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[SCG48-12] Pale blue amygdale minerals in the Higashi-Matsuura basalt from Kabeshima Island, Saga Prefecture, SW Japan
Keywords:Kabeshima Island, copper, sepiolite, alkali basalt
The alkali basalts known as the Higashi-Matsuura basalts are widely distributed on Kabeshima Island, Yobuko area, Saga Prefecture (Kobayashi et al., 1955; Nakamura et al., 1986), and a pale blue mineral has been found in amygdales in the Higashi-Matsuura basalts on the island. Amygdale minerals have been reported previously in Higashi-Matsuura basalts from near Karatsu city (Ishibashi, 1974), but not minerals with a pale blue color. In this study, to identify the pale blue mineral and explain its coloration, we described it and analyzed its chemical composition using a field emission electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry.
The pale blue mineral was found in amygdales, up to 5 cm in diameter, occurring in a dense, blue-gray alkali basalt. The alkali basalt exhibits intersertal texture, with phenocrysts of olivine and plagioclase less than 100 µm in diameter in a groundmass of plagioclase and Fe-Ti oxide. Mantle and crustal xenoliths occur in the alkali basalt and, quartz, and clinopyroxene occur as xenocrysts, and chromite with an exsolution texture and chalcopyrite crystals up to 1 µm in size are also rarely observed in the basalt.
The amygdale minerals in the alkali basalt consist of the pale blue mineral, Fe-Ti oxides, calcite, and Mn oxides. The pale blue mineral is spherical with a diameter of 10–20 µm, and it has a spongy texture with numerous micro-scale pores with a diameter of less than 0.5 µm on the mineral surface. A white mineral similar in occurrence and texture to the pale blue mineral is also found in the amygdales. The pale blue mineral occurs in radial clusters on the host rock and other minerals. Quantitative analysis of the pale blue mineral showed that it consists mainly of SiO2, MgO, and Al2O3, with Mn, Fe3+, Cu, Ti, Cr, Ni, Co, and V as trace transition-metal elements. Its chemical formula is (Mg2.285Al0.390Fe0.032Ca0.100Na0.025K0.060) Σ2.892(Si3.847Al0.153) Σ4.000O10(OH)2, which is within the range of the ideal chemical formula of clay minerals. In chemical composition, it is similar to β-sepiolite, a clay mineral described as a new occurrence in the Higashi-Matsuura basalt by Ishibashi (1974).
The transition-metal elements detected in the pale blue mineral that might cause blue coloration in clay minerals are Co and Cu. In general, blue-colored sepiolite contains Co (Zhang et al., 2018), but only 0.001 wt.% of Co was detected in the pale blue mineral, an amount too low to be considered as the coloring factor. In contrast, 0.145 wt.% of Cu was detected in the pale blue mineral. This result suggests that the transition-metal element Cu is responsible for the pale blue color of the mineral.
Ishibashi (1974) described the β-sepiolite occurring in Higashi-Matsuura basalts as white or light green; no pale blue examples were mentioned. The occurrence of pale blue sepiolite on Kabeshima Island can be explained by the following geological background and petrological characteristics of the alkali basalt. Many mantle xenoliths occur in the alkali basalt that hosts the pale blue mineral (Ejima et al., 2022), and the basalt also includes low amounts of fine olivine phenocrysts and very fine sulfide minerals, which are host minerals of Cu. It is inferred, therefore, that because the alkali basalt contains very few host minerals of Cu, Cu ions derived from the mantle xenoliths were not taken up by the basalt. Instead, they were transported by fluids and accumulated in the amygdales. As a result, pale blue sepiolite occurs in alkali basalts on Kabeshima Island, unlike sepiolite occurring in other Higashi-Matsuura basalts.
The pale blue mineral was found in amygdales, up to 5 cm in diameter, occurring in a dense, blue-gray alkali basalt. The alkali basalt exhibits intersertal texture, with phenocrysts of olivine and plagioclase less than 100 µm in diameter in a groundmass of plagioclase and Fe-Ti oxide. Mantle and crustal xenoliths occur in the alkali basalt and, quartz, and clinopyroxene occur as xenocrysts, and chromite with an exsolution texture and chalcopyrite crystals up to 1 µm in size are also rarely observed in the basalt.
The amygdale minerals in the alkali basalt consist of the pale blue mineral, Fe-Ti oxides, calcite, and Mn oxides. The pale blue mineral is spherical with a diameter of 10–20 µm, and it has a spongy texture with numerous micro-scale pores with a diameter of less than 0.5 µm on the mineral surface. A white mineral similar in occurrence and texture to the pale blue mineral is also found in the amygdales. The pale blue mineral occurs in radial clusters on the host rock and other minerals. Quantitative analysis of the pale blue mineral showed that it consists mainly of SiO2, MgO, and Al2O3, with Mn, Fe3+, Cu, Ti, Cr, Ni, Co, and V as trace transition-metal elements. Its chemical formula is (Mg2.285Al0.390Fe0.032Ca0.100Na0.025K0.060) Σ2.892(Si3.847Al0.153) Σ4.000O10(OH)2, which is within the range of the ideal chemical formula of clay minerals. In chemical composition, it is similar to β-sepiolite, a clay mineral described as a new occurrence in the Higashi-Matsuura basalt by Ishibashi (1974).
The transition-metal elements detected in the pale blue mineral that might cause blue coloration in clay minerals are Co and Cu. In general, blue-colored sepiolite contains Co (Zhang et al., 2018), but only 0.001 wt.% of Co was detected in the pale blue mineral, an amount too low to be considered as the coloring factor. In contrast, 0.145 wt.% of Cu was detected in the pale blue mineral. This result suggests that the transition-metal element Cu is responsible for the pale blue color of the mineral.
Ishibashi (1974) described the β-sepiolite occurring in Higashi-Matsuura basalts as white or light green; no pale blue examples were mentioned. The occurrence of pale blue sepiolite on Kabeshima Island can be explained by the following geological background and petrological characteristics of the alkali basalt. Many mantle xenoliths occur in the alkali basalt that hosts the pale blue mineral (Ejima et al., 2022), and the basalt also includes low amounts of fine olivine phenocrysts and very fine sulfide minerals, which are host minerals of Cu. It is inferred, therefore, that because the alkali basalt contains very few host minerals of Cu, Cu ions derived from the mantle xenoliths were not taken up by the basalt. Instead, they were transported by fluids and accumulated in the amygdales. As a result, pale blue sepiolite occurs in alkali basalts on Kabeshima Island, unlike sepiolite occurring in other Higashi-Matsuura basalts.