| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 93 === X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectrometry were used to characterize structural, morphological, and chemical features of hydrothermal halloysite in relation to the alteration sequence of primary minerals in a two-pyroxene hornblende andesite body in Tatun Volcanics Group, northern Taiwan. The selected samples were collected from Bailaka Range, northwest of the main peak of Tatun Volcanics Group.
The hydrothermally altered andesite contains white and yellow veinlets of halloysite. Both are mainly composed of 10 Å halloysite that are partially dehydrated to form 7 Å halloysite, with the yellow one showing a higher degree of dehydration. These materials exhibit the following characteristics: (1) broader and ill-defined XRD peaks of yellow halloysite implying poorer crystallinity, smaller crystallite size, and higher defect density relative to white halloysite; (2) relatively weak and obtuse ~910 cm-1 (Al-OH stretching) and broad 3460-3556 cm-1 (O-H stretching) FTIR absorption peaks suggesting a relatively high proportion of interlayer water molecules locating at the inner surface of the layer structure of yellow halloysite in contrast to those situated at the centers of the ditrigonal rings of SiO4 tetrahedra of the layer structure of white halloysite; (3) a relatively high iron content in yellow halloysite but little or no iron content in white halloysite as indicated by XRF and X-ray energy dispersive spectrometric analyses. Both white and yellow halloysites display three different types of morphology: (1) tubular particle (little or no iron content), with a circular cross section of 0.2–0.5 µm in diameter; (2) spherical particle (medium iron content) having a diameter of 0.05–0.2 µm; (3) globular particle (high iron content) with a diameter of ≦0.05 µm, as shown by TEM data. The yellow halloysite is composed mostly of globular halloysite (~60 %) and tubular halloysite (~38 %), whereas the white halloysite has a characteristic tubular shape (~ 95 %).
A trace or no iron content is characteristic of the halloysite that occurs in association with partially altered Ca-rich plagioclase cores and matrix feldspars in the least altered rocks. There is no evidence for alteration of hornblende, pyroxenes, and opaque Fe-Ti oxides in the least altered rocks. The reaction of primary minerals follows the sequence of plagioclase → hornblende → Fe-Ti oxides → pyroxenes as the degree of alteration increases. Relatively iron-rich halloysite commonly occurs in the vicinity of Fe-bearing primary minerals and is present only in advanced stages of alteration when Fe-bearing minerals are affected by hydrothermal fluids.
The aforementioned data collectively imply that the formation of relatively iron-poor, high crystallinity, and coarse-grained tubular halloysite is followed by the occurrence of iron-rich, poor crystallinity, and fine-grained globular halloysite in later stages of alteration. The growth characteristics of halloysite appear to be directly related to its iron content which is in turn controlled by the sequence of alteration of primary minerals when the rock/water ratio is high and the fluid composition is dominated by the parent rock, as is the case studied here.
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