Thermodynamic study of calcic amphiboles

The paper reports original thermochemical data on six natural amphibole samples of different composition. The data were obtained by high-temperature melt solution calorimetry in a Tian–Calvet microcalorometer and include the enthalpies of formation from elements for actinolite Ca_1.95(Mg_4.4Fe _0.5 ²⁺ Al₀₁)[Si_8.0O₂₂](OH)₂(–12024 ± 13 kJ/mol) and Ca_2.0(Mg_2.9Fe _1.9 ²⁺ Fe _0.2 ³⁺ )[Si_7.8Al_0.2O₂₂](OH)₂, (–11462 ± 18 kJ/mol), and Na_0.1Ca_2.0(Mg_3.2Fe _1.6 ²⁺ Fe _0.2 ³⁺ )[Si_7.7Al_0.3O₂₂](OH)₂ (–11588 ± 14 kJ/mol); for pargasite Na_0.5K_0.5Ca_2.0-(Mg_3.4Fe _1.8 ²⁺ Al_0.8)[Si_6.2Al_1.8O₂₂](OH)₂ (–12316 ± 10 kJ/mol) and Na_0.8K_0.2Ca_2.0(Mg_2.8Fe _1.3 ³⁺ Al_0.9) [Si_6.1Al_1.9O₂₂](OH)₂ (–12 223 ± 9 kJ/mol); and for hastingsite Na_0.3K_0.2Ca_2.0(Mg_0.4Fe _1.3 ²⁺ Fe _0.9 ³⁺ Al_0.2) [Si_6.4Al_1.6O₂₂](OH)₂ (?10909 ± 11 kJ/mol). The standard entropy, enthalpy, and Gibbs free energy of formation are estimated for amphiboles of theoretical composition: end members and intermediate members of the isomorphic series tremolite–ferroactinolite, edenite–ferroedenite, pargasite–ferropargasite, and hastingsite.     

Statistical chemistry of calcic amphiboles

Principal components analysis is used to study the chemistry of 639 calcic amphiboles. Eigenvectors representing multiple partial correlation coefficients give various sets of substitutional relationships. The relative significance of each set can be noted by the percent variation of the data it represents. The highest percent variation (36%) is associated with the substitutions $$Si + Mg \rightleftharpoons Al^{IV} + Al^{VI} + Ti + Fe^{3 + } + Fe^{2 + } + Na + K$$ . Other expected substitutions among the ions such as Al^IV + Na ? Si, the positive correlation between Al^IV and Al^VI etc. are shown statistically. The substitution of Al in T ₁ and T ₂ positions imposes an ordering in the M ₁, M ₂ and M ₃ sites. Variability of OH in the amphiboles is found to be significant. There is no definite correlation between OH and Fe³⁺ but OH and Ti are positively correlated. Under certain conditions and provided the concentration of Al^IV does not change significantly, Fe and Mg may be assumed to mix ideally in the amphibole solid solution.     

Characteristics of asbestiform and non-asbestiform calcic amphiboles

In terms of morphology there are four major types of calcic amphibole; massive, prismatic, finely acicular and asbestos. Representatives of each of these types have been examined by optical microscopy, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Massive specimens (nephrite) consist of randomly oriented clusters of fine, roughly lath-shaped, sub-microscopic crystals; within each cluster the lath lengths (z) are approximately aligned but neighbouring laths are rotated with respect to one another. Finely acicular specimens (“byssolites”) have well-formed crystals bounded mainly by {110} (100) and (010) faces and characteristically have striations parallel to their lengths. Asbestiform varieties range from finer (flexible) to coarser (more brittle) specimens and many specimens contain a mixture of fine and coarse fibrils. The fibrils in a bundle are aligned parallel to z but are in a range of azimuthal orientations. It is inferred that they are formed by multiple independent nucleation and growth parallel to z rather than through parting or cleavage on {110} planes. (100) defect or twin planes, or on (010) planar defects.

The {110} cleavage in amphiboles is well reported but (100) features are rarely mentioned in the literature. Our observations reveal the importance of (100) as a cleavage or parting as well as the tendency in nephrites, byssolites and asbestos towards a lath-like (parallel to z) morphology with flattening on (100). In the latter varieties therefore, the y-direction is that of second fastest crystal growth, after z.

When subjected to moderate grinding, the comminution of asbestos fibres proceeds more by separation of fibrils and less by fracturing to shorter lengths as compared with prismatic and byssolite specimens. Prolonged grinding does, however, shorten lengths of even the least brittle asbestos.

