Ryuji Kitagawa

Stacking structures in pyrophyllite revealed by high-resolution transmission electron microscopy (HRTEM)

Abstract Stacking structures in pyrophyllite, Al2Si4O10(OH)2, were investigated mainly by using high-resolution transmission electron microscopy (HRTEM). The specimens examined were large lath-shaped crystals (Berosovska, Urals, Russia) and massive aggregates of fine platy crystals (Nohwa, southwest Korea). Both specimens showed powder X-ray diffraction (XRD) patterns similar to those reported previously as the 2M polytype. The common stacking sequence in the two specimens is not monoclinic with two-layer periodicity as previously reported, but a uniform orientation of the 2:1 layers and near complete disorder of two alternative directions of interlayer displacement, i.e., lateral displacement between the two tetrahedral sheets across an interlayer region. The directions of interlayer displacement are about ± 2π/3 from that of the intralayer shift (lateral displacement between the two tetrahedral sheets within a 2:1 layer). Simulation of powder XRD patterns by this stacking model closely approximates the experimental pattern. Elongation of the lath-shaped Berosovska crystals corresponds to the direction of the intralayer shift, as seen in illite-1M. 2:1 layers with different orientations, and interlayer displacement almost parallel to the intralayer shift, were occasionally observed as stacking faults. Such disorder occurs more frequently in the massive Nohwa specimen than in the Berosovska specimen. Sub-micrometer domains of the 2M stacking sequence with regular alternation of the two directions of interlayer displacement were found in the Nohwa specimen.

XRD and HRTEM analyses of stacking structures in sudoite, di-trioctahedral chlorite

Abstract The stacking structures of sudoite, di-trioctahedral chlorite from two different localities are investigated by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analyses using a Gandolfi camera revealed that the structures of both specimens are similar, corresponding to a one-layer IIbb-4 (1A) polytype. HRTEM observations indicate that the stacking sequence is characterized by a largely uniform intralayer shift of a/3 in the -X1 direction (Xi represent the directions along the pseudohexagonal axes) and by an interlayer displacement of similar magnitude in either the -X2 or -X3 direction. Stacking disorder is primarily caused by the mixing of interlayer displacements in the two directions. This disorder is more common in the lath-shaped crystalline specimen from Berezovsk, Russia than in the fine platy crystalline specimen from Ottré, Belgium. DIFFaX simulations of the powder XRD patterns for this stacking model reproduced the observed features well for both specimens. The stacking configuration of these sudoite specimens is considered to be controlled by the corrugation of basal oxygen planes in the 2:1 layer and the corresponding deformation in the brucite-like interlayer sheet.

Microtopography of Regularly-Interstratified Mica and Smectite

The gold decoration technique of electron microscopy was used to observe the microtopography of natural (001) surfaces of 1:1 regularly-interstratified mica/smectite minerals (expandable layer: 40–45%) collected from four different pyrophyllite deposits in Japan. The specimens are characterized by parallel growth steps of malformed, circular or polygonal forms with varying step separations. Many particles exhibit paired steps that seem to show spiral growth. Microtopographic observations suggest that the growth of regular interstratification (at least for the specimens investigated in this study) normally takes place by an interlacing of paired steps. If the height of a single step corresponds to that of a mica or a smectite layer, the particles are estimated to be normally 40–300 Å in thickness. If the particles on which a spiral center is observed are single crystals of interstratified mica and smectite, then some crystals investigated in this study are far thicker than fundamental particles. The results of this study are interpreted to suggest that these regularly-interstratified mica/smectites were formed by hydrothermal metasomatism from their respective host rocks.

Chlorite in metabasites from the Mikabu and North Chichibu Belts, Southwest Japan

Chlorites formed as a replacement of phenocrysts in metabasites from the pumpellyite-actinolite to lower-greenschist facies Mikabu and North Chichibu belts in southwest Japan were studied by X-ray powder diffraction, electron microprobe analysis (EMPA) and high-resolution transmission electron microscopy (HRTEM). The metabasites contain a small quantity of fine-grained smectite and corrensite in the <1 pm size fraction. The chlorite also contains trace amounts of Ca, Na and K, which generally appear to be associated with smectite stacked in chlorite packets. The smectite layers comprise up to 13% of the chlorite domains. Theoretical estimates of the smectite ratio by the Wise method using EMPA data coincide well with the ratio determined based on HRTEM observations in most chlorites. However, in some chlorites with high proportions of Ca, Na and K, the cations cannot be reasonably attributed to smectite alone. Based on the present analyses, Ca, Na and K cations are also hosted in discrete interstitial phases of fine-grained smectite and corrensite as possible retrograde metamorphic products. These findings suggest that care should be taken in application of the Wise method to estimate the smectite ratio, and that the whole-rock chlorite composition may not be suitable as a geothermometer.

Comparative study of rock weathering affected by two lichen species; Ramalina yasudae Rasanen and Lecidea sp.

地衣類による岩石基物の生物学的風化現象(物理 的な破壊作用および化学的な溶解作用)は,岩石・鉱 物の観点からだけでなく,文化財を含む石造建築物の 保護事業,外壁の景観と耐久性を考える都市工学とい った観点からも注目される現象である.被害を懸念し て現在のところ除去一辺倒の対策(機械的に削る,薬 品で溶かすなど)がなされているが,地衣類の除去に は基物の破損を伴うことが多い.今後は地衣類の被覆 が景観に与える美的側面を見直し,除去を最小限に留 めた対策が望まれる.地衣類除去の是非を議論するに は地衣類が基物に与える生物学的風化作用の詳細な 影響評価が不可欠である.本研究では同一基物に混在 して生育している形態の異なる 2 種の地衣類に着目 し,それぞれの接触部における風化現象を比較した. 流紋岩質凝灰角礫岩の転石を広島県三和町から採 取した .表面には Ramalina yasudae Rasanen および Lecidea sp.の生育が見られた.それぞれの接触部の様 子を構造学的に比較するために,光学顕微鏡および EPMA(BSE 像)による断面観察,SEM による表面観察 を行った(図1).樹枝状形態の R.yasudae Rasanen の 接触部では,スポット的に深く進行する特徴的な溶解 の形跡が認められた.ただし,穿孔特性は無い.一方,固 着性形態の Lecidea sp.の接触部では溶解の形跡は小 規模で断続的だった.浅く広い面積にわたって進行す ると考えられる.どちらの場合にも溶解の進行してい る部分では石基中の空隙が多くなっていた.次にこれ ら溶解促進部分における流紋岩(石基部分)の組織 変化を議論するために EPMA による化学分析および 面分析を行った.粘土鉱物など二次鉱物への変質が予 想されたが,化学組成に変化は認められなかった..な お,Lecidea sp.の接触部で起こっている現象について 地衣類による作用であることを示唆する形態的な特 徴に乏しいが,地衣類に覆われていない流紋岩断面お よびその表面と比較することで,非生物的な化学風化 とは区別されるものであることを確認した.