Chung-Cherng Lin

Genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh-pressure gneiss from Erzgebirge, Germany

Abstract The common association of microdiamonds with the phase assemblage: phlogopite, apatite, paragonite and α-quartz (containing amorphous Na–Al silicate inclusions), as inclusions in garnets of quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany, was studied by analytical electron microscopy. The assemblage implies a precedent melt, which coexisted with the microdiamonds before and after entrapment in the garnet host, and subsequently crystallized. The formation of microdiamonds in this metamorphic rock could be explained by cotectic-induced partial melting of a subducted continental slab at about 4–6 GPa and 1000°C, as constrained by the occurrence of TiO 2 with an α-PbO 2 -type structure at the peak metamorphic conditions, and by the catalytic effect of siderophile and chalcophile elements.

Elasticity of single-crystal calcite and rhodochrosite by Brillouin spectroscopy

Abstract The single-crystal elastic moduli of natural samples of both calcite (CaCO3) and rhodochrosite (MnCO3) have been measured by Brillouin spectroscopy under ambient condition. Based on the trigonal unit cell, the elastic constants C11, C33, C44, C12, C13, and C14 are 149.4(7), 85.2(18), 34.1(5), 57.9(11), 53.5(9), -20.0(2), and 223.9(15), 132.6(41), 44.5(9), 93.4(21), 76.0(23), -17.3(6) GPa for CaCO3 and MnCO3, respectively. Our data for calcite are in good agreement with earlier data obtained by ultrasonic experiments. The off-diagonal elastic constants (C12, C13, and C14) for rhodochrosite have systematically larger values than the trend defined by other isostructural carbonates, in all of which the divalent cations are alkaline-earth metals. This is a distinctive signature of transition- metal-bearing oxides, which is present in silicates and simple oxides as well.

Sol-gel synthesis of zinc orthosilicate

Abstract A two-step sol-gel process between a nucleophilic addition on zinc by the SiOH group was explored to prepare Zn 2 SiO 4 powders from organometallic compounds. Hydrolysis of tetraethyl orthosilicate under ⩽ 3N HNO 3 followed by reaction with diethylzinc and then condensation under 0.1 to ∼ 15M NH 4 OH resulted in the formation of amorphous Zn 2 SiO 4 particles ∼ 10–20 nm in size. Infrared and Raman spectroscopic results suggest that this Zn 2 SiO 4 consists mainly of a SiOZn network which remained amorphous to 600°C. Heating (10°C/min) above ∼ 700 and 900°C caused the formation of β- and α-Zn 2 SiO 4 powders, respectively, which appeared to coalesce but not sinter. A high concentration of NH 4 OH is preferred for the synthesis of a high-density gel which transforms at lower temperatures to crystalline polymorphs.

POST-ARAGONITE PHASE TRANSITIONS IN STRONTIANITE AND CERUSSITE—A HIGH-PRESSURE RAMAN SPECTROSCOPIC STUDY

Abstract By using Raman spectroscopy, phase behaviors of strontium and lead carbonates under quasi-hydrostatic conditions have been studied in a diamond-anvil cell up to 420 kbar at room temperature. The aragonite-type SrCO 3 (strontianite) and PbCO 3 (cerussite) were found to transform to their high-pressure polymorphs at ∼ 350 and ∼ 170 kbar, respectively. Mode softening was observed for some of the Raman bands of both SrCO 3 and PbCO 3 . For certain modes of both SrCO 3 and PbCO 3 , furthermore, frequency shifts with the compression and decompression processes form a hysteresis loop. The causes of the mode softening and hysteresis loop have been suggested to correlate with the spatial hindrance on the movement of the CO 3 groups. Combined results of present and previous work suggest that all observed MCO 3 I ↔ MCO 3 II transitions (M = Sr, Pb, and Ba) at room temperature may operate with the same mechanism, and that the phase boundaries of these transitions possess a negative Clapeyron slope. By extrapolation, a hydrostatic pressure greater than ∼ 1000 kbar may be required for CaCO 3 to form the same post-aragonite modification at room temperature.

Raman spectra of phase C (superhydrous phase B) at various pressures and temperatures

Variations of Raman spectra of phase C, or the superhydrous phase B, were investigated up to about 420 kbar at room temperature and in the range 80–873 K at atmospheric pressure. With the exception of the 3355 cm -1 OH band, the frequency of all observable Raman bands of phase C increases monotonously with increasing pressure. However, the frequency for the two well-defined OH bands at pressures greater than about 100 kbar depends strongly on the presence of shear stress. With the exceptions of the three OH bands and the ambient 325 cm -1 band, the frequency of all other Raman bands of phase C decreases with increasing temperature within the experimental uncertainties and the range investigated. Phase C appears to be intact up to the highest temperature attainable in the present study. However, Raman spectra obtained at various conditions seem to suggest that amorphization may start to occur in phase C above 773 K, but dehydration is not completed at the highest temperature studied. It is interesting to find that amorphization occurs at lower temperature than dehydration in phase C. The pressure and temperature effects on the Raman frequencies of phase C are different in many aspects from those observed earlier in phases A and B.