Transmission electron microscopy revealed extensive sub-grain boundaries and dislocation networks (suggesting a deformation history) in all prismatic and nephrite specimens. Fine multiple (100) twinning was observed in asbestos but not in other varieties. Although chain-width defects [on (010)], with visibility enhanced by beam damage, were most abundant in nephrites and fibrous tremolites, there appears to be no completely consistent relationship between such features and morphological type.

Electron probe analyses showed that specimens that contain more than a very small amount of aluminium do not have asbestiform habit. Asbestos specimens also have lower contents of Mn, Na and K and have formulae closer to the ideal Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂. Small departures from this in asbestos involve Na in the A site compensated by Na for Ca rather than Al for Si whereas the reverse is true in byssolites. Chemical substitutions in prismatic specimens are much less constrained.

The characteristics of the four morphological sub-groups correlate reasonably well with what is known of their geological environments.     

Compositions and phase relations of calcic amphiboles in epidote- and clinopyroxene-bearing rocks of the amphibolite and lower granulite facies,central Massachusetts,USA

Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe³⁺-rich epidote-group minerals suggests the presence of Fe³⁺ in most of these amphiboles. An empirical Fe³⁺ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) ^c =-1.479 Na(M4) ^c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) ^u and ΣFM(M4) ^u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) ^u +1.479 ΣFM ^u ). From the deficient oxygen the Fe³⁺ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe³⁺ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe²⁺/(Fe²⁺+Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe²⁺/(Fe²⁺+Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X _Ab/X _Ed, where X _Ab=Na/(Na+Ca) in plagioclase and X _Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi_-1)^Hbl+H₂O.     

Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry

Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria:

1. edenite+4quartz=tremolite+albite
2. edenite+albite=richterite+anorthite

For A and B all possible within-site and cross-site interactions among the species □?K?Na?Ca?Mg?Fe²⁺?Fe³⁺?Al?Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.     

The distribution of certain elements among coexisting calcic pyroxenes, calcic amphiboles and biotites in skarns

Skarns composed predominantly of Ca pyroxene, Ca amphibole, and biotite occur locally in the Grenville metamorphic terrain of the Canadian pre-Cambrian Shield. The enclosing rocks are Grenville gneisses and limestones, gabbroic rocks, veined gneisses, and granitic rocks. The metamorphic grade of the skarns is identical to that of the enclosing rocks, namely the upper region of the amphibolite facies.

Twelve skarn specimens were selected for chemical study. An investigation is made of the concentrations of Al, Fe, Mg, Mn, Ti, Ca, Na, K, Ba, V, Cr, Zr, Y, and Sc in eleven Ca pyroxenes, ten Ca amphiboles, and eleven biotites.

Consideration is given to the distribution of elements among coexisting minerals. Regular relationships appear when

1. (1) the concentration of an element in a mineral is related to the concentration of the element in a coexisting mineral by a linear or non-linear function

2. (2) the linear or non-linear function is itself a function of the concentration of another element in one or both of the coexisting minerals. These relationships indicate that, for the most part, minerals within each skarn specimen closely approached a state of chemical equilibrium.

A preliminary attempt is made to explain the distribution relationships in terms of crystal chemistry.     

Optical spectroscopy study of natural Fe, Ti-bearing calcic amphiboles

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O^2?→ Fe³⁺, Fe²⁺, O^2?→ Ti⁴⁺ and Fe²⁺ + Ti⁴⁺→ Fe³⁺ + Ti³⁺). A broad intense Y-polarized band ～22?000?cm^?1 (ν_1/2?≈?3700?cm^?1) in spectra of two low iron amphiboles with a relatively low Fe³⁺/Fe_total ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe²⁺(M3) + Ti⁴⁺(M2)→Fe³⁺(M3)+Ti³⁺(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti⁴⁺ and Fe²⁺ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.     

Magmatic-subsolidus and oxidation trends in composition of amphiboles from silica-saturated peralkaline igneous rocks

Summary Sub-calcic alkali amphiboles from silica-saturated peralkaline rocks exhibit two main patterns of compositional variation, which we term the magmatic-subsolidus and oxidation trends. The first is a continuous change from magmatic to subsolidus amphibole from barroisite through richterite to arfvedsonite, involving substitutions under reducing conditions, mainly of the type Al^ivCaSi(Na,K). The second trend is towards riebeckite, under the influence of oxidizing hydrothermal fluids, according to a reaction such as Fe²⁺ SiFe³⁺ Al^vi or (K, Na)^A Fe²⁺Fe³⁺. Textural evidence suggests that fluid characteristics are as important as host-rock compositions in controlling the chemical characteristics of amphiboles in peralkaline intrusive and volcanic rocks.

