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1.
接触交代夕卡岩型矿床中石榴子石和辉石成分特点及其与矿化的关系 总被引：1，自引：0，他引：1
本文对中国十四个接触交代钙夕卡岩矿床和钙-镁夕卡岩矿床中的三百多个样品的石榴子石和辉石成分进行了电子探针分析。不同矿床类型的石榴子石和辉石成分代表着钙夕卡岩矿床的十个矿种(Fe、Fe-Cu、Pb-Zn、W、Sn、Sn-Mo-Bi-W、、W-Bi-Cu-Mo、Cu-Zn、Cu-Sn、W-Zn-Cu)和钙-镁夕卡岩矿床的三个矿种(Fe-Cu、Mo、Pb-Zn)。石榴子石和辉石成分变化范围大,大多数石榴子石是含锰铝榴石+铁铝榴石+镁铝榴石小于15％(摩尔百分数)的钙铝榴石-钙铁榴石固溶体;大多数辉石是含小于5％的锰钙辉石的透辉石-钙铁辉石固溶体。有些Pb-Zn钙-镁夕卡岩矿床中的辉石显示出Mn含量有所增加。只有Sn和W钙夕卡岩矿床及Pb-Zn钙-镁夕卡岩矿床含(Sps+Alm+Pyr)总量大于15％的石榴子石。石榴子石和辉石成分与夕卡岩矿床金属矿化类型之间有某些联系。 相似文献
2.
广西拉么Zn—Cu矿床中夕卡岩的主要矿物特点与Ca—Fe—Si—C—O体系T—lgfO2图解 总被引：1，自引：0，他引：1
用电子探针数据研究了该矿床中主要夕卡岩矿物特点及其变化规律。采用理想结晶溶液固溶体位置混合模式,计算了石榴子石和辉石矿物对中钙铁榴石和钙铁辉石的摩尔分数。在计算纯固相和流体相参加的反应抵达平衡时的边界方程中,加上理想结晶固溶体中端元组分摩尔分数的修正项后,我们便可以计算出有纯固相、流体相和理想结晶溶液固溶体参加反应的在给定T、P条件下的lgfo_2值。计算结果表明:无变度点4周围的Hed+And+Wol组合对本矿区具有重要的地质地球化学意义。 相似文献
3.
对柿竹园矽卡岩成因及其成矿作用的新认识 总被引：10，自引：0，他引：10
本文将报导柿竹园Ｗ，Ｓｎ，Ｍｏ，Ｂｉ矿床中矽卡岩矿物－石榴子石和辉石成分和石榴子石中熔融包裹体研究结果，对不同中段的１６块矽卡岩样品（９４％的标本采自第Ⅲ矿带）中的石榴子石和辉石进行了电子探针分析，共获得２３０个测点的数据，此外，还对上述中段大部分样品里的石榴子石和辉石进行了从边部到核心成分测定，共获得１３个样品的９７个测点数据，研究结果表明，绝大多数石榴子石属于含Ａｌｍ＋Ｓｐｓ＋Ｐｙｒ〈１３％（ 相似文献
4.
A newly identified skarn occurrence is described from the Neoproterozoic rocks of the SW Arabian shield. It is exposed to the SE, E and NE of the Al-Madhiq town. The skarn attributes correspond to those typical of the calcic skarns that host W-deposits. It is characterized as an exoskarn of the proximal type, related to a granitoid contact close to an impure quartzite bed within the regional metamorphic rocks of mixed sedimentary and volcanic derivation. The skarn is localized along a shear zone parallel to the regional faults and other major shear zones. Samples from the studied area contain characteristic skarn minerals that include both the prograde (brownish red grossular, ferrosalite, aluminian titanite-grothite, albite-oligoclase, scapolite), and retrograde (epidote, quartz, hornblende, calcite) assemblages. The pyroxenes are ferrosalites, Mn-bearing, and more like those from “oxidized” skarns; although garnets indicate it to be a “reduced” type skarn. Epidote mimicks that from typical skarns, as it bears a pistacite content of 15.9–20.7%. Grossular composition reflects a largely reduced genetic environment; as it is in solid solution with 6.5–21.6% andradite, 0–0.15% uvarovite, 0–0.47% pyrope, 4.33–18.75% almandine, and 0.4–8.58% spessartine molecules. Titanite composition varies from aluminian titanite to grothite, that may be analogous to the newly described Al-rich titanite from the low-pressure calc-silicate rocks. 相似文献
5.
