Azuma Iijima

Berthierine and chamosite in coal measures of Japan

Berthierine (formerly chamosite) occurs as concretions, lenses, and bands in carbonaceous, kaolinitic shale of freshwater coal-swamp deposits in Paleogene and Upper Triassic coal measures of Japan. Textural relations in thin sections of the Triassic berthierine rocks and a siderite-kaolinite-berthierine-quartz assemblage in Paleogene rocks indicate that the berthierine formed by reaction of siderite with kaolinite. The transformation of siderite and kaolinite to berthierine and quartz occurs progressively under reducing conditions between 65° and 150°C and at burial depths of 2–5 km. Utatsu berthierine is an aluminous, low-Mg variety as compared with berthierine pellets in modern marine and estuarine sediments and in ancient marine ironstones. Fe is the dominant octahedral cation with Fe2+ ≫ Fe3+. The composition of the berthierine varies between different morphological types. Utatsu berthierine transformed to ferrous chamosite when kaolinite in the host shale changed to pyrophyllite. These transformations are estimated to have occurred at ∼160°C and at a burial depth of ∼3 km.РезюмеБертиерин (раньше хамосит) выступает в виде конкреций, ленсов, и полос в карбонатных, каолинитовых сланцах в свежеводных, углеболотных осадах в палеогенных и выше-триасовых каменноугольных пластах Японии. Текстурные отношения в тонких секциях триасовых бертие-риновых пород и скоплений сидерит-каолинит-бертиерин-кварц в палеогенных породах указывают на то, что бертиерин образовался путем реакции сидерита и каолинита. Трансформирование сидерита и кординита в бертиерин и кварц выступает прогрессивно в восстановленных условиях между 65° и 150°С на глубине погребения 2 до 5 км. Бертиерин из Ютатсу является алюминиевого, низко-Мg сорта по сравнению с бертиериновыми таблетками в современных морских и эстуарных осадках и в древних морских железных рудах. Ре является основным октаэдрическим катионом с Ре2+ ≫ Ре3+. Состав бертиерина различен для разных морфологических типов. Бертиерин из Ютатсу преобразовался в железистый хомосит, когда каолинит в материнском сланце изменился в пирофиллит. Оценивается, что эти преобразования осуществились при температуре 160°С и на глубине погребения порядка 3 км. [Е.С.]ResümeeBerthierit (früher Chamosit) tritt als Konkretionen, Linsen, und Bänder in kohligem, Kaolinithaltigem Schieferton von Süßwasser-Kohlelagerstätten in paläogenen und obertriassischen Kohleschichten von Japan auf. Die Gefügemerkmale in Dünnschliffen der triassischen Berthierit-Gesteine und eine Siderit-Kaolinit-Berthierit-Quarz-Vergesellschaftung in paläogenen Gesteinen deuten darauf bin, daß sich der Berthierit durch die Reaktion von Siderit mit Kaolinit bildete. Die Umwandlung von Siderit und Kaolinit in Berthierit und Quarz findet in zunehmendem Maße unter reduzierenden Bedingungen zwischen 65° und 150°C und bei einer Überlagerung von 2–5 km statt. Der Berthierit von Utatsu ist verglichen mit Berthierit-Pellets in jungen marinen und ästuarinen Sedimenten und alten marinen Eisensteinen eine Al-haltige Varietät mit wenig Mg. In oktaedrischer Koordination tritt vor allem Fe auf, wobei Fe2+ ≫ Fe3+. Die Berthieritzusammensetzung schwankt zwischen den einzelnen morphologischen Typen. Der Utatsu Berthierit wandelte sich in Fe-haltigen Chamosit um, wenn sich der Kaolinit im Muttergestein in Pyrophyllit umwandelte. Es wird angenommen, daß diese Umwandlungen bei ∼160°C und bei einer Überlagerung von ∼3 km stattfanden. [U.W.]RésuméLa benthiérine (autrefois la chamosite) est trouvée en concrétions, en formes lenticulaires, et en bandes dans du shale kaolinitique de dépôts de charbon-marécage d’eau douce dans des mesures de charbon d’âge paléogène et haut triassique du Japon. Des relations texturales dans des sections minces des roches benthiérine triassiques et un assemblage benthiérine-kaolinite-quartz dans les roches paléogènes indiquent que la benthiérine a étè formée par la réaction de sidérite avec la kaolinite. La transformation de sidérite et de kaolinite en benthiérine et quartz se passe progressivement sous des conditions de réduction entre 65° et 150°C et à des profondeurs d’ensevelissement de 2–5 km. La benthiérine Utatsu est une variété alumineuse, à bas Mg comparée aux boulettes de benthiérine dans des sédiments marins et estuarins modernes et dans d’anciennes roches ferreuses marines. Fe est le cation octaèdral dominant avec Fe2+ ≫ Fe3+. La composition de la benthiérine varie entre differents types morphologiques. La benthiérine Utatsu s’est transformée en chamosite ferreuse lorsque la kaolinite dans le shale hôte s’est changée en pyrophylite. On estime que ces transformations se sont passées à 160°C et à une profondeur d’ensevelissement de ∼3 km. [D.J.]

