Hanan J. Kisch

Nomenclature of amphiboles : additions and revisions to the International Mineralogical Association's amphibole nomenclature

The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 a.f.p.u. (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of B(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which, here, is changed slightly, but significantly, from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized.

Use of the lattice parameter b 0 of dioctahedral illite/muscovite for the characterization of P/T gradients of incipient metamorphism

The P/T gradient or “baric type” of low-grade metamorphic terranes can be characterized on the basis of cumulative frequency curves of the lattice parameter b0 of dioctahedral micas (Sassi and Scolari 1974), which essentially reflects celadonite content and increases with pressure. The method is here applied to illite/muscovite of sub-greenschist (“incipient” metamorphism) and low-greenschist facies terranes from the marginal zones of the Swedish Caledonides, Swiss Alps and Venezuelan Andes, where the grade of metamorphism was determined on the basis of illite “crystallinity”.Complications in the determinations of b0 arising from (a) shifts in the spacing of the apparent 060 diffraction peaks of the illites due to the presence of inter-stratified expandable layers, and (b) persistence of more “crystalline” clastic micas are pointed out.In contrast to the earlier-reported decrease of b0 with progressive metamorphism at higher grades, b0 tends to increase with grade during incipient metamorphism: it is markedly lower in the “diagenetic” and low-grade anchimetamorphic grades in the Cambro-Silurian of the Caledonides of Jämtland, western Sweden (zone A), and the Lower Tertiary of the Helvetic zone of the Swiss Alps, than in the higher grade anchizone and “epizone” of the former terrane.Mean b0 values for the intermediate and highest (“epizone”) grades of the Swedish Caledonides are respectively 9.030 and 9.037 Å compared to the mean value of 9.035 Å for the medium-high pressure, Barrovian-type, terrane of Otago, N.Z. Mean b0 of 9.005 Å for the mainly epizonal slates of the Venezuelan Andes lies between those for the low-pressure terranes of Bosost (∼ 8.992 Å), and the low-medium pressure terrane of northern New Hampshire (∼9.010 Å). These baric estimates agree well with those for the metamorphic facies series at higher grade produced by the same events.Cumulative b0 curves are concluded to be useful for the characterization of P/T gradients of incipient metamorphism, particularly in the higher grade part of the anchizone and the “epizone”.

The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

The statistical b 0 method is based on the fact that the cumulative b 0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b 0 values (and related celadonite content) in WKM increase with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b 0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made a large amount of data available, revision of this b 0 scale is now opportune, and the primary need is a better definition of the reference b 0 values and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustres of W Alps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilufer area of NW Turkey. In addition, WKM chemical and b 0 data were used from various sources. The new chemical and b 0 data (EMPA and XRPD) allowed us: ( i ) to establish the reference b 0 and chemistry of WKM from HP/LT metapelites: b 0 turns out to be around 9.040 A; ( ii ) to confirm and extend to HP/LT metapelites the petrological basis of the b 0 method, particularly the control of mineral assemblages on b 0 values; ( iii ) to glimpse more complex relationships between b 0 and Si than those currently known, and particularly the importance of the Fe 2+ /(Mg + Fe 2+ ) ratio; ( iv ) to ascertain the validity of the b 0 method for detecting generic HP/LT conditions; (ν) to ascertain the inability of the b 0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b 0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation.

The relationship of phyllosilicate orientation, X-ray diffraction intensity ratios, and c/b fissility ratios in metasedimentary rocks of the Helvetic zone of the Swiss Alps and the Caledonides of Jämtland, central western Sweden

Phyllosilicate preferred orientations, X-ray diAraction intensity ratios, and c/b fissility ratios were determined from 23 metasedimentary rocks from the Helvetic zone of the Swiss Alps and from the Caledonides of Jamtland, Sweden. The relationships between these parameters of phyllosilicate orientation depend strongly on the lithology and the cleavage morphology. DiAerences in orientation direction and in degree of preferred orientation reflect both deformation intensities and lithologies. Whereas polyphase deformed, fine-grained samples generally have phyllosilicates that are oriented parallel to cleavage showing high orientation intensities, phyllosilicates in less deformed and/or coarse-grained rocks are oriented parallel to bedding with low orientation intensities. Weak orientation intensities in samples with phyllosilicates that are oriented parallel to bedding reflect compaction strain. The higher strain in samples with micas and chlorites that are parallel to cleavage is a combination of compaction and tectonic strain due to the development of crenulation cleavage. # 2000 Elsevier Science Ltd. All rights reserved.

An algorithm for finding composition, molar volume and isochors of CO2-CH4 fluid inclusions from Th and Tfm (for Th < Tfm)

Abstract A modified Redlich-Kwong equation of state is used to calculate the solubility of CO 2 in methane at various temperatures and pressures. From the solubility of CO 2 in CH 4 at the triple point and at final melting ( T h T fm ), and the molar volume of solid CO 2 , the volume of solid at the triple point, and the molar volume of the inclusion can be calculated using a mass balance. The pressure at the melting point is calculated from the equation of state. The algorithm predicts composition, molar volume, pressure at final melting and the isochor pressure (for a given temperature of trapping) for CO 2 -CH 4 fluid inclusions for the case T h T fm , given T h , T fm and experimental data on P h and d co 2 (solid) at T h .

Magnesiocummingtonite-P21/m: A Ca- and Mn-Poor Clino-Amphibole from New South Wales

AbstractA Ca- and Mn-poor clino-amphibole with Mg/Mg+Fetot+Mn (atomic ratio)=0.81 is described. The structural formula is

Geoarchaeological tsunami deposits at Palaikastro (Crete) and the Late Minoan IA eruption of Santorini

Na_{0.09} (Ca_{0.19} Mg_{5.45} Fe_{1.23}^{2 + } Mn_{0.04} Fe_{0.00}^{3 + } Ti_{0.01} Al_{0.07} )_{6.99} [(Si_{7.83} Al_{0.17} )_{8.00} O_{22} /(OH)_2 ].

NOMENCLATURE OF AMPHIBOLES: ADDITIONS AND REVISIONS TO THE INTERNATIONAL MINERALOGICAL ASSOCIATION’S 1997 RECOMMENDATIONS

The unit-cell constants area0=9.49 Å,b0=18.00 Å,c0=5.30 Å, β=102.0°,V0=886 Å3, the refractive indices αNa=1.621, βNa=1.632, and γNa=1.643. These values, when plotted against the Mg/Mg+Fe ratio, fit the extrapolations towards Mg7[Si8O22/(OH)2] from recently published determinative curves for the cummingtonite series. The clino-amphibole, or part of it, has space groupP21/m rather thanC2/m. The most magnesian cummingtonites reported thus far have Mg/Mg+Fe+Mn ratios around 0.7, but recently more magnesian Ca-poorP21/m clino-amphiboles have been reported. Although Ca and Mn have been claimed to stabilize cummingtonite as against anthophyllite, most magnesian cummingtonites appear to have <0.24 Ca, and <0.1 Mn per formula unit.The nomenclature of the cummingtonite series is discussed. Retaining the subdivision of the cummingtonite series at Mg/Mg+Fe=0.5, the author proposes to reviveTilley’s (1939) name magnesiocummingtonite for members with Mg/Mg+Fe >0.5. Grunerite is reserved for members with Mg/Mg+Fe <0.5. The space group,C2/m orP21/m, may be indicated with a suffix, if known.