Anastasia Chopelas 《Physics and Chemistry of Minerals》2006,33(6):363-376
Raman and infrared spectroscopic data at ambient and high pressures were used to compute the lattice contribution to the heat capacities and entropies of six endmember garnets: pyrope, almandine, spessartine, grossular, andradite and uvarovite. Electronic, configurational and magnetic contributions are obtained from comparing available calorimetric data to the computed lattice contributions. For garnets with entropy in excess of the computed lattice contribution, the overwhelming majority is found in the subambient temperature regime. At room temperature, the non-lattice entropy is approximately 11.5 J/mol-K for pyrope, 49 J/mol-K for almandine, and 19 J/mol-K for andradite. The non-lattice entropy for pyrope and some for almandine cannot be accounted for by magnetic or electronic contributions and is likely to be configurational in nature. Estimates of low temperature non-lattice entropies for both spessartine and uvarovite are made in absence of calorimetric measurements and are based on low temperature calorimetry of other minerals containing the Mn2+ and Cr3+ cations as well as on solid solution garnets containing these cations. The estimate for uvarovite non-lattice entropy is approximately 18 J/mol-K, while for spessartine, approximately 45 J/mol-K. Neither of these cations is expected to provide electronic contributions to the entropy. For both iron-bearing garnets, a small electronic or magnetic entropy contribution continues above ambient temperatures. High pressure data on pyrope, grossular and andradite permit calculation of the thermodynamic parameters at high pressures, which are important for computation of processes in the Earth’s mantle. Thermal expansion coefficients of these materials were found to be 1.6, 1.5, 1.6×10−5 K−1 at 298 K, respectively, using a Maxwell relation. These closely match the literature values at ambient conditions. 相似文献
6.
The i.r. spectrum of 13 analyzed garnets of the pyralspite group has been investigated in the 1400-200 cm–1 region, and correlations have been found between the spectrum and the chemical composition. The results include: typical features in the spectrum of the end-members pyrope, almandine and spessartine; relationships between the spectrum and the pyrope percentage in pyrope-almandine solid solutions; and the influence of the CaO (grossularite) amount on the shape of the low-frequency absorption bands. These data allow a semiquantitative determination of the pyrope percentage in pyrope-almandine solid solutions. 相似文献
7.
R. Newberry 《Mineralium Deposita》1987,22(3):207-215
The polymetallic Pb-Zn-Ag-W-Cu-Au skarn and vein ores of the Darwin area, California, represent a metal association not common in skarn deposits. Detailed and regional geologic mapping combined with modal and chemical analyses of igneous rocks and electron microprobe and assay studies of calc-silicate rocks shows that there are three separate mineralizing systems which have been superimposed by structural and intrusive events. Scheelite-bearing skarns are associated with the alkaline Darwin stock; small Cu±Au skarns are adjacent to sheared (thrust-emplaced) granite-granodiorite masses; and Pb-Zn-Ag vein skarns are zoned around small granite porphyry dikes and plugs. Calc-silicate mineral compositions are also indicative of three different mineralization systems: W-skarns have high Fe pyroxenes and moderate Fe garnets; Cu-skarns have low Fe-pyroxenes and high Fe-garnets; and Pb-Zn skarns have high-Mn pyroxenes and garnets (patterns seen in other skarn deposits). Vein ores contain metals compatible with the subjacent skarns and apparently represent lower-temperature remobilization of skarn ores. 相似文献
8.
This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized. 相似文献
9.
