Osumilite is an important rock-forming mineral at l0 exposures in the Late Archean Napier complex of coastal Enderby Land (50-52'E), Antarctica. Mineral assemblages of low compositional variance (all with quartz-rplagioclase) are osumilite-garnet-sillimanite -r spinel, osumilite-sapphirine-sillimanite, osumilite-garnet-orthopyroxene-sillimanite, osumilite-orthopyroxene-biotite, and in a rock nearly free of iron (atomic Mg/(Mg+Fe) = 0.89) from Reference Peak (67'15'S, 50"29'E), osumilite-enstatite (EnerFse)-sillimanite. The Antarctic osumilite departs from the theoretical composition (K, Na) (Mg,Fe,Mn)zAlr (SiroAlto3o in that (Mg+Fe+Mn) ranges from2.20to2.39; A1,4.45 to 4.67; and Si, 10.05 to 10.30. These variations approximately obey the substitution (Mg+Mn+Fe) + Si : 2Al. Atomic Me/(Mg+Mn+Fe) ratio (X14j of Antarctic osumilite ranges from 0.79 to 0.97, and is greater than X1a, of associated cordierite and biotite. Antarctic osumilite-bearing have moderate to high Mg/Fe ratios (atomic Mg/(Mg+Fe) = 0.43 to 0.89) and are peraluminous. A petrogenetic grid in the system K2O-FeO-MgO-AlzOrSiO2 for quartz (present in all assemblages), osumilite, K-feldspar, garnet, sapphirine, cordierite, orthopyroxene, and sillimanite can be constructed given the constraint that the univariant reaction osumilite = cordierite * enstatite + K-feldspar + quartz in the iron-free subsystem intersects the stable portion of the univariant reaction in the MgO-Al2O3-SiO2 system: cordierite : enstatite + sillimanite -t qtartz. Pressure-temperature slopes of reactions involving osumilite but not cordierite are positive; slopes of reactions involving both osumilite and cordierite are negative, or in one case, parallel to the pressure axis. Osumilite is predicted to be stable in rocks of appropriate bulk composition at temperatures above 750'C and at total (lithostatic) pressures less than 8 or 9 kbar. M. Olesch and F. Seifert's recent experimental work and Olesch's calculations indicate that osumilite would be stable under hydrous conditions only when P11r6 = Prot.l < I kbar; at higher total pressures, H2O partial pressure must be much less than total pressure. Minimum temperatures for the appearance of osumilite-garnet and osumilite-sillimanite assemblages are in general higher than those needed for osumilitecordierite-orthopyroxene, and pressures of 5 or 6 kbar may be needed to stabilize osumilite-garnet or osumilite-sillimanite at geologically accessible temperatures. As the physical conditions proposed here for osumilite stability overlap the estimated conditions in many granulite-facies terranes, osumilite can be expected to occur at more localities than have been reported to date.

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Osumilite in the sapphirine-quartz terrane of Enderby Land, Antarctica; implications for osumilite petrogenesis in the granulite facies

Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2).



The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel).



The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H2O).



Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors.



Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.

Reaction history of garnet‐sapphirine granulites and conditions of Archaean high‐pressure granulite‐facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

