The identification of manganese dioxide minerals in continental manganese deposits and in marine environment is fraught with difficulties, because they are often very fine-grained, poorly crystalline, and intimately intergrown with other minerals. Although various high techniques are recently used for it, we tried to reconsider the information obtained under microscope as the most basic and important technique for the identification and characterization of opaque minerals. We searched specially the information of reflectance (R) and microhardness (VHN) of natural manganese dioxide minerals from a lot of references and classified the minerals to four (I to IV) groups based on the similarity of R and VHN in consideration of our measurement result. The magnitude of the R and VHN is I>II>III>IV groups. The minerals belong to each group are as follows: I group, pyrolusite and nsutite; II group, cryptomelane-hollandite-coronadite-manjiroite, romanechite, woodruffite, and ramsdellite; III group, chalcophanite-aurorite, birnessite, lithiophorite, todorokite, asbolane, and janggunite; IV group, vernadite, rancieite-takanelite, akhtenskite, and buserite. We conclude that the identification of the manganese dioxide minerals is probably done by observing polished sections using polarizing reflected-light microscope, considering above relationship and other optical and mineralogical properties.
This paper reports a technique of quantitative micro-analysis of zeolite group minerals by electron-probe microanalyzer with an energy-dispersive spectrometer system. Natural analcite, laumontite, heulandite and stilbite were used as reference samples. To avoid radiation damages of zeolites, beam diameters of quantitative analysis for each zeolite are set as follows; analcite and laumontite can be analyzed by defocused beam with ca. 5 microns in diameter, our laboratory routine analytical conditions, heulandite and stilbite have to be analyzed by scanning beam with 12.5 microns square or more. Heulandite and stilbite have the same anhydrous formula of CaAl2Si7O18; A water content of heulandite with ideal formula is 15.74 wt.% and that of stilbite with ideal formula is 17.89 wt.%. In analyzing these zeolites, total weight percent of anhydrous oxides sometimes falls between the values expected for two zeolites with ideal formula. As precision of analysis of the crystalline water for two zeolites, which were estimated by the difference between 100 and total weight percent of anhydrous oxides determined, is less than 2 wt.%, the varying anhydrous total is considered real. Thus, the zeolites which have intermediate composition between heulandite and stilbite can not be identified by the present procedure.
Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite. Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of “Geiyo volcanic rocks” to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area. On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.
Plagioclases in common volcanic rocks (basalt to rhyolite) were analyzed using an electron probe microanalyzer (JCXA-733) under the following condition: accelerating voltage=25 kV, beam current at Faraday cup=0.02 μA, line=Lα, crystal=PET, and measurement time=120 sec for the peak (line) and 60 sec (×2) for the backgrounds. The statistical lower limit of detection is SrO=0.05%, and the standard deviation (1σ) is 0.03% for SrO≤0.5%. The analytical results imply that Sr content of plagioclase generally decreases as crystallization progresses in closed magma systems of common volcanic rocks. Also some applications using this analytical method have been briefly discussed.
This paper reviews recent studies on the microtextures in the Ca-Mg-Fe pyroxenes whose compositions are shown on the common ‘pyroxene quadrilateral'. Two types of microtextures due to phase changes are focused on here; one is the exsolution texture formed by nucleation or spinodal decomposition process, and another is anti-phase domain structure. In this paper, we described brief supplements on the formation process of these textures. We also emphasize the importance to study on the microtextures for further understanding of the origin of the earth and planetary materials from the view points of the microtexture analysis.
Characteristic structure of Cl-amphibole is reviewed, and its application to chemical zoned amphibole from the Ramnes cauldron is summarized. Large ionic radius of Cl− brings geometrical and chemical effects to the amphibole structure. In the Cl-amphibole, the double chain is significantly deformed to fit expanded octahedral strips, which is achieved by substitution of IVAl for Si (geometrical constraint). The Cl− projects toward the A site and there is a interaction between Cl− and cations in the A site (A site constraint). The short range order of Fe2+-Cl− is observed in Cl-amphibole (chemical constraint). These constraints simultaneously control the Cl− content in amphibole and imply that Cl− substitutes for OH− only in the ferro-paragasite or ferro-hastingsite component. Based on these structural considerations on Cl-amphibole, Cl− variations in the zoned amphibole from the Ramnes cauldron were interpreted. The zoning were developed under different fluid conditions in chemistry and temperature.
Unique finds from placer deposits in the western Bulgaria are shown to discuss the interrelation among types of articles, the sites of gold-manufacturing, the placer deposits as the source of gold and the archeological significance of the discovery. The articles found in this area are characterized by both the gold and the precisely fine crafts. Most of them are identified archeologically with those at the same age of the Chalcolithic Necropolis near Varna in the eastern Bulgaria, where the metallic articles including copper and gold products were found in 1972 and have been considered to be of the oldest metal culture (before 4000 BC) in the world. The finding in the western Bulgaria are also valuable in following two facts that the sites of manufacturing gold-articles were specified to be very near to the gold-placer deposits and that the articles are comparable in various natures with those of the Varna Necropolis.