F-OH substitution in natural tremolite,talc, and phlogopite

The distribution of F between tremolite and talc has been determined in metamorphosed siliceous carbonates from the Grenville Province, Ontario. Wavelength dispersive electron microprobe analyses of contiguous, texturally compatible tremolite-talc pairs indicate that the substitution of F for OH is the most significant deviation from end-member stoichiometry in the samples studied. Mixing of F and OH components has been represented by an ideal solution model for F in tremolite and an asymmetric model for F in talc. Both linear and nonlinear regression techniques have been used to derive activity coefficients for the exchange of one equivalent of OH and F components in talc. The following expressions are the result of nonlinear regression of 32 analyses from coexisting mineral pairs: $$\begin{gathered} \ln \gamma _{TC(OH)} = X_{TC(F)}^2 [2.447 - 2.845X_{TC(OH)} ] \hfill \\ \ln \gamma _{TC(F)} = X_{TC(OH)}^2 [1.024 + 2.845X_{TC(F} ] \hfill \\ \end{gathered} $$ Isobaric \(T - X_{CO_2 } \) sections constructed using these equations show an enhanced stability for the assemblages talc+calcite and phlogopite+quartz+calcite with F substituting for OH. Projection of isothermal invariant points into P-T space predicts a shift in the stability of the assemblage talc-calcite from lower grade into the sillimanite field with increasing substitution of F for OH in talc.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Phengite geobarometry based on the limiting assemblage with K-feldspar,phlogopite, and quartz

Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K₂O-MgO-Al₂O₃-SiO₂-H₂O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl_2–xMg_x[Al_1–xSi_3+xO₁₀] (OH)₂ with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Velde's assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u.The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H₂O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid.For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented.     

Stability of phlogopite in granitic melts, an experimental investigation

Water-saturated and water-undersaturated experiments (a _{H2 O} = 1.0 and 0.5) were performed in the temperature range 780–1040°C at 2 and 5 kbar in order to determine the upper thermal stability of phlogopite in granitic melts. Starting compositions were: (A) subaluminous mixtures of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous haplogranitic glass; (B) peraluminous mixtures (normative corundum  = 4 %) of 20 wt % synthetic phlogopite and 80 wt % synthetic anhydrous peraluminous haplogranitic glass. The molar quartz: albite: orthoclase ratio of the glasses of the 2␣kbar runs was 35:39:26 and that of the 5 kbar runs 30:42:28. In the subaluminous system, phlogopite is stable up to 820°C at a _{H2 O} = 1.0 and up to 780°C at a _{H2 O} = 0.5. At higher temperatures, it is replaced by enstatite. In the peraluminous system phlogopite has a remarkably higher thermal stability (up to 1000°C at 5 kbar and a _{H2 O} = 1.0) and there is a temperature interval of 80°C at a _{H2 O} = 1.0, and 90–100°C at a _{H2 O} = 0.5 between the first appearance of enstatite and the disappearance of phlogopite. In the peraluminous system, phlogopite is a solid solution (ss) of phlogopite, muscovite, talc and eastonite components. The crystalline product of the phlogopite_ss breakdown reaction is an aluminous enstatite. The MgO-content of the melt depends on the normative corundum content of the starting material and the run temperature. It is independent of pressure. In the subaluminous system, the MgO-content ranges between 0.05 and 0.3 wt % in the temperature interval 780–880°C at both investigated water activities. The MgO-content of the peraluminous melts at a _{H2 O} = 1.0 ranges between 0.4 and 1.7 wt % and at a _{H2 O} = 0.5 between 0.2 and 1.4 wt % in the temperature range 780–980°C. Received: 28 August 1995 / Accepted: 6 August 1996     

A high pressure-high temperature study of TiO2 solubility in Mg-rich phlogopite: implications to phlogopite chemistry

The solubility of Tio₂ in phlogopites has been experimentally determined in the system K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg₄TiAl₂Si₆O₂₀(OH)₄-K₂Mg₅TiAl₄Si₄O₂₀(OH)₄ between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg_4.5TiAl₃Si₅O₂₀(OH)₄ which represents a combination of the Mg^[VI]2Si^[IV] = Ti^[VI]2Al^[VI] and $\text{2Mg}{}^{\mathrm{[VI]}}\text{=}\text{Ti}{}^{\mathrm{[VI]}}\text{□}{}^{\mathrm{[VI]}}$ substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio₂ content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K₂O dissolved in H₂O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄], eastonite [K₂Mg₅Al₄Si₅O₂₀(OH)₄], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K₂(Mg₄Ti□)Al₂Si₆)O₂₀(OH)₄, and Ti-eastonite [K₂Mg₅TiAl₄Si₄O₂₀(OH)₄]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H₂O + 2 K₂O. Lack of knowledge of H₂O and K₂O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.     