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Magmatische Subsolidus-und Oxydations-Trends der Zusammensetzung von Amphibolen aus Si-gesättigten peralkalinen Magmatiten

Zusammenfassung Alkali-Amphibole aus Si-gesättigten peralkalinen Gesteinen zeigen zwei definierte Trends chemischer Variation, die hier als magmatisch-subsolidus und Oxydations-Trend bezeichnet werden. Der erstere stellt einen kontinuerlichen Übergang von magmatischen zu subsolidus Amphibolen dar, und zwar von Barroisit über Richterit bis zu Arfvedsonit, wobei Substitutionen unter reduzierenden Bedingungen, hauptsächlich des Typs Al^iv CaSi(Na,K), zu beobachten sind. Der zweite Trend zeigt, unter dem Einfluß oxydierender hydrothermaler Lösungen, eine Tendenz auf Riebeckit hin, entsprechend einer Reaktion wie z.B. Fe²⁺ Si Fe³⁺ Al^vi oder (K, Na)^AFe²⁺ Fe³⁺. Mikroskopische Befunde weisen darauf hin, daß die Eigenschaften dieser Lösungen genauso wichtig für die chemische Zusammensetzung der Amphibole in peralkalinen Intrusiv-Gesteinen und Vulkaniten sind, wie die Zusammensetzung der Wirtsgesteine.

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With 6 Figures     

Exsolution of Ca-amphibole from glaucophane and the miscibility gap between sodic and calcic amphiboles

Transmission and analytical electron microscopy (TEM/AEM) of glaucophane from glaucophane + Ca-amphibole-bearing blueschist and eclogite from two Vermont localities (Ecologite Brook and Tillotson Peak) and one California locality (Cazadero) has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles. The Tillotson Peak samples and the Cazadero samples contain coarsegrained glaucophane—actinolite pairs, while the Eclogite Brook samples contain coexisting glaucophane and actinolitic hornblende. Ca-rich glaucophanes contain abundant fine-scale lamellae of Ca-rich amphibole. These lamellae are usually oriented near (100) and . High-resolution TEM (HRTEM) images show them to be coherent. The exsolution lamellae are so narrow, beyond the resolution of AEM, that their true Ca contents are obscured by analytical contributions from the surrounding host. The AEM data suggest that the lamellae are either winchite or actinolite, depending on the true Ca concentration. In most cases, the exsolution lamellae have very curved interfaces and show variable orientation. This is attributed to the close similarity of unit-cell parameters for the two amphiboles. Three-dimensional optimal phase-boundary calculations using EPLAG (Fleet 1982) show that the observed 100 and orientations are consistent with the minimization of area strain between the two lattices along the interface. Some samples show evidence for incipient exsolution in the form of homogeneously distributed, fine-scale precipitates. These results suggest that Ca-rich glaucophanes from other glaucophane—actinolite assemblages may be exsolved at the TEM scale. The coexisting amphiboles from Eclogite Brook also have been studied using the electron microprobe (EMP). The compositional gap defined by the Eclogite Brook pairs is consistent with previously reported results, but shows a wider break along the glaucophane—actinolite pseudobinary join, suggesting very limited solid solution up to temper-atures of about 500–550°C. The glaucophanes are relatively poor in Ca, except for one anomalous grain containing Ca-amphibole lamellae. Ferric iron, estimated by normalization to fixed cation numbers, is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component.     

Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles

Abstract Crystal-chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high-pressure parageneses:
1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).
2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).
3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).
P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.
Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from Na^M4= 0.80 in actinolite to Na^M4= 1.70 in glaucophane at T = 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.
The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members.     

Chemistry of calcic amphiboles from the area around Terakanambi,southern Karnataka

Chemial analyses of twelve amphiboles from the area around Terakanambi are presented. Results indicate that they are ferro-hornblende and ferro-pargasitic hornblende types in banded iron formations; magnesio hornblende in ultramafic rocks and edenite; and ferroan pargasite and ferroan pargasitic hornblende types in calcamphibolites. Titanium content in the amphiboles of the present study is relatively low compared to results from similar zones elsewhere. The lower titanium content of the amphiboles may be attributed to either bulk chemical composition or to low oxygen fugacity. Mg/Fe ratios vary considerably and it is mainly controlled by host rock composition. The plots of calcic amphiboles on (lOONa/Ca + Na)/(100 Al/Si + Al) and Al^iv/Al^vi diagrams indicate that they are of medium to low pressure type.     

Geotectonic significance of ultramafic rocks

Ultramafic rocks can be divided into 5 principal geotectonic groups, as follows: (1) Layered gabbro-norite-peridotite masses, commonly associated with all ages of terrane, possibly connected with accretionary plate margins. (2) Concentrically zoned Alaska-type bodies, possibly associated with consuming plate margins. (3) Ultramafic lavas and associated intrusions, nearly exclusively in Archaean terranes, of uncertain tectonic position. (4) Alpine peridotites, in linear Phanerozoic and Late Precambrian deformed belts, indicative of activity both at accretionary and consuming plate margins. (5) Nodules and other ultramafic rocks associated with alkalic volcanic centers, which in some cases represent mid-plate activity, as well as direct samples of mantle rocks underlying the volcanic area. A key question in interpretation of the geotectonic significance of ultramafic rocks is the nature or even existence of plate-tectonic activity prior to that reflected in the present ocean basins.     

