閃玉之拉曼光譜之研究

Raman Spectroscopic Study of Nephrite

黃怡禎(Eugene Huang)

《國立臺灣博物館學刊》 69卷1期 (2016/03) Pp. 31-50

https://doi.org/10.6532/JNTM.2016.69(1).04

摘要

我們利用拉曼光譜儀測量不同成份閃玉的圖譜變化，結果顯示拉曼光譜在低頻部分 (ν ＜ 1000 cm^(–1))及高頻部份(ν ＞ 3000 cm^(–1))大部分振動模頻率隨鐵之含量增加，呈負相關變化。此為當鐵離子逐漸佔據M1及M3位址時，導致此兩個八面體體積變大，使鍵長增長而鍵結變弱所致。振動模隨鐵含量增加的線性關係，可以用來做為閃玉系列礦物鐵－鎂相對含量之半定量測量。閃玉之鐵含量逐漸增高時，高頻部份之OH振動模由單一振動模變為兩個，然後再變為三個振動模。此現象乃因鐵離子在置換鎂離子時，使晶格中M-O八面體對OH造成不對稱的拉力，導致OH鍵由單一振動模分裂成三個振動模。我們建議可利用固溶液呈現三個OH振動模的成份點，做為定義透閃石和陽起石在固溶體系列之分界點。閃玉之系統性拉曼光譜研究結果，除了可建立造岩礦物的資料庫供參考外，亦可用以做為非破壞性檢定及研究寶石或古玉之極佳工具。

關鍵字

拉曼光譜 ； 閃玉 ； OH振動模 ； 透閃石－陽起石

並列摘要

Natural specimens of nephrite in the tremolite-actinolite solid solution series were studied by Raman spectroscopy. Among 28 samples analyzed, seventeen of them are nephrite. A total of 23 Raman modes were detected in the low-wavenumber range (＜ 1200 cm^(-1)), among which the Si-OSi bending mode at near 672 cm^(-1) is the most prominent one. The wavenumber of most of the vibrational modes shifts to lower wavenumber with an increase in Fe-content. This is attributed to the weakening of chemical bonding in M-O octahedron by the substitution of Mg by Fe in octahedral sites. The linear relationship of wavenumber versus Fe-content can be used as a semiquantitative measurement of the Mg-Fe ratio in nephrite of unknown chemical composition. In the high-wavenumber range (~3400 cm^(-1)), we found that the number of OH-stretching modes increases from 1 (at Mg#97.8) to 3 (at Mg#92.4). This is interpreted as due to the degeneration of OH-stretching modes caused by the loss of symmetry in M4 sites as Fe substitutes for Mg in the lattice. The splitting of OH-stretching modes can be used as a criterion for the classification of mineral species in tremolite-actinolite series. We propose that the term ＂tremolite＂ be used for minerals in this series with an Mg# higher than 93 where only one OH-stretching mode is present. For minerals in the tremoliteactinolite series with an Mg# lower than 93 where three OH-stretching modes present, actinolite is used. The results in this study not only provide scientific meaningful demarcation for the classification of minerals in the tremolite-actinolite series, but also find its application in the non-destructive identification of nephritic artifacts, which is of vital importance in the examination of archaic jades.

並列關鍵字

Raman spectroscopy ； nephrite ； OH vibration mode ； tremoliteactinolite

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