Phase relations of talc and tremolite in metamorphic calcite-dolomite sediments in the southern portion of the Damara Belt (South West Africa)

The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite:

1. 3 dolomite+4 quartz+1 H₂O ? 1 talc+3 calcite+3 CO₂
2. 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO₂+2 H₂O
3. 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO₂+1 H₂O
4. 5 dolomite+8 quartz+1 H₂O ? 1 tremolite+3 calcite+7 CO₂
5. 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO₂.

The common paragenesis of four mineral assemblages tc+cc+dol+qtz¹ and tre+tc+ cc+qtz with increasing temperature over an extended area show that the reactions must have taken place along the equilibrium curve or when fluid pressure is not constant along the equilibrium plane of reactions (1) or (2). The described occurrence of the five mineral assemblage tre+tc+cc+dol+qtz can be stable only on the isobaric intersection point, or when P _f is variable on the univariant intersection curve of the equilibrium planes of all five reactions. The genetic relations of the described parageneses are illustrated with the help of a phase diagram. Minimum P-T conditions which prevailed during metamorphism in this part of the Damara Orogen have been estimated to be about 590° C and 5 kb.     

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO₂‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X_Mg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative ε_Nd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).     

Serpentinization of phlogopite phenocrysts from a micaceous kimberlite

Phenocrysts of phlogopite from a micaceous kimberlite contain finely interlayered serpentine. These phenocrysts occur in the kimberlite groundmass and are altered along the mica layers and are slightly deformed. Lizardite is the predominant serpentine mineral, but chrysotile and mixed structures also occur. The lizardite occurs as lamellae within phlogopite, oriented such that (001) layers of the two minerals are parallel and the [100] direction of lizardite is parallel to the [100] or 110 directions of phlogopite. The serpentinized regions of phlogopite are localized and extensive along the (001) layers. Chrysotile fibers and chrysotile-like curled serpentine occur within regions of disrupted material, and polygonal structures occur in folded lizardite lamellae. Textural relations suggest three events: 1) replacement of phlogopite by lizardite, 2) deformation of the phenocrysts, and 3) partial transformation of the lizardite to chrysotile-like structures. Deformation features include openings along (001), folds, and regions of disrupted or broken material. The folded and broken material consists of lamellar lizardite and phlogopite, indicating that lamellar replacement preceded deformation. Intergrowths of lizardite and curled serpentine are associated with cleavage openings and voids in disrupted material, suggesting that a partial transformation of lizardite to chrysotile occurred within openings created by deformation. Clay minerals also occur within phlogopite as either a minor product of serpentinization or of late-stage alteration.     

Weathering of phlogopite in simulated bioleaching solutions

The purpose of this study was to examine structural alterations of finely ground phlogopite, a trioctahedral mica, when exposed to acid, iron- and sulfate-rich solutions typical of bioleaching systems. Phlogopite suspensions were supplemented with ferrous sulfate and incubated with iron- and sulfur-oxidizing bacteria (Acidithiobacillus ferrooxidans) at 22 °C. As bacteria oxidized ferrous iron, ferric iron thus formed partially precipitated as K-jarosite. K-jarosite precipitation was contingent on the preceding ferrous iron oxidation by bacteria and the release of interlayer-K from phlogopite. This chemically and microbially induced weathering involved alteration of phlogopite to a mixed layer structure that included expansible vermiculite. The extent of phlogopite weathering and structure expansion varied with duration of the contact, concentration of ferrous iron and phlogopite, and the presence of monovalent cations (NH₄⁺, K⁺, or Na⁺) in the culture solution. NH₄⁺ and K⁺ ions (100 mM) added to culture suspensions precipitated as jarosite and thereby effectively prevented the loss of interlayer-K and structural alteration of phlogopite. Additional Na⁺ (100 mM) was insufficient to precipitate ferric iron as natrojarosite and therefore the precipitation was coupled with interlayer-K released from phlogopite. When ferrous iron was replaced with elemental sulfur as the substrate for A. ferrooxidans, the weathering of phlogopite was based on chemical dissolution without structural interstratification. The results demonstrate that iron oxidation and the concentration and composition of monovalent ions can have an effect on mineral weathering in leaching systems that involve contact of phlogopite and other mica minerals with acid leach solutions.     