Philippa M. Black 《Contributions to Mineralogy and Petrology》1973,38(3):221-235
A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has
been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict.
The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and
pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised
by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments;
calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional
rocks.
Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear
in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments
and metabasalts.
45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser.
The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores
enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites
and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5
mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect
similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of
parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of
garnets.
Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective
of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are
almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation
ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions
than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral
assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular
than garnets from lower-grade pelites.
The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement
of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified
equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine. 相似文献
10.
Martin Okrusch 《Contributions to Mineralogy and Petrology》1971,32(1):1-23
Three garnet-biotite pairs and eleven garnet-cordierite-biotite triplets from the Steinach aureole (Oberpfalz, North-East Bavaria) were analyzed using an electron probe microanalyzer.The regional metamorphic muscovite-biotite schists contain garnets strongly zoned with Mn-Ca-rich centers and Fe-rich edges, the average composition being almandine 67 — spessartine 4 — pyrope 4 — grossular (+andradite) 25.The first contact garnet that is formed in mica schists of the outermost part of the aureole is small, virtually unzoned, and has an average composition of almandine 52 — spessartine 37 — pyrope 8 — grossular (+andradite) 3. With increasing metamorphic grade, there is a consistent trend to form garnets richer in Fe ending up with a composition almandine 84.5 — spessartine 5.5 — pyrope 7.5 — grossular (+andradite) 2.5. This trend is accompanied by a general increase in grain size and modal amount of garnet. Associated cordierites and biotites also become richer in Fe with increasing grade. While the garnets in the highest grade sillimanite hornfelses are poorly zoned, the transitional andalusite-sillimanite hornfelses contain garnets with distinct but variable zonation profiles.These facts can possibly be explained by the time-temperature relationships in the thermal aureole. In a phase diagram such as the Al-Fe-Mg-Mn tetrahedron, the limiting mineral compositions of a four-phase volume or a three-phase triangle are fixed by T and P (the latter remaining effectively constant within a thermal aureole). Thus, in garnet-cordierite-biotite assemblages, garnet zonation should be controlled by temperature variation rather than by a non-equilibrium depletion process. Taking into account the experimental data of Dahl (1968), a zoned garnet from a transitional andalusite-sillimanite hornfels would reflect a temperature increase of about 40° C during its growth. A hypothetical P-X diagram is proposed to show semi-quantitatively the compositional variation of garnet-cordierite pairs with varying pressures (T constant). 相似文献
11.
《Chemical Geology》2006,225(3-4):336-346
We present results of high temperature, high pressure atomistic simulations aimed at determining the thermodynamic mixing properties of key binary garnet solid solutions. Computations cover the pressure range 0–15 GPa and the temperature range 0–2000 K. Through a combination of Monte-Carlo and lattice-dynamics calculations, we derive thermodynamic mixing properties for garnets with compositions along the pyrope–almandine and pyrope–grossular joins, and compare these with existing experimental data. Across the pressure–temperature range considered, simulations show virtually ideal mixing behaviour in garnet on the pyrope–almandine join, while large excess volumes and enthalpies of mixing are predicted for garnet along the pyrope–grossular join. Excess heat capacities and entropies are also examined. These simulations shed additional light on the link between the behaviour at the atomic level and macroscopic thermodynamic properties: we illustrate the importance of certain atomistic Ca–Mg contacts in the pyrope–grossular solid solutions. For simulation techniques of this type to become sufficiently accurate for direct use in geological applications such as geothermobarometry, there is an urgent need for improved experimental determinations of several key quantities, such as the enthalpies of mixing along both joins. 相似文献
12.
Raman spectra of silicate garnets 总被引：2，自引：1，他引：1
The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine,
grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder
spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison
of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized
with 26Mg. A general order of mode frequencies, i.e. R(SiO4)>T(metal cation)>T(SiO4), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies
as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO4-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations
reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode
behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine
join.