Structure and dissolution of CaO–ZrO2–TiO2–Al2O3–B2O3–SiO2 glass (II)

Abstract The purpose of this study is to determine the structure and dissolution resistance of the CaO–ZrO 2 –TiO 2 –Al 2 O 3 –B 2 O 3 –SiO 2 glass fabricated in Part (I). Vibrational spectroscopy indicated the structure units of the glass are SiO 4 , TiO 4 , BO 4 and their groupings for thermal exposure up to 750°C. The surface of an etched sample contained amorphous-phase-separated droplets which developed in multiple stages and clustered upon heating near the glass transition temperature, T g (ca. 640°C). Room temperature (30±1°C) dissolution of glass powders in aqueous solutions (pH=0, 1, 3, 5, 10 and 12) showed size-dependent leaching of cations Ca, Al, Ti and Zr, with Zr the most dissolution-resistant component. The leaching of the cations Ca and Al was linear with time in the initial dissolution stage and each had a logarithmic dissolution rate versus pH dependence; the slope being negative in acidic while positive in basic condition with a minimum dissolution rate in the pH range of 6.5 to 8.5. Dissolution of a partially devitrified glass slab under acidic (pH=0) or basic (pH=12) conditions indicated that glass/crystal (ZrO 2 derived) interfaces were preferentially dissolved with the crystals being more resistant. An Al-depleted layer and ridge-free hillocks developed on the glass surface at pH=12 and 0, respectively.

High-pressure phase transformations of carbonates in the system CaoMgOSiO2CO2

The phase behaviour of the carbonates in the system MgOCaOSiO2CO2 have been studied in a diamond-anvil press employing YAG laser heating from about 40 to 260 kbar at ∼ 1000°C. It is already known that at the CaCO3MgCO3 join calcite (CaCO3) is stable only at relatively low pressures (< 30 kbar), that aragonite (CaCO3) is stable between 30 and 400 kbar, and that magnesite (MgCO3) is stable between 7 and 550 kbar at ∼ 1000°C. In the present study it has been found that the intermediate mineral dolomite (CaCO3 · MgCO3) decomposes into the mixture aragonite + magnesite in the pressure range 60–70 kbar at ∼ 1000°C. Another intermediate mineral, huntite (CaCO3 · 3MgCO3), transforms to a new phase at pressures below 40 kbar, and then decomposes to the mixture dolomite + magnesite at pressures greater than ∼ 60 kbar. The new phase with the huntite composition possesses an orthorhombic cell with ao = 9.933 ± 0.002, bo = 6.712 ± 0.001 and co = 24.06 ± 0.02 A at ambient conditions. There are only two well-known silicate-carbonate minerals in the CaOSiO2CO2 system. These are spurrite and tilleyite. Both were found to decompose into their component silicates and carbonates at pressures below 40 kbar at ∼ 1000°C. On the basis of the present study it can be concluded that aragonite and magnesite are the only two carbonates in the entire CaOMgOSiO2CO2 system which are stable at pressures greater than about 70 kbar at ∼ 1000°C, if no interaction occurs between carbonates and silicates at still higher pressures.

RAMAN SPECTRA OF PHASE E AT VARIOUS PRESSURES AND TEMPERATURES WITH GEOPHYSICAL IMPLICATIONS

Abstract Variations in Raman spectra of phase E were investigated up to 340 kbar at room temperature and in the range 83–870 K at atmospheric pressure. Phase E appears to remain intact up to at least 870 K. However, the OH Raman band of phase E was not observable at temperatures above 473 K during heating, and it was not detected in the recovered sample. The temperature variation in Raman frequencies of phase E also showed abnormal behavior above 473 K. Thus, it is suggested that phase E dehydrated and converted to an imperfect forsterite above 473 K. Only the two most intense Raman bands of phase E were reliably measured in the high- pressure experiments. With the exception of the OH Raman band, the Raman frequencies of phase E increase with increasing pressure but decreased with increasing temperature. The frequency of the OH Raman band increases with increasing temperature, which parallels that observed in both phase A and the hydrous β-phase. The temperature dependency was observed to be linear below 500 K and the pressure dependency to be non-linear within the experimental uncertainties and the range investigated. It appears that phase E may be regarded as the hydrous form of forsterite and it is further suggested that phase E and the hydrous β-phase may be competitive hydrous phases in the upper mantle and transition zone.

Raman spectra of phase a at various pressures and temperatures

Abstract Variations of the Raman spectra of phase A (Mg 7 Si 2 O 14 H 6 ) were investigated up to about 400 kbar at room temperature and in the range 80–853 K at atmospheric pressure. Phase A appears to undergo a reversible phase transition around 180 kbar at room temperature, and it becomes amorphous above 813 K at atmospheric pressure. The Raman frequencies of the two strong OH bands of phase A decrease linearly with increasing pressure, but increase non-linearly with increasing temperature. The frequencies of the other Raman bands of phase A increase non-linearly with increasing pressure but decrease both linearly and non-linearly with increasing temperature within the experimental uncertainties and the range investigated. The trends of the pressure and temperature effects on the Raman frequencies of phase A parallel those observed in the hydrous β-phase, but nonlinear behaviour was not observed in the latter.

Raman study of phase D at various pressures and temperatures

Samples of phase D were synthesized in high pressure and high temperature experiments with a multi-anvil press. Phase D may have a chemical range of Mg/Si = 0.65 ± 0.05 and H2O in the range 6∼14 wt% on the basis of electron microprobe analyses. The nature of phase D was characterized by both powder X-ray diffraction and Raman spectroscopy. The resolution of the Raman spectra of phase D is in general very poor, though the X-ray diffraction data suggest that it is well crystalline. No clear OH Raman band of phase D was demonstrated, however. Variations of Raman spectra of phase D were investigated up to 18 GPa at room temperature and in the range 81–373 K at atmospheric pressure. Lower temperatures and/or high hydrostatic pressures did not improve the quality of the spectra. Phase D is extremely unstable when it was irradiated by laser and/or heated above 373 K at atmospheric pressure. Among the silicate vibrations, the pressure variation has been studied for only two bands and the temperature variation for four bands. The Raman frequencies for these bands of phase D increase with increasing pressure but decrease with increasing temperature, which are the same as the other dense hydrous magnesium silicates so far studied.