Experimentelle Untersuchung der Metamorphose von kieselig dolomitischen Sedimenten

During an experimental investigation of the metamorphism of siliceous dolomites the equilibrium data of the heterogeneous bivariant reaction 1 $$3{\text{ dolomite + 4 quartz + 1 H}}_{\text{2}} O \rightleftharpoons + 3 calcite + 3 CO_2 $$ were determined for the total fluid pressures of 1,000, 3,000 and 5,000 bars. The equilibrium conditions were found by experiments in which dolomite, quartz and water react to form talc, calcite and CO₂, as well as by experiments with reversible reaction direction. Results are shown on the temperature- \(X_{CO_2 } \) -diagram of Fig. 3. The temperature of formation of talc and calcite depends to a considerable extent on the composition of the CO₂-H₂O-gas phase; this can be read straight off the isobaric (P _f =const.) equilibrium curves in Fig. 3. In addition a strong dependence of the equilibrium temperature on the total pressure P _f was established (see Fig. 5). At a total gas pressure of 1,000 bars dolomite and quartz can react, according to the composition of the CO₂-H₂O-gas phase, to talc and calcite over the whole of the temperature range between about 350° and 490° C. This indicates that at low pressures the formation of talc and calcite takes place in the field of the albite-epidote-hornfels facies. At a pressure of 3,000 bars dolomite and quartz are stable up to about 550° C if the fluid phase is rich in carbon dioxide and correspondingly poor in water. Thus, this paragenesis can occur up to the stability field of staurolite [see annotation (5)] if the partial pressure of CO₂ is large. At the higher total gas pressure of 5,000 bars dolomite and quartz react even at medium CO₂-concentrations only at about 580° C to give talc and calcite. Therefore it is expected that in regional metamorphism at about 5,000 bars pressure or more the paragenesis dolomite plus quartz exists up to and within the stability field of staurolite and reacts only here to form talc and calcite after reaction (1) or tremolite and calcite after the following reaction (2)¹: $$5 dolomite + 8 quartz + 1 H_2 O \rightleftharpoons 1 tremolite + 3 calcite + 7 CO_2 $$ . The exact physico-chemical conditions under which dolomite, quartz and water react on the one hand to form talc, calcite and CO₂, and on the other hand to form tremolite, calcite and carbon dioxide, will be discussed later when our experimental investigations on the formation of tremolite are completed. First results were already published in a short note by Metz, Puhan and Winkler (1968).     

An absarokite from a phlogopite lherzolite source

An absarokite (SiO₂ 47.72 wt %, K₂O 3.41 wt %) occurs in the Katamata volcano, SW Japan. The rock carries phenocrysts of olivine, phlogopite, clinopyroxene, and hornblende. Chemical compositions of bulk rock (FeO^*/ MgO 0.73) and minerals (Mg-rich olivine and phlogopite, Cr-rich chromite) suggest that the absarokite is not differentiated. Melting experiments at high pressures on the Katamata absarokite have been conducted. The completely anhydrous absarokite melt coexists with olivine, orthopyroxene, and clinopyroxene at 1310° C and 1.0 GPa. The melt with 3.29 wt % of H₂O also coexists with the above three phases at 1230° C and 1.4 GPa; phlogopite appears at temperatures more than 80° C below the liquidus. On the other hand, the melt is not saturated with lherzolite minerals in the presence of 5.13 wt % of H₂O and crystallizes olivine and phlogopite as liquidus phases; the stability limit of phlogopite is little affected at least by the present variation of H₂O content in the absarokite melt. It is suggested that the absarokite magma was segregated from the upper mantle at 1170° C and 1.7 GPa leaving a phlogopite lherzolite as a residual material on the basis of the above experimental results and the petrographical observation that olivine and phlogopite crystallize at an earlier stage of crystallization sequence than clinopyroxene. The contribution of phlogopite at the stage of melting processes is also suggested by the geochemical characteristics that the absarokite is more enriched in Rb, K, and Ba and depleted in Ca and Na than a typical alkali olivine basalt from the same volcanic field.     

