The First Occurrence of Suolunite in Taiwan: Its Physical Properties, Chemical Composition, and Genesis

臺灣首次發現的直水矽鈣石：其物理性質、化學成分及成因之探討

何恭算(Kung-Suan Ho) ； 陳汝勤(Ju-Chin Chen) ； 游鎮烽(Chen-Feng You) ； 許世傑(Shih-Chieh Hsu) ； 黃敏惠(Ming-Huei Huang)

《Collection and Research》 22期 (2009/12) Pp. 1-13

https://doi.org/10.6693/CAR.2009.22.1

摘要

The hydrated calcium silicate, suolunite, is an extremely rare mineral and was found in serpentinites near Shanyuan, of the southern Coastal Range in southeastern Taiwan. Its physical properties and chemical compositions were studied by x-ray diffraction, electron-microprobe analysis, thermal ionization mass spectrometry, and inductively coupled plasma mass spectrometry. Suolunite was associated with tobermorite and occurred as irregularly shaped veinlets in metasomatized rodingite and albitite. It is colorless, with white streaks, a vitreous luster, and white fluorescence under shortwave ultraviolet light. The average contents of SiO_2, CaO, and water obtained from three suolunite samples were 44.61, 40.98, and 13.62 wt%, respectively. Small amounts of Na, Mg, K, and Fe replace the Ca, and Al substitute for the Si to a minor extent. Calculation of the analytical data showed that suolunite from Taiwan has a simplified chemical formula of Ca_(1.96)Fe_(0.01)Si_(1.99)Al_(0.01)O_5(OH)_2．H_2O. In addition, trace amounts of Li, Sc, V, Cr, Mn, Co, Ni, Zn, Sr, Y, Zr, Nb, Ba, Ce, Nd, Dy, Hf, Pb, and Th also existed within the suolunite crystals. The isotopic composition of Sr in the three suolunite samples was markedly heterogeneous, with the ^(87)Sr/^(86)Sr ratios varying from 0.706895 to 0.709342. When these suolunite samples were heated to 320 °C for 1 h, some specimens lost their water and were transformed to xonotlite. Suolunite was formed in metasomatized rocks as a secondarily altered mineral due to interactions between calcium-bearing solutions and Ca-plagioclase. The conditions of formation imply hydrothermal activity at temperatures below 140 °C; these conditions are similar to those favoring the occurrence of tobermorite as suggested by Gabrovsek et al. (1993).

關鍵字

suolunite ； genesis ； serpentinite ； southern Coastal Range ； southeastern Taiwan

並列摘要

直水矽鈣石又名索倫石，它是一種非常罕見的礦物，筆者在臺灣東南之海岸山脈南段杉原附近的蛇紋岩中發現。本文針對它的物理性質和化學成分，利用X光繞射儀、電子微探儀、熱離子質譜儀和感應耦合電漿質譜儀進行分析與研究。直水矽鈣石與雪矽鈣石一起伴生，並在異剝鈣榴輝長岩和鈉長岩等換質岩石內呈不規則細脈狀產出。礦物為無色，具白色條痕和玻璃光澤，在短波段紫外線照射下會發出白色螢光。本文挑選三件直水矽鈣石標本分析結果顯示，其平均二氧化矽、氧化鈣和水含量分別為44.61, 40.98 和13.62 wt.%。其晶體內另有少量的鈉、鎂、鉀和鐵取代鈣的位置，少量的鋁置換矽的位置。從分析數據計算臺灣直水矽鈣石的化學式為Ca_(1.96)Fe_(0.01)Si_(1.99)Al_(0.01)O_5(OH)_2．H_2O。此外，在直水矽鈣石中也賦存微量的鋰、鈧、釩、鉻、錳、鈷、鎳、鋅、鍶、釔、鋯、鈮、鋇、鈰、釹、鏑、鉿、鉛和釷等元素。至於鍶同位素組成則呈現相當的不均質，三件標本的^(87)Sr/^(86)Sr比值從0.706895變化至0.709342。當直水矽鈣石被加熱至攝氏320度且持續1小時後，部分標本會失去結晶水而轉為另一種礦物—硬矽鈣石。直水矽鈣石在換質岩石內以次生換質礦物生成係因含鈣溶液與鈣斜長石作用的結果，推測其形成的溫度類似Gabrovsek et al. (1993)實驗測得雪矽鈣石產出的條件—低於攝氏140度。

並列關鍵字

直水矽鈣石 ； 成因 ； 蛇紋岩 ； 海岸山脈南段 ； 臺灣東南