Factors controlling the nucleation and growth of aragonite and calcite in pumping hot spring
Date Issued
2010
Date
2010
Author(s)
Lu, Yi-Chia
Abstract
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.
Subjects
hot spring
sinter
carbonate
aragonite
calcite
porosity
colors
File(s)![Thumbnail Image]()
Loading...
Name
ntu-99-R97224206-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):e87cf0eecbeecce4a151007f6dc0d8cc