Talc paragenesis in some siliceous dolomitic rocks,and its sedimentologic significance

Thin (0.5–4 mm), contorted stringers of talc, associated with apatite and minor pyrite, are containdy Formation in eastern Alabama. The form, position and lithologic distribed within generally saccharoidal dolomite-quartz marbles of the Cambrian Shaution of the stringers strongly suggest an algalstromatolitic origin, with interlaminar trapped dolomitic muds. Metamorphic formation of talc plus apatite proceeded only within the stringers, whereas surrounding marble remained as unreacted dolomite plus quartz. Talc generation is best explained by the reaction $${\text{dolomite}} + {\text{silica}} + {\text{water}} + {\text{P}}_{\text{2}} {\text{O}}_{\text{5}} = {\text{talc}} + {\text{apatite}} + {\text{CO}}_{\text{2}}$$ in which the phosphate was supplied to the reaction from organic matter contained within the stromatolitic layers. The system was probably open to CO₂ during metamorphism, and \(P_{{\text{CO}}_{\text{2}} }\) remained relatively low.     

Kulkeite,a new metamorphic phyllosilicate mineral: Ordered 1∶1 chlorite/talc mixed-layer

Kulkeite occurs as platy, colorless, porphyroblastic, single crystals up to 2 mm in size in a low-grade dolomite rock associated with a Triassic meta-evaporite series at Derrag, Tell Atlas, Algeria, It is associated with sodian aluminian talc, unusual chlorite polytypes, and both K and Na phlogopite. Kulkeite is optically biaxial, negative, n _x=1.552, n _y=1.5605, n _z=1.5610, 2V_z=24° (obs.). Based on microprobe analysis the empirical formula is (Na_0.38K_0.01Ca_0.01)(Mg_8.02Al_0.99)[Al_1.43Si_6.57O₂₀](OH)₁₀ with some variation in Na, Si, and tetrahedral Al. The crystals are monoclinic with a=5.319(1), b=9.195(2), c=23.897(10) Å, β=97° 1(3)′; Z=2; the calculated density is 2.70 g cm^?3. The four strongest lines in the X-ray powder pattern are (d, I, hkl): 7.90, 100, 003; 1.533, 100, 060; 7.42, 80, 002; 3.38, 80, 007; the 001 reflection with 23.7 Å has intensity 10. Transmission electron microscopy confirms the nature of a regular 1∶1 mixed-layer, which consists of 14 Å chlorite (clinochlore) sheets alternating with sheets of one-layer (9.5 Å) talc characterized by the lattice substitution NaAl→Si just as in the talc occurring as a discrete mineral co-existing with kulkeite. Kulkeite is intergrown with lamellae of clinochlore that represent two-layer and five-layer (70 Å) polytypes with optical birefringence exceeding the normal value for clinochlore by a factor of 3. The origin of kulkeite is due to low-grade metamorphism with temperatures probably not exceeding 400° C. As the clinochlore lamellae and sodian aluminian talc are found in mutual contact, kulkeite seems to represent a metastable mineral at least during the latest phase of metamorphism. However, at an earlier stage, prior to clinochlore formation, kulkeite might have been stable, and the incorporation of Na and Al into its talc component could indeed be the decisive factor for the formation of the mixed-layer.     

