溫泉區的霰石和方解石成核與成長機制探討

Abstract

本研究取花蓮瑞穗虎爺溫泉會館的霰石結垢與知本東美溫泉飯店的方解石結垢為研究對象，藉由水樣化學成分探討影響碳酸鈣晶型的因素。並以各種精密的儀器如X光繞射分析儀、電子背散射式繞射分析儀、Ｘ射線螢光掃描分析、能量頻譜分析儀與波長頻譜分析儀等，分析結垢的晶型、晶軸方向、晶界角度、元素分布、孔隙率和結晶大小等，來比較方解石和霰石晶體形成的差異，並探討虎爺溫泉會館結垢顏色和條帶的成因，以及其結晶生長的條件。 本研究認為：影響虎爺和東美碳酸鈣晶型的主要因素，很可能就是溫泉水中的鎂離子濃度。因虎爺溫泉水含高濃度的鎂離子，不利方解石的生長。 結垢研究方面：霰石結垢有兩種晶形，一為以細顆粒的圓球聚集成的條帶。另一則是針狀排列。方解石結垢只有一種晶形：呈塊狀且相互鑲嵌。霰石的晶界角度有三個高峰：15度內、50-60度、60-70度，方解石的晶界角度則大多在20度以內。霰石的孔隙率差異極大，可在0%到25%左右(與其排列的方式有關)，而方解石孔隙率極低。霰石易存在離子半徑較鈣大的鍶、鋇等離子，方解石易存在離子半徑較鈣小的鎂、鐵、錳等離子。這個現象不受溫泉水各離子濃度的影響。與方解石為堆積較密的六方晶系，霰石為較寬鬆的斜方晶系有關。 虎爺溫泉結垢的顏色變化與元素分布、晶軸方向、晶型並無對應關連，而與孔隙率和結晶大小有關。因最白的地方有最大的孔隙率和細顆粒晶體組成的條帶，最黑的地方孔隙率極低且無細顆粒晶體組成的條帶。這可以光學角度來解釋：若晶體為長條針狀彼此緊密排列，則光線照射到晶體時較容易穿透，而鮮少有光線折射和反射回眼睛，看起來偏暗黑。但倘若由許多細小的球粒狀晶體所組成，則光線再照射到晶體表面的時候，即可能有許多折射和反射，顏色就偏亮白色。 虎爺溫泉霰石結垢的產狀和孔隙率不同可能是抽水頻率所導致。冬天因大量抽水，導致地底溫泉水的二氧化碳大量逸失，使得pH快速上升、離子溶度積(Ksp)驟降，而比夏季更易達高度過飽合，因短時間大量成核，霰石結垢易呈現細小球狀，其後長出之針狀霰石也較易排列混亂，造成較大的孔隙率。

The carbonate sinters of this study are aragonite and calcite which collected from Ho-Ya SPA hotel, located in Rui-Shui, Hualien County and Dung-Mei Hot Spring Hotel, in Chi-Pen, Tai-Tung County, respectively. The aim is to study what kinds of factors controlling precipitation of carbonate phase by compositions of hot springs. Chemical and physical properties of sinters, such as phases, orientations, misorientation angle distributions, ion species and concentrations, porosity and the size of crystal were analyzed by X-Ray Diffraction, Electron Backscatter diffraction, μ-XRF analysis, energy dispersive spectrometers and wavelength dispersive spectrometers. Those data could help us understand the differences of origin between aragonite and calcite, the factors controlling the color strips of aragonite in pumping pipe and conditions of crystal formation in hot springs. The results show that the factor controlling carbonate phase of Ho-Ya and Dung-Mei hotspring could be the concentration of Mg2+. The high concentration of Mg2+ in Ho-Ya hot spring is adverse for calcite. There are two particle morphologies of aragonite in Ho-Ya SPA sinter. One is small and round crystal gathered to strips, while the other is needle. However, crystal form of calcite in Dung-Mei Hot Spring sinter is characterized by mosaic granulated grains. The peaks of misorientation angle in aragonite are <15°, 50°-60°, and 60°-70°, while, in calcite is usually less than 20°. The porosity of aragonite range from 0% to 25%, but for calcite is very low to zero. Because calcite is hexagonal and aragonite is orthorhombic, the latter is larger than the former in crystal lattice. Calcite accepts cations with radii smaller than Ca2+ (eg, Mg2+, and Fe2+) in lattice. Aragonite, on the other hand, accepts cations with radii larger than Ca2+ (eg, Sr2+, and Ba2+). The phenomena are not changed by the concentrations of cations in water. Precipitated minerals of pipe sinter from Ho-Ya hot spring are predominantly composed of aragonite (>99%) with preferred orientation analyzed by Electron Backscatter Diffraction (EBSD). We took the concentrations of Mg, Ba, Sr, Ca, Fe, Cr, Zn, Cu, Zr, Sc and Ti by ICP-AES and μXRF. The results show that there are few differences between strips with these ions. Images of crystal size and color strips show that the smallest crystal size is located in the whitest zone, which may be generated by oversaturation of solution. On the contrary, the darkest zones do not have the oversaturation strips. Relationships of SEM image, porosity and pore size display that the whitest zone has the highest porosity and larger pores. Oppositely, the darkest zone has the lowest porosity and average pore size. Those results can be explained by optic theory. Well elongate or higher porosity aragonites allow more light to penetrate and less light to be reflected from them to display a darker zone, while random or lower porosity crystals reflect more light back to show white color strips. In this case, we propose that the higher pumping rate causes the rapid depressurization to over-saturate quickly, then crystals nucleate and grow. This process may generate higher porosity and rounded shape of aragonite.