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蒂芬妮石和摩拉多紫色蛋白石的礦物學研究

A Mineralogical Study of Tiffany Stone and Morado Opal

摘要


近年來,市面上出現一種特殊花紋的紫色蛋白石,主要是以紫色螢石混有白色透明的二氧化矽礦物充填,其二氧化矽礦物主要以蛋白石為主,另含少量玉髓與石英。研究發現這類寶石的二氧化矽礦物逐漸置換原先存在的螢石,因而又被稱為蛋白石化螢石。其最著名的產地有兩處:1.美國猶他州斯波山(Mt. Spor)、2.墨西哥的哈利斯科州(Jalisco)。美國猶他州所產的普遍稱為蒂芬妮石(Tiffany Stone),而產自墨西哥哈利斯科州者多稱為摩拉多蛋白石(Morado Opal)。兩者的外觀與礦物組成十分相似,很容易混淆。本研究使用一般鑑定機構較容易取得的蓋格計數器及長-短波紫外燈,以及偏光顯微鏡、顯微X光螢光光譜儀、顯微拉曼光譜儀,試圖區分兩者間的差異。從X光螢光光譜的分析結果顯示兩者主元素成分大致相同,然而猶他州產的蒂芬妮石所含的微量元素種類較墨西哥產的摩拉多蛋白石豐富。兩者最顯著的差異在於蒂芬妮石含有砷,且鈾含量及放射性普遍高於摩拉多蛋白石。螢光反應兩者都非常明顯,蒂芬妮石在長波紫外光下呈現藍色螢光,短波紫外線下少部分呈現綠色螢光。摩拉多蛋白石在短波紫外線下,呈現出較強的綠色螢光反應。兩者主要的特徵差異,蒂芬妮石常見拼圖狀之角礫狀組織,在顯微鏡下可見良好晶形的紫色螢石充填孔隙中;摩拉多蛋白石中較難發現晶形良好螢石晶體,但偶而可觀察到受紫色螢石置換的板狀礦物假晶。

並列摘要


In recent years, a purple opal with special patterns has appeared in gemstone markets. The purple opal is composed of purple fluorite mixed with white and transparent silica minerals. The silica minerals consist dominantly of opal with minor traces of chalcedony and quartz. Studies have shown that the formation of these gemstones occur through the gradual replacement of fluorite with silica minerals, hence giving them the alternative name of opalized fluorite. There are two famous localities for this gemstone: (1) Spor Mountain, Utah, USA and (2) Jalisco, Mexico. Gemstones originating from the former locality are commercially known as "Tiffany Stone", while those found in the latter are known as "Morado Opal". The appearance and mineral composition of these two gemstones are very similar, so it is confusing in the gem market. In this study, we used Geiger counters and long-short-wave ultraviolet lamps, popular in identification institutions, along with a polarized microscope, Micro-XRF, and Micro-Raman to distinguish the differences between them. The XRF results reveal mostly matching elements in the gemstone compositions of the two localities. However, the Tiffany Stone of Utah contains arsenic, and its uranium content and radioactivity are overall higher than the Morado Opal. Fluorescence is evident in samples from both localities. The Tiffany Stone shows blue fluorescence under longwave UV light, with a small portion of samples presenting green fluorescence under shortwave UV light. On the contrary, the Morado Opal shows very strong green fluorescence under shortwave UV light. As for the main difference in the crystal structure, a puzzle-like, brecciated feature is found in the Tiffany Stone, with its fissures filled with euhedral purple fluorites. On the other hand, euhedral fluorites were seldom observed in the Morado Opal, with only the occasional observation of platy pseudomorphous purple fluorite.

並列關鍵字

opal fluorite fluorescence radioactivity Morado Opal Tiffany Stone

參考文獻


Curtis, N. J., Gascooke, J. R., Johnston, M. R. and Pring, A. 2019. A Review of the Classification of Opal with Reference to Recent New Localities. Minerals, Vol. 9, No. 5, pp. 299–318, https://doi.org/10.3390/min9050299.
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Fritsch, E., Mocquet, B., Williams, B., Williams, C. and Turner, S. 2019. Purple Fluorite in Opal from Utah, USA. The Journal of Gemmology, Vol.36, No. 5, pp. 404–407

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