The role of fluids in the formation of talc deposits of Rema area,Kumaun Lesser Himalaya

Talc deposits of Rema area in the Kumaun Inner Lesser Himalaya are hosted within high magnesium carbonates of the Proterozoic Deoban Formation. These deposits occur as irregular patches or pockets mainly within magnesite bodies, along with impurities of magnesite, dolomite and clinochlore. Textures represent different phases of reactions between magnesite and silica to produce talc. Petrography, XRD and geochemistry reveal that the talc has primarily developed at the expense of magnesite and silica, leaving dolomite largely un-reacted. Early fluid inclusions in magnesite and dolomite associated with talc are filled with H₂O+NaCl+KCl ± MgCl₂ ± CaCl₂ fluids, which represent basin fluid system during diagenesis of carbonates. Their varied degree of re-equilibration was although not pervasive but points to increased burial, and hence requires careful interpretation. H₂O-CO₂ fluid with XCO₂ between 0.06 and 0.12 was equilibrated with talc formation. The reaction dolomite+quartz → talc was not extensive because T-X_CO2 was not favourable, and talc was developed principally after magnesite+quartz.     

The formation of mantle phlogopite in subduction zone hybridization

Extrapolation and extension of phase equilibria in the model system KAlSiO₄-Mg₂SiO₄-SiO₂-H₂O suggests that at depths greater than 100 km (deeper than amphibole stability), hybridism between cool hydrous siliceous magma, rising from subducted oceanic crust, and the hotter overlying mantle peridotite produces a series of discrete masses composed largely of phlogopite, orthopyroxene, and clinopyroxene (enriched in Jadeite). Quartz (or coesite) may occur with phlogopite in the lowest part of the masses. The heterogeneous layer thus produced above the subducted oceanic crust provides: (1) aqueous fluids expelled during hybridization and solidification, which rise to generate in overlying mantle (given suitable thermal structure) H₂O-undersaturated basic magma, which is the parent of the calc-alkalic rock series erupted at the volcanic front; (2) masses of phlogopite-pyroxenites which melt when they cross a deeper, high-temperature solidus, yielding the parents of alkalic magmas erupted behind the volcanic front; and (3) blocks of phlogopite-pyroxenites which may rise diapirically for long-term residence in continental lithosphere, and later contribute to the potassium (and geochemically-related elements) involved in some of the continental magmatism with geochemistry ascribed to mantle metasomatism.     

Reaction kinetics of dolomite rim growth

Reaction rims of dolomite (CaMg[CO₃]₂) were produced by solid-state reactions at the contacts of oriented calcite (CaCO₃) and magnesite (MgCO₃) single crystals at 400 MPa pressure, 750–850 °C temperature, and 3–146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgCO₃ interface with length ranging from about 6 to 41 µm. At the same time, a 5–71 µm wide rim of equiaxed granular dolomite grew at the contact with CaCO₃. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12–80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the $[11\bar{2}0]$ equivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO are E _a (CaO) = 192 ± 54 kJ/mol and E _a (MgO) = 198 ± 44 kJ/mol, respectively.     

Experimentelle Untersuchung der Reaktion Dolomit + Kalifeldspat + H2O ⇌ Phlogopit + Calcit + CO2

The equilibrium curve for the reaction 3 dolomite + 1 K-feldspar + 1 H₂O=1 phlogopite + 3 calcite + 3 CO₂ was determined experimentally at a total gas pressure of 2000 bars using two different methods.

1. In the first case water alone was added to the reactants. The CO₂ component of the gas phase was producted solely by the reaction under favourable P-T conditions. This manner of carrying out the reaction is called the “water method”. With this method sufficient time must be allowed for the gas phase to attain a constant composition (see Fig. 1). Reverse reactions were carried out using reaction products of the forward reaction.
2. In the second case silver oxalate + water were added to the reactants. Breakdown of the silver oxalate leads to formation of a CO₂-H₂O gasphase of definite composition. At constant temperature and gas pressure the \(X_{{\text{CO}}_{\text{2}} } \) determines whether the reaction products will be phlogopite + calcite or dolomite + K-feldspar. In this case it is not necessary to wait for equilibrium to be attained. This method is abbreviated the “oxalate method”. Reactants for reverse reactions are not identical with the products of the forward reaction.

