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1. Introduction
The Higashimatsuura basalts, distributed throughout the Higashimatsuura Saga Prefecture of Japan, consists of five lithological units based on the period of activity, and the most recent stage has yielded three new minerals; kimuraite (CaY2(CO3)4•6H2O, Nagashima et al., 1986), kozoite- (Nd) (Nd(CO3)(OH), Miyawaki et al., 2000)?Ckozoite-(La) (La(CO3)(OH), Miyawaki et al., 2002). The Higashimatsuura basalts are the only basalts reported to include these rare earth minerals, and the regional distribution and detailed mineralogy of these minerals have yet to be investigated.
2. Experimental
In the present study, samples were collected from 31 sites (15 sites in the 5th-stage basalt, 1 site from 3rd-stage basalt, 13 sites in the 2nd-stage basalt and 2 sites in 1st-stage). Samples were analyzed with respect to chemical compositions and crystallography by X-ray diffractometer, SEM-EDS, TEM-EDS (Kyushu University HVEM Lab.), EPMA and CHN analyzer in order to clarify the regional distribution and mineralogy of rare earth minerals in these basalts.
3. Results
3-1. Regional distribution
REE minerals were fond from 7 sites include type localities of new minerals. In previous studies, REE minerals were fond from only 5th-stage basalt. This is the first time that REE mineral found from 1st-stage basalt and 2nd-stage basalt. REE phosphate was found from 1st-stage basalt and lanthanite-(Nd) and kimuraite were also identified in the 2nd-stage unit of the Higashimatsuura basalt. 3-2. The unknown Ca-Y carbonate
The unknown Ca-Y carbonate was found from Mitsukoshi that is 5th-stage basalt and type locality of kozoite-(La). It occurs as druse or fissure mineralization in the basalt, is white in color with vitreous to silky luster, and forms spherical aggregates of small plate crystals with perfect cleavage. Lokkaite and tengerite are closely associated with the unknown Ca-Y mineral. Chemical composition was Ca1.78REE6.18(CO3)11.2•3.8H2O. Periodicities of 0.61 nm and 0.93 nm are observed from the normal to cleavage plane by transmission electron microscopy, similar to the values for tengerite and kimuraite. A 6.3 nm periodicity parallel with the cleavage plane is also observed from the X-ray and electron diffraction data, suggesting an alternating layer structure consisting of a kimuraite layer (2.4 nm) and a lokkaite layer (3.9 nm). 3-3. REE phosphates
REE phosphates were found from 5 sites (1 site from 1st-stage basalt and 2nd-stage basalt each and 3 sites from 5th-stage basalt). It occurs as druse mineral and forms radial aggregates of hexagonal crystal. There are 3 types of phosphates that mainly contain Y, La and Nd. One of them identified for a new type of rhabdophane mainly contains Y by chemical composition (Y0.33,La0.31,Nd0.21,Gd0.05, Ce0.02)1.01Ca0.09(PO4)0.96•1.09H2O) obtained by EPMA and electron diffraction pattern (hexagonal, a = 0.70 nm, c = 0.64 nm).
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