Petrology and Diagenetic Changes of Neogene Siliceous Rocks in Northern Japan

ABSTRACT Diagenetic changes in textures and mass properties of Neogene marine noncalcareous siliceous rocks corresponding to silica-phase transformations (opal-A opal-CT quartz) were studied in subsurface and surface sections of northern Japan. In opal-A diatomite, siliceous tests are well preserved, no cementation is observable, and pores exist chiefly as inter- and intragranular micropores (2 10µ). During the opal-A to opal-CT transformation, siliceous tests are extensively dissolved to form fine aggregates of opal-CT in matrix and cement of opaline porcelanite. An abrupt decrease of as much as 15% in porosity occurs between opal-A and opal-CT zones in subsurface sections, a decrease which may be due to the destruction of intragranular pores in siliceous tests by dissolution. Ultramicropores (10 µ) as molds and chambers of siliceous tests in quartzose porcelanite. Porosity of Neogene noncalcareous siliceous rocks in subsurface sections of northern Japan gradually decreases with the increase of burial depth, and silica-phase transformations have a relatively small effect on it. This gradual decrease in porosity, which corresponds well to that of Neogene mudstone and clean, fine-grained sandstone, indicates that little additional silica cementation occurs during burial (Iijima and Tada, 1981). At a depth of 4.5 km and a temperature of 130° C, quartzose porcelanite holds porosity of 10-15%. Burial depth of around 10 km might be required to form dense quartzose chert without porosity by burial diagenesis of siliceous rocks if extrapolating from the burial depth-porosity relation. This depth is too large. Some additional mechanisms are required ther than mechanical compaction, silica-phase transformations and clay alteration. Pressure solution-reprecipitation of quartz is the most probable one.

Mechanism of sedimentation of rhythmically bedded chert

Abstract The biogenic chert-shale sequences in the Upper Paleozoic and Lower Mesozoic Inner Chichibu terrain and in the Tertiary Setogawa terrain formed on a bank or in the central part of a marginal sea adjacent to a nearby land. The rhythmic layering results from periodic turbidity currents. In spicule-radiolarian bedded cherts of the Inner Chichibu terrain, most chert beds are of symmetrical triple-layered type composed of a middle siliceous layer and upper and lower argillaceous layers. The amounts of hydrogenous base metals, such as Fe, Mn, Cu, Ni, V and Zn, are higher in chert than in shale partings. The MnO/Al 2 O 3 ratio and sedimentation rate diagram in Fig. 4 shows that the average sedimentation rate is 7.1 m m.y. −1 for chert and 45 m m.y. −1 for shale partings. These lines of evidence suggest that the shale partings formed from distal turbidity currents and are interbedded with hemipelagic biogenic chert. On the other hand, in the Tertiary diatomaceous bedded cherts of the Setogawa terrain, most sets of chert beds and overlying shale partings were deposited as siliceous turbidites, like in many bedded cherts in other orogenic belts of the world. The frequency of occurrence of such periodic turbidity currents is calculated as one per 11,200-17,700 yrs in the Inner Chichibu terrain and one per 2600-10,000 yrs in the Setogawa terrain.

Sulphur isotopic composition in the Palaeogene coal of Japan

Abstract Sulphur isotopic compositions of organic and inorganic sulphur are reported for 39 coal samples from three major coalfields of Japan. The δ 34 S values are from 2.7 to 21.1% and from 2.6 to 20.3% for organic and pyritic sulphur respectively, in low-sulphur coal beds ( δ 34 S value of organic sulphur of the Kado coal (2.7–3.9%), which was formed in a freshwater environment, are probably the δ 34 S values of the primary sulphur in the Palaoegene (Eocene) coals of Japan. The δ 34 S values show a downward decrease in the low-sulphur coal beds in the Ishikari and Miike coalfields. In contrast, the δ 34 S values are from 0.2 to 30.6% and from −9.6 to 43.1% for organic and pyritic sulphur respectively, in high-sulphur coal beds (> 1% total sulphur) in the Miike coalfield, and show three distinctive vertical variations: (1) a little downward and remarkable downward increase in organic and pyritic sulphur respectively; (2) a large vertical variation having the maximum value near the middle of the bed in both organic and pyritic sulphur, and (3) a downward decrease associating major fractionations near the top and bottom of the bed in both organic and pyritic sulphur. Both organic and pyritic sulphur, even in the low-sulphur coal beds except for the Kado coal, probably originated from the reduction of seawater sulfate, which infiltrated from above and partially from below the bed. Two major stages of sulphur incorporation were inferred; the early stage occurred at the peat-forming stage and the late stage occurred after burial at a few hundred meters depth. A distinctive relationship between the δ 34 S values and the sulphur contents is observed among individual coal samples. In the low-sulphur coals, organic sulphur is enriched in δ 34 S relative to pyritic sulphur and is more abundant than pyritic sulphur. In high-sulphur coals, three types are recognized: (1) when the δ 34 S values of both organic and pyritic sulphur are over 20%, organic sulphur is depleted in 34 S relative to pyritic sulphur, and is more abundant than pyritic sulphur; (2) when the δ 34 S values of organic sulphur are over 20% but those of pyritic sulphur are not, organic sulphur is enriched in 34 S relative to pyritic sulphur, and is more abundant than pyritic sulphur; and (3) when the δ 34 S values of both organic and pyritic sulphur are less than 20%, organic sulphur is enriched in 34 S relative to pyritic sulphur, but no general pattern in the sulphur contents exists.