At high temperatures the results of the two different methods agree well (see Tables 1 and 2). Equilibrium was attained in one case at 490° C and \(X_{{\text{CO}}_{\text{2}} } \) of approximately 0.77, and in the other case at 520° C and \(X_{{\text{CO}}_{\text{2}} } \) of 0.90. At lower temperatures there are considerable differences in the results. With the water method an \(X_{{\text{CO}}_{\text{2}} } \) of about 0.25 was reached at 450° C. With the oxalate method dolomite K-feldspar and water still react with each other at even higher \(X_{{\text{CO}}_{\text{2}} } \) values. Phlogopite, calcite and CO₂ are formed together with metastable talc. There are no criteria to indicate which of the methods is the correct one at lower temperatures and in Fig. 2, therefore, both equilibrium curves are plotted.     

The Petrography and composition of phlogopite micas from the Blue Ball Kimberlite,Arkansas: A record of chemical evolution during crystallization

Summary The Blue Ball kimberlite, Scott County, Arkansas, contains between 23 and 36 modal % phlogopite. Phlogopite is present as phenocrysts, a groundmass phase, and as coronas around serpentinized olivine phenocrysts. Intermediate and reverse pleochroism, seen in phenocryst cores and the majority of groundmass phlogopites, can be correlated with the mineral chemistry. Reverse pleochroism, exhibited by phenocryst rims, rare groundmass grains, and phlogopites forming coronas around olivine, is generally accompanied by an increase in total Fe (as FeO) and a decrease in Al₂ O₃. Analyses of those phlogopites whoch exhibit reverse pleochroism have high cation totals, indicative of the presence of Fe³⁺. We suggest that these pleochroic schemes are a function of Fe³⁺ in the tetrahedral site, which is expressed in terms of T = [8 - (Si + Al)]. All Blue Ball phlogopites have positive values for T, but reverse pleochroism is found only when T > 0.6, indicative of increased Fe³⁺ in the residual magma. It is concluded that phlogopite was on the liquidus throughout practically all of the kimberlite crystallization, because of the wide range in phlogopite compositions and the general decrease in Ba from micas exhibiting intermediate to reverse pleochroism. Variations of MG #, Ti, Ba, Si, and Al within the phlogopites have been used to trace the crystallization of other phases present in the kimberlite, namely olivine and spinel (chromite and titanomagnetite). Olivine was an early crystallizing phase, causing a decrease in MG # and Si, and an increase in Ti and Al in the phlogopites, a reflection of residual magma composition. Minor chromite also crystallized at this time, but was not of sufficient quantity to override the effect of olivine fractionation on the residual magma. Widespread Titano-magnetite crystallization occurred as olivine fractionation ceased, causing an increase in MG # and Si, and a decrease in Ti and Al. By examining these chemical variations with T, the evolution of the Blue Ball kimberlite has been determined.

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Petrographie und zusammensetzung phlogopitischer Glimmer aus dem Blue Ball Kimberlit, Arkansas: chemische evolution während der kristallisation

Zusammenfassung Der Blue Ball Kimberlit, Scott County, Arkansas, enthält zwischen 23 und 36% Phlogopit (modal). Dieser kommt in idiomorphen Kristallen, als Grundmasse, und als Coronas um serpentinisierte Olivinkristalle vor. Verschiedene Arten von Pleochroismus, die man in den Kernen von Kristallen und in der Mehrzahl der Phlogopite in der Grundmasse erkennen kann, lassen sich mit der Mineralehemie korrelieren. Reverser Pleochroismus ist an den Rändern von Kristallen, selten in Grundkörnern der Grundmasse, und in Phlogopiten die Coronas in Olivin bilden, zu beobachten. Diese Erscheinung wird im allgemeinen von einer Zunahme des Gesamt-Eisens (als FeO) und einer Abnahme der Al₂ O₃-Gehalte begleitet. Analysen jener Phlogopite, die reversen Pleochroismus zeigen, haben hohe Gesamtwerte an Kationen und dies weist auf die Anwesenheit von Fe³⁺ hin. Wir interpretieren diesen Pleochroismus als eine Funktion des Fe³⁺ in der Tetraeder-Position, die sich als T = 8 - (Si + Al) ausdrücken läßt. Alle Phlogopite von Blue Ball haben positive Werte für T, aber reverser Pleochroismus tritt nur dort auf, wo T > 0.6, was widerum auf eine Zunahme von Fe³⁺ im residualen Magma hinweist. Phlogopit war praktisch während der gesamten Kristallisation des Kimberlits am Liquidus, und diese Annahme wird auch durch das weite Feld der PhlogopitZusammensetzungen und durch die allgemeine Abahme der Ba-Gehalte in Glimmern mit intermediärem bis reversem Pleochroismus unterstützt. Variationen von MG #, Ti, Ba, Si und Al in den Phlogopiten wurden benutzt, um die Kristallisation von Olivin und Spinellen (Chromit und Titanomagnetit) zu verfolgen. Olivin hat früh kristallisiert und führte zu einer Abnahme von MG# und Si und einer Zunahme von Ti und Al in den Phlogopiten. Auch kleinere Mengen von Chromit kristallisierten zu dieser Zeit, dies war jedoch nicht ausreichend, um den Effekt der Olivin-Fraktionierung auf die Restschmelze aufzuheben. Titanomagnetit-Kristallisation setzte mit aufhörender Olivinfraktionierung ein und führte zu einer Zunahme von Mg und Si und einer Abnahme von Ti und Al. Eine Überprüfung der chemischen Variationen und Vergleich mit T Werten tragen wesentlich zur Kenntnis der Evolution des Blue Ball Kimberlites bei.