Fe-Mg partition and miscibility gap between coexisting calcic amphiboles from the southern Abukuma Plateau,Japan

Lamellar intergrowths of actinolite and hornblende or aluminous actinolite occur in metamorphosed igneous rocks in the Hitachi metamorphic district, southern Abukuma Plateau. Electron microprobe analyses of five pairs are presented. The Fe-Mg partitioning and the miscibility gap are discussed in terms of an Mg-Fe-Al^IV diagram. The Fe-Mg partition coefficients depend on the Al^IV contents in hornblendes in a manner indicating that the pairs are close to equilibrium. Calcic amphibole pairs of high Fe/Mg ratio are richer in Al^IV than those with low ratio. The Al^IV content of the coexisting actinolite first increases with rising temperature, but then decreases as the temperature increases further. On the other hand, the Al^IV content of coexisting hornblende-aluminous actinolite successively decreases with rising temperature.     

Strontium isotope and alkali element abundances in ultramafic rocks

The concentrations of the trace elements Na, K, Rb and Sr and the isotopic composition of Sr have been measured in a suite of ultramafic rocks, including alpine-type intrusions, inclusions in basalts and kimberlite pipes, zones from stratiform sheets, and a mica peridotite. From these data and those available in the literature the following conclusions can be drawn. Alpine-type ultramafic material appears to be residual in nature and can be neither the source material for the derivation of basalts nor the refractory residue of modern basalts. Alpine-type ultramafic intrusions appear to have no relationship with ultramafic zones in stratiform sheets and were probably derived from the upper mantle. A genetic relationship exists between basalts and their ultramafic inclusions, but it is extremely doubtful that this inclusion material could give rise to basalts by partial fusion. There is a possible genetic relationship between basalts and ultramafic inclusions in kimberlite pipes, and this ultramafic material is a potential source for the derivation of basalts. Ultramafic inclusions in basalts are probably not fragments of an alpine-type ultramafic zone in the mantle. An attempt has been made to synthesize the data and interpretations of this study by way of speculations on the role of ultramafic rocks in the differentiation history of the earth.     

Alkali amphiboles intermediate in composition between actinolite and riebeckite

Alkali amphiboles of an intermediate composition in the magnesioriebeckite-eckrite series have been found in the metamorphic terrane of Leros Island. A complete compositional gradation has been demonstrated by a series of electron microprobe scans and analyses of spots. Electron microscopic examination revealed no exsolution.The present analyses and other data indicate a closing of the Na-Ca amphiboles gap towards the magnesian end members at temperatures higher than that usually associated with the blueschist facies.     

Strontium isotope composition of alpine-type ultramafic rocks in the Lake Chatuge District,Georgia—North Carolina

The ⁸⁷Sr/⁸⁶Sr ratios for a series of ultramafic rocks from the Lake Chatuge region range from 0.7023 to 0.7047, suggesting a direct upper mantle source and precluding a multiple differentiation origin for these alpine-type rocks. Higher ⁸⁷Sr/⁸⁶Sr ratios (0.7058–0.7068) for serpentinized rocks from this suite apparently reflect the influx of radiogenic ⁸⁷Sr from the surrounding gneisses and schists during serpentinization.     

Metamorphic olivine in ultramafic rocks from Western Australia

Textural relationships plus associated mineralogy are compatible with a metamorphic origin for some olivines in altered ultramafics. Upper amphibolite facies PT conditions are indicated.     

高压-超高压变质带中超基性岩的成因类型及其流体活动

超基性岩的地质过程提供了地幔岩在造山带形成过程中所作的贡献,并记录了地质构造以及壳-幔之间相互作用的信息。根据现有的研究,可将俯冲带橄榄岩大致分为陆下地幔橄榄岩、基性-超基性堆晶杂岩和大洋地幔橄榄岩。文中简要评述了不同类型造山带橄榄岩的岩石学和地球化学特征。不同类型的橄榄岩所经历的地质历史不同,而留有不同的岩石学和地球化学特征。大多数造山带橄榄岩经历了高压-超高压变质作用,并受到蛇纹岩化等多期次流体和融体的交代作用,因而俯冲造山带的辉石岩和橄榄岩无论在岩石学的组成、结构和地球化学等特征方面通常表现得复杂多变。