Received: 3 December 1996 / Revised, accepted: 13 April 1997 相似文献
13.
On recasting analyses of garnet into end-member molecules 总被引：5，自引：0，他引：5
P. C. Rickwood 《Contributions to Mineralogy and Petrology》1968,18(2):175-198
Most published analyses of garnet deviate from structural ideality. Consequently, compositions expressed as molecular percentages of end-member molecules may differ if different re-calculation sequences have been used. A suitable standard calculation procedure is presented, and is demonstrated to be satisfactory by its application to 69 published analyses of garnets both common and rare.It is seldom necessary to use molecules other than pyrope, almandine, spessartine, grossular, andradite, uvarovite and hydrogrossular, and most analyses can be recast into four or less molecules which exceed 3% of the garnet. This means that most analyses can be visually displayed in a composition tetrahedron.It is suggested, that the percentage number of cations which can be allocated to garnet molecules is a figure useful for assessment of analytical quality. More than 95% of the cations can be so allocated in the majority of the analyses considered.Full details of the proposed scheme are appended together with a worked example which demonstrates the abbreviated procedure which applies to most common garnets. A compilation is given of the common physical properties which have been measured for synthetic end-member garnets of the types used in the calculation scheme.S.A. UMP Publication No. 5. 相似文献
14.
15.
库车坳陷三叠系发育良好,出露齐全,主要由陆相碎屑岩组成。本文运用电子探针微区成分分析方法,对库车坳陷北部三叠系砂岩中石榴石、电气石、铬尖晶石进行了矿物化学成分分析。结果显示,碎屑石榴石主要富含铁铝榴石,其次为镁铝榴石、锰铝榴石,钙铝榴石含量较低,他们主要来自于低级-高级变质岩和花岗岩;电气石主要来自于变质沉积岩和花岗岩;铬尖晶石则主要源自岛弧玄武岩、洋岛玄武岩和与俯冲相关的橄榄岩。综合石榴石与电气石研究结果表明,上三叠统碎屑物质更多的来自于高级变质岩和花岗岩,而下三叠统碎屑物质主要源自低级变质岩和花岗岩。通过对比西天山榴辉岩、片麻岩中石榴石成分,本文所研究的高镁石榴石以低钙铝榴石含量与榴辉岩中石榴石相区别,而与片麻岩中石榴石成分相似。西天山榴辉岩在三叠纪时期可能尚未剥露至地表,但片麻岩已有相当范围的出露。上三叠统的碎屑铬尖晶石可能主要来自于中天山及南天山的岛弧岩浆岩及蛇绿岩,部分源自洋岛玄武岩,为南天山为多岛海造山提供了沉积学证据。 相似文献
16.
Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton.
Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are
1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high XMn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape
of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The
core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine
and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69.
The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59.
Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and
0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor
biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite
melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is
related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone. 相似文献
17.
Garnetiferous basic granulites occur, as parts of hornblende-pyroxene- and pyroxene granulites, in a Precambrian terrain around Saltora. The chemistry of the garnetiferous basic granulites is broadly similar to that of the hornblende-pyroxene granulites, their immediate precursors, but in detail they have distinctly higher Fe/Mg ratios. The compositions of the major mafic silicates of the garnetiferous varieties do not reflect higher pressures of formation: the Jd/Ts ratios in calcic pyroxenes are similar to those from the non-garnetiferous varieties, and the pyrope contents of garnets are low. Exchange equilibrium in respect of major elements was established among the mafic silicates in spite of garnets being late overprints. The orthopyroxene — calcic pyroxene pairs from the garnetiferous granulites show lower values of K D(Mg-Fe) opx-cpx than those from the non-garnetiferous granulites, pointing to lower temperature of equilibration. The K D(Mg-Fe) opx-hbl K D(Mg-Fe) cpx-hbl relations show that the more magnesian triads equilibrated at lower temperatures; viewed against experimental data regarding the effect of Mg/Fe ratios on the appearance of garnets in basic rocks, formation of garnets by cooling is strongly indicated. Several intergrowth textures, especially garnet-ilmenite and garnet-quartz (±albite) symplectites, and modal relations argue in favour of composite reactions of the type hornblende+ quartz-→calcic pyroxene+garnet+albite+H2O, which couple hornblende breakdown reactions with orthopyroxene+anorthite→garnet reactions. The approximate range of pressure and temperature conditions, estimated from experimental data, are 6–8.5 kb and 750–830° C. Since garnets formed by cooling in iron-rich granulites, the garnetiferous granulites do not represent higher pressure subfacies of the granulite facies. 相似文献
18.
