Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the surrounding lenticular coarser-grained (100-200 μm) aggregate (seam) with the same mineral assemblage of the symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field-emission gun SEM (FE-SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The symplectite is composed of two segments with a large misorientation angle of ∼ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with gradual lattice distortion smaller than a few degrees/mm and intra-sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of pyroxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The symplectite minerals in each sector show systematic crystallographic orientations (topotaxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.
This paper shows that Kikuchi pattern analysis in a transmission electron microscope (TEM) using recent techniques is superior to conventional electron diffraction analysis for determining crystal orientation in an assemblage of small crystals such as those of biogenic origin. A CCD camera with a wide view and large dynamic range was used together with a dedicated program to analyze the patterns and enable real-time and unique determination of crystal orientation. Convergent illumination of the incident beam was effective in enhancing the Kikuchi patterns and in improving spatial resolution. Two example biominerals are characterized by Kikuchi pattern analysis; one is spinel-law twins in several tens of nanometers width magnetite crystals from magnetotactic bacteria. The other is crystal orientation and the alignment of calcite fragments in coccolith.
The Western Ethiopian Shield is an exposed Neoproterozoic metamorphic belt and forms part of the Arabian-Nubian Shield. The metamorphic belt consists of high-grade biotite gneisses, low-grade volcanogenic sediments, and mafic-ultramafic complexes. The Bikilal-Ghimbi gabbro is a mafic body surrounded by these gneissic rocks, and is located 440 km west of Addis Ababa. The gabbro is elliptical in shape and covers an area of 350 km2. It consists of olivine gabbro in its center and hornblende gabbro and hornblendite at the perimeter. The olivine gabbros are very fresh and undeformed, but hornblende-bearing suites have deformational textures. Each rock type can be divided into apatite-bearing and apatite-free subtypes. The major element geochemistry shows that despite the differences between the olivine and the hornblende gabbros, there is no systematic chemical contrast between the lithotypes except for the fluid mobile elements, suggesting an origin from a common parental magma. Only the perimeter is affected by metasomatism. An estimation of the parental magma composition using the trace element abundance in fresh clinopyroxenes and fresh olivine gabbro bulk rock suggests an intraplate-type tholeiite. Crystallization model calculations using a tholeiitic parental magma suggest that the gabbros crystallized in a manner where small amounts of interstitial melt were retained. The apatite-bearing varieties are always associated with Mg-rich mineral phases, suggesting an origin from the supercooling of replenished basalt into an evolved low temperature magma chamber. The supercooling caused saturation of the apatite in the basalt melt, along with Mg-rich crystals, and these later mixed together with the more evolved crystals that had precipitated previously. The intraplate-type tholeiitic parental magma suggests plume-type magmatism for the origin of the Bikilal-Ghimbi gabbro body.
Charlesite was found in a calcite vein that developed along the boundary between crystalline limestone and gehlenite-spurrite skarns at the Fuka mine in Okayama Prefecture. It occurred as a flattened hexagonal dipyramid up to 6 mm across in association with calcite. An electron microprobe analysis of the charlesite showed a marked variation in its composition from the core to the rim. The average chemical composition indicated the empirical formula (Ca5.77Na0.02K0.07)Σ5.86(Al1.23Si0.79Mg0.01Mn0.01Fe0.01)Σ2.05[(CO3)1.16(SO4)0.93]Σ2.09[B(OH)4]0.98[(OH)10.62O1.38]Σ12.00·25.41H2O based on a total of 11 cations (anhydrous part). The unit cell parameters of charlesite are a = 11.097(5) and c = 21.22(3) Å. The mineral is optically uniaxial negative, with the indices of refraction being ω = 1.498(2) and ε = 1.462(2). Its Vickers microhardness is 96.0 kg/cm2 (10-g load), and its Mohs hardness number is 2.5. The measured density is 1.84 g/cm3. It is likely that charlesite in the Fuka mine was formed primarily under a low-temperature hydrothermal condition.