+ spinel = sapphirine. This produced coarse-grained coronas of sequence sapphirine around the corundum and spinel. The earliest assemblage at Hakurutale was orthopyroxene + sillimanite + sapphirine. Garnet growth occurred via the reactions orthopyroxene + sillimanite + sapphirine = garnet and orthopyroxene + sillimanite = garnet + quartz. Locally, garnet was partially destroyed by a INTRODUCTION variety of corona-forming continuous reactions. The most important Rocks of highly aluminous and magnesian bulk comare garnet = orthopyroxene + sillimanite + sapphirine and position commonly record a part of the P‐T path close garnet + sillimanite = sapphirine + cordierite (both localities) to peak temperatures (e.g. Hensen, 1987), where the and, at Hakurutale, garnet + quartz = orthopyroxene + sense of the P‐T path—clockwise or counter-clockwise— sillimanite, garnet + sillimanite + quartz = cordierite and may be determined. These rocks generally contain a garnet + quartz = orthopyroxene + cordierite. In places, subset of the minerals garnet (Gar), orthopyroxene (Opx), orthopyroxene + sillimanite + quartz reacted to form cordierite, cordierite (Crd), Al-spinel (Sp) and sapphirine (Spr), in and orthopyroxene + sillimanite reacted to form cordierite + combination with sillimanite (Sil), corundum (Cor) or sapphirine coronas. At Hakurutale, inclusion textures indicate that quartz (Qz). They have been increasingly used in the sapphirine was present in the earliest recorded assemblages (prograde), last few years to deduce parts of the P‐T paths witnessed but also formed later as various corona-forming reactions occurred. by granulite-facies terranes (e.g. Droop & BucherNurminen, 1984; Droop, 1989; Bertrand et al., 1992;

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Petrology of Sapphirine-bearing and Associated Granulites from Central Sri Lanka

Channel CO2 and H2O has been re-introduced into a channel evacuated Mg-cordierite Ko.or Nao.o:(Mg1.e1Fe6.6eMno or)All e6Si5.s1O16 from White Well, Australia. At 600'C and pressures up to 6 kbar, a maximum of 3.13 wt.Vo CO2 and 2.6 wt.% H2O re-enter cordierite's channels. The H2O molecule orients preferentiallywithits H-H vectorparallelto c (opticorientation: c = X,b: Y,a: Z).Wirh increased H2O content, the refractive indices y and B increase more strongly than a whereas the as cell edge decreases, b0 perhaps increases slightly, and cs first increases and then levels off. The linear CO2 molecule orients chiefly parallel to cordierite's a-axis so that, as CO2 content increased, refractive index Tincreased more strongly than did aand B. As a result, 2V^ increased with CO2 content so as to exceed 90". Simultaneously, c6 increased, as decreased, and be remained constant. The distortion index A for these crystals changed with H2O and COz content.

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Orientation and effects of channel H 2 O and CO 2 in cordierite

