以地球化學方法區分澳亞群玻隕石、捷克玻隕石與粒狀黑曜岩等天然玻璃

The Distinction of Natural Glass: Australasian Tektite, Moldavite and Granular Obsidian by Geochemical Method

林書弘(Shu-Hong Lin) ； 陳惠芬(Hui-Fen Chen) ； 李孟陽(Meng-Yang Lee)

《國立臺灣博物館學刊》 75卷3期 (2022/09) Pp. 23-42

https://doi.org/10.6532/JNTM.202209_75(3).02

摘要

天然玻璃係指天然產出的二氧化矽非晶質固體，依其成因可分為火山源與非火山源兩類，前者包括玄武岩玻璃與黑曜岩，後者包含分別由閃電作用、斷層作用與隕石撞擊作用所產生的閃電熔岩、假玄武玻璃、焦石英與玻隕石等。其中，黑曜岩與玻隕石常因外觀或成分相似而最容易混淆，故本研究將分析來自美洲、亞洲與澳洲的天然玻璃樣本，其中包含了已知黑曜岩和玻隕石之樣本。另有三個新產地之未分類天然玻璃，透過X光螢光分析、顯微拉曼分析、比重與折射率等方法，比較黑曜岩與玻隕石之間的差異，並嘗試區分天然玻璃種類與產地來源。研究發現黑曜岩類都含有大量微晶包體，經顯微拉曼光譜儀分析後，確認包含鋯石、斜長石、褐錳礦、黑雲母與磷灰石等礦物；相反地，玻隕石樣本中則僅觀察到氣泡和糖渦紋等典型特徵。哈克圖解析X光螢光分析數據之結果顯示，黑曜岩相比於玻隕石具有相對較高的鋁和鉀，而有相對較低的鈣和鈦，且捷克玻隕石和澳亞群玻隕石則具有明顯不同的地化特徵。傳統寶石學方法、顯微包體分析以及哈克圖的解析結果驗證了本研究中所收集來自印尼、哥倫比亞和美國亞利桑納等三個產地的天然玻璃是黑曜岩而非玻隕石。此為初步研究，未來或可結合統計學方法進一步區分不同產地之黑曜岩與澳亞群玻隕石。

關鍵字

玻隕石 ； 黑曜岩 ； X光螢光分析 ； 哈克圖

並列摘要

Natural glass refers to natural silica-rich amorphous solids which are grouped into two categories: volcanic and non-volcanic origins. The former includes basaltic glass and obsidian; the latter includes fulgite, pseudotachylite, lechatelierite and tektite whcich are respectively produced by lightning, faulting and meteorite impact.. Among them, obsidian and tektite are often confused because of their similarity in appearance or composition, so this study analyzed natural glass samples from the Americas, Asia and Australia which included obsidian, tektites and unclassified natural glasses from Indonesia, Arizona, USA. and Colombia. These samples were analyzed through EDXRF analysis, micro-Raman analysis, specific gravity and refractive index to compare the differences between obsidian and tektites from different origins. Micro-Raman spectroscopy revealed that all the obsidians contained various xenocrysts including zircon, plagioclase, braunite, biotite or apatite, etc. On the contrary, bubbles and swirly structures were the only typical textures observed in the tektites. The Harker diagram from EDXRF data showed that obsidian had higher aluminum and potassium content, but relatively lower calcium and titanium content compared with tektite. Besides, Moldavite and Australasian tektites had significantly different geochemical features. The results of traditional gemological tests, inclusions, and Harker diagrams verified that the natural glasses from Indonesia, Colombia and Arizona collected in this study were obsidian rather than tektites. This is a preliminary study, and then statistical methods may be combined to further distinguish obsidian from different origins and Australasian tektites in the future.

並列關鍵字

tektite ； obsidian ； EDXRF ； Harker diagram

參考文獻

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