K. Krambrock F. S. Guimarães M. V. B. Pinheiro R. Paniago A. Righi A. I. C. Persiano J. Karfunkel D. B. Hoover 《Physics and Chemistry of Minerals》2013,40(7):555-562
Fine gem-quality, purplish-red garnets from the Tocantins State, Brazil, were investigated for their crystal chemistry and optical properties by several spectroscopic techniques, including electron microprobe analysis, Mössbauer, Raman spectroscopy and optical absorption. Although most garnets are purplish-red, some specimens show color zoning, with deep red color in the core and purple in the outer parts. Electron microprobe analysis showed that these garnets are principally almandine–pyrope solid solution at the rim. However, at the red core, they contain also up to 7 % of spessartine. Mössbauer spectroscopy reveals that the iron content is predominantly Fe2+ (>99 %) in the natural garnets. The optical absorption spectra are dominated by spin-allowed and unusual high-intense spin-forbidden transitions from eightfold coordinated Fe(II) in the near infrared and visible spectral region, respectively. For the red core, in addition, three sharp bands centered in the blue part of the visible spectral range and a broad charge transfer band in the near-UV region are observed. All garnets with purplish colors show also a remarkable color-changing effect from purple in daylight light to red in incandescent light called alexandrite-like effect. Heat treatments in the 700–900 °C temperature range in oxidizing and reducing atmospheres lead to reversible and irreversible color changes which are discussed based on the microscopic changes in the Fe ion coordination and valence states. 相似文献
19.
Origin of garnet phenocrysts in calc-alkaline rocks 总被引：2，自引：0，他引：2
A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb
, but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle. 相似文献
20.
The compression of synthetic pyrope Mg3Al2 (SiO4)3, almandine Fe3Al2(SiO4)3, spessartine Mn3Al2 (SiO4)3 grossular Ca3Al2(SiO4)3 and andradite Ca3Fe2 (SiO4)3 was studied by loading the crystals together in a diamond anvil cell. The unit-cell parameters were determined as a function
of pressure by X-ray diffraction up to 15 GPa using neon as a pressure transmitting medium. The unit-cell parameters of pyrope
and almandine were measured up to 33 and 21 GPa, respectively, using helium as a pressure medium. The bulk moduli, K
T
0, and their first pressure derivatives, K
T
0
′, were simultaneously determined for all five garnets by fitting the volume data to a third order Birch-Murnaghan equation
of state. Both parameters can be further constrained through a comparison of volume compressions between pairs of garnets,
giving for K
T
0 and K
T
0
′ 171(2) GPa and 4.4(2) for pyrope, 185(3) GPa and 4.2(3) for almandine, 189(1) GPa and 4.2 for spessartine, 175(1) GPa and
4.4 for grossular and 157(1) GPa and 5.1 for andradite, where the K
T
0
′ are fixed in the case of spessartine, grossular and andradite. Direct comparisons of the unit-cell volumes determined at
high pressures between pairs of garnets reveal anomalous compression behavior for Mg2+ in the 8-fold coordinated triangular dodecahedron in pyrope. This agrees with previous studies concerning the compression
behaviors of Mg2+ in 6-fold coordinated polyhedra at high pressures. The results show that simple bulk modulus–volume systematics are not obeyed
by garnets.
Received: 29 July 1998 / Revised, accepted: 7 April 1999 相似文献