Joseph V. Smith

The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite

Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al2O3 and Cr2O3, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe2O3 indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe2O3 than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized.

Pyroxene-Garnet Transformation in Coorara Meteorite

Majorite is a new garnet in a veinlet of the Coorara meteorite. Its chemical composition is compatible with derivation mostly from original pyroxene, not from olivine as originally reported. Silicon is partly in sixfold coordination. Ringwoodite, a spinel of olivine composition, occurs as purple grains set in a matrix of fine-grained garnet. The similar mineralogy and texture of the Coorara and Tenham meteorites suggest a common parent body.

Na, K, P and Ti in garnet, pyroxene and olivine from peridotite and eclogite xenoliths from African kimberlites

Electron microprobe analyses sensitive to 20ppmw (2σ) were made for Na, P, K and Ti in garnet, pyroxenes and olivine from peridotite and eclogite xenoliths from African kimberlites and volcanic rocks in Tanzania. Average concentrations (ppmw) in peridotite (mostly garnet lherzolite) are: Na2O gt 340 ol 90 opx 1070 cpx 2.1 (wt.%); P2O5 gt 460 ol 130 opx 50 cpx 350; K2O gt <20 ol <20 opx 30 cpx 170; TiO2 gt 1470 ol 130 opx 480 cpx 1630. For eclogites and a cpx megacryst with gt inclusions: Na2O gt 610 cpx 4.3 (wt.%); P2O5 gt 530 cpx 300; K2O gt <20 cpx 370; TiO2 gt 1990 cpx 1980. In garnet, Na can be explained by coupled substitution with P and Ti, and there is no need to invoke six-coordinated silicon. The Na distribution between garnet and clinopyroxene correlates with the Fe/Mg distribution for both eclogites and peridotites, and for the peridotites correlates with estimates of pressure and temperature from pyroxene composition. When calibrated experimentally, the Na distribution may be a useful indicator of physical conditions at depths for which the Fe/Mg distribution is insensitive; furthermore the Na distribution may be less sensitive to oxidation state.

Structural features of aluminophosphate materials with

Abstract The aluminophosphates with Al P = 1 form a sub-group of the wide range of phosphates important in biochemistry, soil chemistry, geology and industrial chemistry. The Al and P positions of each 1:1 aluminophosphate structure can be related to alternating nodes of a four-connected 3D net. Descriptions are given of: berlinite (net 90); AlPO4-cristobalite (net 1); AlPO4-tridymite (net 2); AlPO4-20 (sodalite, net 108); variscite (net 5); metavariscite (net 3); AlPO4-5 (net 81); AlPO4-1.5H2O (net 24a); AlPO4-21 and -25 (net 401); AlPO4-15 (net 400); AlPO4-14 (analogous to GaPO4-14, net 399); AlPO4-EN3 (net 402); AlPO4-12 (related to net 398 by changing a linkage). The crystal chemistry of the 1:1 aluminophosphates is different from that of the zeolites because of electrostatic neutrality of the AlPO4 moiety, occurrence of some Al atoms in five or six coordination with oxygen species including OH and H2O, and strict alternation of Al and P atoms on tetrahedral nodes around even-numbered circuits. The aluminophosphate structure types range from dense phases through the hydrates and semi-dense phases to the newly-synthesized microporous materials, which become molecular sieves upon removal of encapsulated material. There is no simple answer to the extent that encapsulated material acts as a template during synthesis. Polytypic variations have not been reported. Integrowths related by simple structural changes are theoretically possible. Observed and calculated X-ray powder patterns are given for seven phases.

Storage of F and Cl in the upper mantle: geochemical implications

Abstract Electron microprobe analyses yielded mean values of F 0.43 and Cl 0.08 wt. % for primary-textured phlogopites in coarse, depleted garnet-lherzolite xenoliths from kimberlites. Most secondary-textured phlogopites have too low Cl (0.01–0.08 wt.%) to be metamorphic precursors of primary-textured phlogopites. MARID-suite phlogopites and many megacrysts in kimberlites have low Cl ( ∼ 0.02 wt. % ), and some but not necessarily all secondary micas may result from infiltration of kimberlite into peridotite xenoliths. A good correlation between P and F in some oceanic basalts and gabbros might suggest that these elements are derived mainly from F-rich apatite inthe mantle, and that whitlockite is not present in the source region. Mantle-derived mica and amphibole have such low Cl that it is necessary to attribute Cl in oceanic basalts and gabbros either to substantial Cl in the source apatite, or to Cl from invading solutions, or both: three apatites from the mantle contain 0.8–1.0 wt.% Cl, and others contain lower amounts. The halogen contents of kimberlitic magmas can be explained by incorporation of Cl-bearing mica and F-rich apatite during melting of peridotites, but compositional constraints are weak.

