多明尼加藍-綠色針鈉鈣石致色成因

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題名:	多明尼加藍-綠色針鈉鈣石致色成因 The Color Origin of Cyan Pectolite from the Dominican Republic
作者:	黃浩珉
貢獻者:	地學研究所地質組
關鍵詞:	拉利瑪多明尼加針鈉鈣石藍綠色釩定年 Larimar Dominican Republic pectolite cyan vanadium dating
日期:	2021
上傳時間:	2023-02-15 13:37:56 (UTC+8)
摘要:	近些年，寶石市場上出現一種產自多明尼加的藍-綠色半寶石，俗稱「Larimar」，中譯為「拉利瑪」及「海紋石」，經分析確認為一種針鈉鈣石 (pectolite)。針鈉鈣石主要成分為NaCa2Si3O8(OH)，通常為白色，唯一於多明尼加發現有呈現偏藍色的針鈉鈣石，甚至部分偏綠色。礦物的致色成因可分為：1.礦物包裹體的雜質造成的顏色、2.微量的過渡金屬元素造成的顏色、3.色心結構造成的顏色 (黃怡禎，2002)。由於此產地的針鈉鈣石常與自然銅共生，二價銅 (Cu2+)又常為藍-綠色礦物致色元素，因此過去多數研究認為銅為其致色成因。根據分析，樣本在偏光顯微鏡與SEM的觀察下並無後發現包裹體，為證實多明尼加藍-綠色針鈉鈣石是否由銅元素致色，本研究使用雷射剝蝕感應耦合電漿質譜儀分析微量元素成分。發現銅元素與其色調無相關性，與其藍-綠色調相關性較高的元素為釩及鐵，藍-綠色調比例取決於釩及鐵的濃度，藍色可能由四價釩 (V4+)造成，綠色則可能由二價鐵 (Fe2+)主導。另外，本研究發現除化學元素影響致色，放射狀纖維晶體的排列方向與脫水作用也會影響顏色，此種藍-綠色針鈉鈣石特色之一的「海紋」即是由晶體排列方向的改變造成不同方向看到的色差與透光度的差異，而脫水導致孔隙增多所形成的漫反射效應也會使得顏色變更淺。根據我們對於母岩和包覆針鈉鈣石的紅土之化學分析結果，顯示前期可能有酸性熱液溶解並帶來了矽、鈣、鈉、釩、鐵、銅等元素，熱液可能受到周遭石灰岩體影響逐漸轉為鹼性，一開始先沉澱出自然銅和少數的鈉沸石，接著沉澱較多的方解石與針鈉鈣石。最後藉由方解石的鈾釷定年法分析結果，推論藍-綠色針鈉鈣石的形成時間不會超過40萬年前。 Recently, a cyan colored semi-gemstone found in the Dominican Republic , namely “Larimar”, has appeared in the gem market. It is called “Larimar” or ”sea-wave stone” in Chinese. Chemical analysis results identified Larimar as a type of pectolite. The chemical composition of pectolite is NaCa2Si3O8(OH) and the mineral is mostly found in white color. Blue color pectolites and the ones that show shades of green have only been discovered in the Dominican Republic. Origins of a mineral’s color include: 1. Impurities inside the mineral, 2. Trace amounts of transition metals, and 3. Crystal structure defects. Natural copper is often found along with pectolites in the Dominican Republic. Since Cu2+ has always been considered the main cause of the blue-green color shown in minerals, most of the previous studies have attributed this particular color to the element-copper. Observations of our samples under both the polarized optical microscope and the SEM show no inclusions in the minerals. In order to prove whether copper is the main reason for the origin of cyan pectolites, we utilized an LA-ICP-MS to analyze the trace elements of our pectolite samples. The results show that copper plays an irrelative role in the coloring of pectolite. Instead, vanadium and iron are the main contributors to the cyan color of mineral. The depicted final mixed color of blue and green depends on the ratio between these two colors, which as a result is the concentration ratio of vanadium and iron. The blue color is possibly generated by V4+, and Fe2+ mainly contributes to the green color. In this study, we also discovered that not only will the chemical elements affect the color of the mineral, but the orientation of the radiation fibers and the dehydration processes also play critical roles. “Sea-wave” texture, one of the unique features of this type of cyan color pectolite, is caused by the differences in color and transmittance of visible light if viewed from different angles that results from changes in crystal orientations. Also, dehydration processes increase the porosity of the mineral, which then causes a scattering effect in the crystal and reduces the richness of the blue color. According to the chemical composition analyses of the parent rocks and the red soil surrounding the pectolite, dissolution of basic tholeite by alkaline hydrothermal solution causes precipitations of natrolite, calcite, and pectolite, which are often filled in rock pores. The composition of the parent rock is the main source of Si, Na, Ca, V, and Fe. Finally, based on the uranium-thorium dating result of calcite, we suggest that the origin of our pectolite samples is less than 400,000 years.
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