Chapter 3 Diagenetic Transformations of Minerals As Exemplified By Zeolites and Silica Minerals-A Japanese View

Publisher Summary This chapter examines the diagenetic transformations of minerals as exemplified by zeolites and silica minerals. Japan is one of excellent localities in the world to study the occurrence and genesis of natural zeolites, because volcanic glass of varying composition that is the best raw material of zeolites occurs widely in volcaniclastic rocks of younger geologic ages. Zeolites originate from many kinds of precursor minerals, such as volcanic glass, impact glass, non-crystalline and crystalline clays, feldspar, feldspathoid, and even zeolite itself. Water is indispensable to zeolites so that the geologic occurrence of zeolites can best be classified based on the states of interstitial water in zeolitic rocks. Much work in the field and in laboratories has proved that temperature is a major factor in controlling zeolite formation. Chemistry of pore water plays another important role of zeolite formation. As a result of the presence of a geothermal or chemical gradient, zeolites commonly occur in a vertically or laterally zoned arrangement, which is commonly mappable as zeolite zones.

Chapter 4 Recent Developments in the Sedimentology of Siliceous Deposits in Japan

Publisher Summary This chapter discusses the recent developments in the sedimentology of siliceous deposits in Japan. Fine-grained siliceous deposits are involved to varying extents in all Phanerozoic strata of Japan except the Cambrian. The diversity of occurrences and lithologic types and the complex geologic history of Japan as a compound orogenic belt in the Circum-Pacific region provide one of the most favorable areas in which siliceous deposits are investigated. Economic and scientific interests have stimulated the investigation of siliceous rocks; some are used as silicastone ores, some contain metallic ores such as copper and manganese, and some are oil source rocks and reservoirs. Consolidated fine-grained siliceous deposits of Japan have various appearances and are called different names. Their classification is mainly based on the degree of consolidation, the rate of mixing with clay, and the dominant kind of siliceous skeletons. Chert is a dense and very hard siliceous sedimentary rock composed of cryptocrystalline and microcrystalline opaline silica, quartz or a mixture of the two. The chapter also describes the distribution of siliceous deposits in space and time in the Japanese islands.

Chapter 11 Chemical Sedimentology of Some Permo-Jurassic and Tertiary Bedded Cherts in Central Honshu, Japan

Publisher Summary Most bedded cherts are composed principally of siliceous skeletons such as radiolarian shells, spines, sponge spicules, and diatom frustules. This chapter compares the major and minor element compositions of some bedded and massive cherts, siliceous shale, and related rocks in the Setogawa Terrain and the Chichibu Geosyncline with modern marine sediments. The chapter also discusses the depositional environments and the mechanism of sedimentation of bedded cherts in comparison with modern marine sediments. In the Chichibu Geosyncline, samples were collected from the Triassic to Jurassic section along the Kiso river in the Inuyama district, where folded Middle Triassic to Middle Jurassic bedded cherts, Middle to Upper Jurassic red siliceous shale with sporadic Mn-carbonate lenses, and Upper Jurassic arkosic sandstone and slump beds occur as thrust slices. In addition, interelemental relations among nineteen elements in the bedded cherts from the Setogawa Terrain and the Chichibu Geosyncline were investigated by means of the principal component analysis (PCA).

Chapter 14 Identification of Mixtures of Opaline Silica Phases and Its Implication For Silica Diagenesis

Abstract Three −4 A silica phases–tridymite, opal-CT and Cristobalite–are identified in Neogene siliceous sediments in northern Japan, in which two or three of them may coexist. X-ray diffraction analyses of artificial mixtures of tridymite–opal-CT, opal-CT–Cristobalite and tridymite–Cristobalite were performed to observe the effect of mixing on the strongest ~4 A peak of each phase. Though it never splits, the peak of the tridymite–opal-CT mixtures may be shouldered, acute or rounded. the peaks of tridymite–Cristobalite and opal-CT–cristobalite mixtures are usually tailed toward the low angle side and even split in one case. in addition to these visual distinctions, peak shape analyses of artificial mixtures were performed and peak width, skewness and acuteness were determined. We can distinguish mixtures of two opaline silica phases with specific ratios by these visual distinctions and peak shape characteristics.