糀谷 浩

Abstract Drop-solution enthalpies of Mg2SiO4 wadsleyite and SiO2 stishovite were determined to be 141.38 ± 1.13 and 4.05 ± 0.38 kJ·mol-1, respectively, by performing drop-solution calorimetry with lead borate solvent at 978 K. Isobaric heat capacity of Mg2SiO4 wadsleyite was also measured using differential scanning calorimetry in the temperature range of 300−820 K. In addition, their self-consistent thermoelastic parameters of thermal expansivity (α), isothermal bulk modulus at the standard state (KT0) and its temperature derivatives [(∂KT0/∂T)P] were reassessed by combining the least squares fitting of a third-order Birch−Murnaghan equation of state with Grüneisen relation equation α = γth,0CV/(KT0V), together with those for Mg2SiO4 forsterite, Mg2SiO4 ringwoodite and SiO2 coesite, where isochoric heat capacity (CV) was calculated using the Kieffer model and thermal Grüneisen parameter at the standard state (γth,0) was constrained from measured α data. Obtained thermodynamic parameters were used to calculate forsterite−wadsleyite and wadsleyite−ringwoodite phase boundaries in Mg2SiO4 and coesite−stishovite phase boundary in SiO2. Finally, the present self-consistent thermodynamic datasets were applied to thermodynamic calculations of phase boundaries in the MgSiO3 system among [ringwoodite + stishovite], [wadsleyite + stishovite] and akimotoite by varying the standard enthalpies of formation for ringwoodite and stishovite within their uncertainties. The calculation results suggest that the stability field of the MgSiO3 akimotoite phase spreads to lower pressure region by 2−4 GPa than what have been accepted so far.

Abstract High‐temperature calorimetry (HTC) originated in the 20th century as a niche method to enable measurements not easily accomplished with acid solution calorimetry, combustion calorimetry, vapor pressure, or EMF methods. Over time, HTC has evolved into a versatile approach to accurately quantify formation, phase transition, surface and interfacial enthalpies of a wide range of materials including minerals and refractory inorganic compounds. This evolution has been the result of numerous adjustments to experimental setups and procedures, followed by rigorous testing. The commercial availability and the scientific success of this technique have led to an increase in the number of laboratories applying HTC. However, the knowledge acquired by researchers over the past 70 years is scattered throughout the literature or only available as laboratory internal documentation and personal experience. This publication is a collaborative effort among several leading HTC laboratories to summarize and unify current state‐of‐the‐art HTC techniques and procedures. The text starts by summarizing various HT techniques that are commonly used for readers with an interest in HTC in general. It is then directed toward HTC users and includes a brief section on data evaluation procedures as well as a comprehensive compilation of reference data utilizing molten sodium molybdate and lead borate solvents. Finally, for experienced HTC users, an in‐depth discussion of some common difficulties and a discussion of uncertainties are presented.

Abstract Three single crystals of CaTi2O4 (CT)-type, CaFe2O4 (CF)-type, and new low-density CaFe2O4 (LD-CF) related MgAl2O4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl2O4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively. The structures of CT-type and LD-CF related MgAl2O4 were analyzed by single-crystal X-ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, (Z = 4, space group: Cmcm) at ambient conditions. This phase has an AlO6 octahedral site and an MgO8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å (Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO6 and a 4+1 AlO5 trigonal bipyramid, in which MgO5 trigonal bipyramids are accommodated. The CF-type MgAl2O4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm3 among MgAl2O4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm3), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl2O4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.

高圧相Mg₂Cr₂O₅の回収試料を粉末X線および透過電顕で調べたところ，基本構造の指数の半整数位置付近に，分裂した新たな反射が生じるとの興味深い現象が観察された．調べた結果，これらの分裂した反射は，構造中に周期的に配列した反位相境界が形成されたことによって生じることが分かった．そこで，これら周期的な反位相境界の形成とこの相の高圧相転移の関係について，考察した．

Mg2Al2O5組成を持つ高圧相が新たに発見された。粉末X線回折パターンが極めて類似しているルードビッヒアイトの構造に基づき、新Mg2Al2O5高圧相の結晶構造モデルを作成した。得られた構造モデルでは、稜共有および頂点共有により繋がった(Mg,Al)6八面体に囲まれた三角型のトンネルができており、その中にMgが位置している。トンネル中のMgは三角柱型MgO6の配位環境を持つ。X線回折ピークの消滅側から、空間群はPba2またはPbamの何れかであることが分かった。リートベルト解析の結果、Pbamを採用した場合のR因子はPba2の場合よりも小さくなることから、空間群はPbamが妥当であると考えられる。

海洋堆積物や玄武岩の高圧相中に存在するCaAl4Si2O11組成のCAS相の安定領域を決定するために、高圧実験と熱測定実験を行った。その結果、CAS相は約13GPa以上、1100℃以上で安定であることが示された。また測定されたエンタルピーデータと文献値を組合わせることにより、CAS相がCaペロブスカイト、コランダム、スティショバイトに高圧で分解する境界線を計算した。その結果、地温勾配を仮定するとき、CAS相は遷移層から下部マントル最上部で安定であることが推定された。

マルチアンビル装置で24GPaまで、NaAlSi3O8の高圧相関係および1400℃でのKAlSi3O8-NaAlSi3O8系ホランダイト固溶体の安定領域を調べた。高圧合成試料を微小部および粉末X線回折法で調べ、SEM-EDSで組成分析を行った。 その結果、NaAlSi3O8組成では800-2000℃の範囲で21-22GPaでNaAlSi2O6ジェダイト＋SiO2スティショバイトがNaAlSiO4カルシウムフェライト＋2SiO2スティショバイトに転移するが、NaAlSi3O8ホランダイトは見出されなかった。また1400℃におけるKAlSi3O8ホランダイトへのNaAlSi3O8の固溶量は40-50mol%であり、1000℃の40mol%に比べて大きな増加が見られなかった。以上のことから、NaAlSi3O8ホランダイトの安定領域は2000℃以上のかなり高温領域にあることが予想される。このことは、隕石中のNaAlSi3O8に富むホランダイト相の生成条件と調和的である

海洋プレートの沈み込みに伴いそのプレートの一部を構成している玄武岩がマントル深部にもたらされる。玄武岩組成を出発物質とする高圧高温実験は、アルミニウム成分に富んだカルシウムフェライト型結晶構造を持つ相が高圧下で安定に存在することを示してきた。その相の主成分であるカルシウムフェライト型MgAl₂O₄に関しては高圧合成の困難さから単結晶法による構造解析はまだなされていない。そこで、本研究ではカルシウムフェライト型MgAl₂O₄を高圧高温合成し、回収試料の粉末X線回折プロファイルを用いてリートベルト解析を行った。解析の結果、格子定数は a = 9.9429(5)Å, b = 8.6455(5) Å, c = 2.7900(1) Åと決定された。また、原子座標や熱振動パラメーターも精密化された。