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With 8 Figures     

A model for phlogopite development in low magnesium rocks

Phlogopites containing anomalously high values of Zn, Mn and Li have been identified in low-magnesium feldspathic schists and quartzites from the Precambrian of northern New Mexico. The phlogopite is volumetrically minor (<2%) and coexists with muscovite. The presence of hematite and piemontite in the phlogopite-bearing samples indicates high oxidizing conditions, but their presence in phlogopite-free mica schists nearby demonstrates that high fo₂ is insufficient to account for the phlogopite occurrences. A model for phlogopite development is presented which is based upon (1) high fo₂ conditions, (2) limited H⁺ during metamorphism, and (3) early mobilization of Mg (and Zn, Mn, Li) during incipient metamorphism. The absence of phlogopite in associated rocks which refect high fo₂ conditions is due to the larger amount of H⁺ available during metamorphism, resulting in the development of abundant muscovite rather than rare phlogopite.     

Genesis and evolution of low-Al orthopyroxene in spinel peridotite xenoliths, Grand Canyon field, Arizona, USA

Orthopyroxene porphyroblasts zoned to interiors abnormally low in Al and Cr and containing numerous inclusions of olivine occur in some spinel peridotite xenoliths from the Colorado Plateau. Rims of these orthopyroxene grains contain 2.5–3.0 wt% Al₂O₃, consistent with equilibration in spinel peridotite at temperatures near 850 °C, but interiors contain as little as 0.20 wt% Al₂O₃ and 0.04 wt% Cr₂O₃. The Al-poor compositions are inferred to have equilibrated in chlorite peridotite, before porphyroblast growth during heating and consequent reactions that eliminated talc, tremolite, and chlorite. The distinctive orthopyroxene textures are inferred to have formed during reaction of talc and olivine. Rare intergrowths of orthopyroxene plus diopside are attributed to olivine-tremolite reaction. Al and Cr have gradients at grain rims that appear little modified by diffusion, but divalent elements are almost homogeneous throughout the porphyroblasts. Judging from the relative gradients, diffusion of Ca was at least 100 times faster than that of Al and Cr at the temperatures near and below 850 °C. Diffusion of Al and Cr was most effective along subgrain boundaries, and along these boundaries it appears to have been at least ten times faster than within the lattice: diffusion along such boundaries may be a dominant mechanism for re-equilibration of orthopyroxene at low mantle temperatures. Orthopyroxene with similar low Al and Cr occurs in chlorite peridotite xenoliths from the Navajo field, 300 km east of the Grand Canyon localities, and in spinel peridotite xenoliths from the Sierra Nevada, 500 km west across the extended Basin and Range province. Chlorite peridotite may therefore have been a significant minor component in much of the mantle lithosphere of western North America, although evidence for it would be erased at the higher temperatures recorded by xenoliths from the Basin and Range. Chemical changes during hydration may have been important in the evolution of these mantle volumes, and the case for addition of Sr is particularly strong. Dehydration reactions during heating could have influenced patterns of extension and crustal magmatism. Received: 1 July 1996 / Accepted: 2 December 1996