite objects.
(A) a dark brown carved figure of a “dancing” woman, (B) a white carved figure of a person wearing a skirt, (C) a simple carved bird.

Figure 10. Raman spectra of a (A) New Zealand greenstone pendant, (B) an archaic rectangular blade with a surface carving of an animal and (C) a sawn nephrite tablet.

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.

Figure 11. ATR spectrum of (A) a nephrite object, compared with infrared spectra of (B) jadeite and (C) nephrite minerals.

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.

Figure 12. The infrared (absorbance) spectra from 1200 to
800 cm^-1 of (A) untreated and (B) treated nephrite jade tablets.

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.

Unequivocal identification of faked archaic jade was not possible as it was found that treatment of nephritic jade by 5M H₂SO₄ produced only minimal spectral differences.

References

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Received 26th September 2001, accepted 14th November 2001.

REF: H.F.Shurvell, L. Rintoul and P.M. Fredericks Int. J. Vib. Spect., [www.irdg.org/ijvs] 5, 5, 4 (2001)