An extensive set of refractive indices determined at λ = 589.3 nm ( n D ) from ~2600 measurements on 1200 minerals, 675 synthetic compounds, ~200 F-containing compounds, 65 Cl-containing compounds, 500 non-hydrogen-bonded hydroxyl-containing compounds, and ~175 moderately strong hydrogen-bonded hydroxyl-containing compounds and 35 minerals with very strong H-bonded hydroxides was used to obtain mean total polarizabilities. These data, using the Anderson-Eggleton relationship
 α T = ( n D 2 − 1 ) V m 4 π + ( 4 π 3 − c ) ( n D 2 − 1 ) where α T = the total polarizability of a mineral or compound, n D = the refractive index at λ = 589.3 nm, V m = molar volume in A 3 , and c = 2.26, in conjunction with the polarizability additivity rule and a least-squares procedure, were used to obtain 270 electronic polarizabilities for 76 cations in various coordinations, H 2 O, 5 H x O y species [(H 3 O) + , (H 5 O 2 ) + , (H 3 O 2 ) − , (H 4 O 4 ) 4− , (H 7 O 4 ) − ], NH 4 + , and 4 anions (F − , Cl − , OH − , O 2− ). Anion polarizabilities are a function of anion volume, V an , according to α − = α − 0 ⋅ 10 − N o / V an 1 . 20 where α − = anion polarizability, α − o = free-ion polarizability , and V an = anion molar volume. Cation polarizabilities depend on cation coordination according to a light-scattering (LS) model with the polarizability given by α ( C N ) = ( a 1 + a 2 C N e − a 3 C N ) − 1 where CN = number of nearest neighbor ions (cation-anion interactions), and a 1 , a 2 , and a 3 are refinable parameters. This expression allowed fitting polarizability values for Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Fe 2+ , Y 3+ , (Lu 3+ -La 3+ ), Zr 4+ , and Th 4+ . Compounds with: (1) structures containing lone-pair and uranyl ions; (2) sterically strained (SS) structures [e.g., Na 4.4 Ca 3.8 Si 6 O 18 (combeite), Δ = 6% and Ca 3 Mg 2 Si 2 O 8 (merwinite), Δ = 4%]; (3) corner-shared octahedral (CSO) network and chain structures such as perovskites, tungsten bronzes, and titanite-related structures [e.g., MTiO 3 (M = Ca, Sr, Ba), Δ = 9–12% and KNbO 3 , Δ = 10%]; (4) edge-shared Fe 3+ and Mn 3+ structures (ESO) such as goethite (FeOOH, Δ = 6%); and (5) compounds exhibiting fast-ion conductivity, showed systematic deviations between observed and calculated polarizabilities and thus were excluded from the regression analysis. The refinement for ~2600 polarizability values using 76 cation polarizabilities with values for Li + → Cs + , Ag + , Be 2+ → Ba 2+ , Mn 2+/3+ , Fe 2+/3+ , Co 2+ , Cu +/2+ , Zn 2+ , B 3+ → In 3+ , Fe 3+ , Cr 3+ , Sc 3+ , Y 3+ , Lu 3+ → La 3+ , C 4+ → Sn 4+ , Ti 3+/4+ , Zr 4+ , Hf 4+ , Th 4+ , V 5+ , Mo 6+ , and W 6+ in varying CN’s, yields a standard deviation of the least-squares fit of 0.27 (corresponding to an R 2 value of 0.9997) and no discrepancies between observed and calculated polarizabilities, Δ > 3%. Using
 n D = 4 π α ( 2 . 26 − 4 π 3 ) α + V m + 1 the mean refractive index can be calculated from the chemical composition and the polarizabilities of ions determined here. The calculated mean values of n D > for 54 common minerals and 650 minerals and synthetic compounds differ by In a comparison of polarizability analysis with 68 Gladstone-Dale compatibility index (CI) (Mandarino 1979, 1981) values rated as fair or poor, we find agreement in 32 instances. However, the remaining 36 examples show polarizability Δ values

Empirical electronic polarizabilities of ions for the prediction and interpretation of refractive indices: Oxides and oxysalts

For natural cordierites (except for indialites), the cell-edge variations which cause variations in the distortion index A more likely represent di-fferences in composition than in AllSi ordering. Back-reflection Weissenberg data from ll cordierite grains representing atomic ratios (Fe + Mn)/(Fe * Mn * Mg) from 0.(x6 to 0.898 reveal that, with increase in R" (the average radius ofthe ions occupying the octahedral sites), cell edges a and b increase linearly whereas c decreases until R" exceeds ca.0.765, then apparently increases. Statistical analyses on data for unheated grains reveal A to decrease markedly as x (mole fraction Na) increases, iacrease moderately as Ro increases, but seem relatively independent of Hro, probably because the strong correlation between Na and HrO content in unheated cordierites partially masked the efect of HrO on A. In contrast, A increased signifcantly when four cordierites were heated at 800'C with HrO being driven off. Inclusion of these data in the regression analysis revealed HrO content, as suggested by Stout (1975), to have a signifcant effect on A. For a natural Na,Be cordierite, Schreyer (1979) confirmed this by changing its A value from 0.12" to 0.26" by dehydration, then returning it to 0.11" by rehydration. The optical data, remeasured for l0 grains after their partial dehydration at 800"C for six hours in a slowly flushing H, atmosphere, showed reduction of all refractive indices and yielded remarkably regular curves relating the indices a, B, ard y to (Fe + Mn)/(Fe * Mn * Mg) ratio. Exceptions were two specimens high in Na and Be whose indices, even after heating, exceed the general trends. As for A, heating increased 2v* (x :666 nm) except for the two optically (+) grains which show slight decreases. Prior to heating, 2V^cotelates inversely with HrO whereas, after partial dehydration at 800oC, 2V^ correlates inversely with Na content and, less certainly, with R..

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K. R. Selkregg

Papers

Cordierites; compositional controls of Δ, cell parameters, and optical properties