Reduced chromium in olivine grains from lunar basalt 15555 - X-ray Absorption Near Edge Structure (XANES)

The oxidation state of Cr in 200 [mu]m regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Cr-bearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results coupled with the apparent f[sub o[sub 2]]-independence of partitioning coefficients for Cr into olivine imply that the source magma was dominated by divalent Cr at the time of olivine crystallization. 29 refs., 8 figs., 1 tab.

Aluminophosphate molecular sieve AlPO4-11: partial refinement from powder data using a pulsed neutron source

Abstract The crystal structure of AlPO 4 -11 is an open aluminophosphate framework containing a 10-ring channel obtained by removing two opposing columns of opposing four-rings from the alternating columns of four- and six-rings that surround the 12-ring channel in AlPO 4 -5.

Metamorphism, Partial Melting, and K-Metasomatism of Garnet-Scapolite-Kyanite Granulite Xenoliths from Lashaine, Tanzania

Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase (An28–41, Or1–2) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and CO2. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses (SiO2 38–56 wt %, Al2O3 15–23, K2O 0.5–8) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to melting of garnet, clinopyroxene, and plagioclase, followed by quench crystallization, and finally by K replacement of Na. One clinopyroxenite with 15% glass and quench spinel lacks garnet and scapolite. All properties are consistent with rapid decompression, quenching and K metasomatism. The estimated pre-emption temperature (1200 K) for the Lashaine granulites lies well above the temperatures at ∼14 kb for a standard shield geotherm (850 K) and even an oceanic geotherm (1060 K). Pressure-temperature estimates for granulite xenoliths from three sites (Delegate, Engeln, Lashaine) fall on a single trend tentatively called an alkaline-province geotherm.

Atomic movements in plagioclase feldspars: Kinetic interpretation

The diffraction, n.q.r. and optical data on plagioclase feldspars are used to derive kinetic interpretations of structural changes induced by laboratory heat treatment and by geological processes.For anorthite, the Si, Al configuration is essentially ordered except for unusual transient processes. Cooperation between Ca atoms, and random nucleation, produces a domain texture in the primitive structure which is highly sensitive to temperature. The rapid inversion from the primitive to the body-centered structure is explained by increasingly rapid “rattling” of the Ca ions in the interstices of the semi-flexible alumino-silicate framework. The weakening of “b” reflections at higher temperatures is ascribed to incipient Si, Al disorder associated with irregular vibration of the alumino-silicate framework and the Ca atoms. Quenching phenomena are explained by variation of the domain boundary texture inherited from disorder at high temperature.For albite, the Si, Al configuration changes sluggishly from an ordered to a disordered pattern, and vice versa. Kinetic data are reinterpreted using a model in which the cell dimensions depend on local rather than distant order: the major change in distant order is deduced to occur at 450–600° C.Sodic plagioclase grown at high temperature shows distant disorder of the atoms, but cell dimensions suggest development of strong local order for calcic compositions.Low-entropy plagioclases of intermediate composition show complex intergrowths and domain structures because of kinetic barriers to atomic diffusion. X-ray diffraction data for slowly-cooled specimens are consistent with nucleation of albite- and anorthite-like regions from a high-temperature disordered phase. Electrostatic energy calculations show that Na and Ca atoms, although they face smaller energy barriers for diffusion, cannot form domains until the Si and Al atoms have moved jointly. The Si, Al ordering patterns of low albite and anorthite are topologically incompatible in a continuous framework if oxygen is not to be bonded to two Al. Therefore domains of low-albite and anorthite must be separated by disordered boundaries.For intermediate compositions, An15-An75, domains remain small. The anorthite-like domains probably form at higher temperatures than the albite-like domains. The latter tend to be about the same size for all bulk compositions. The atomic positions are influenced by neighboring atoms.Upon heating rapidly, Si and Al atoms remain in position and provide a memory for reformation of an identical structure upon cooling. The framework changes shape, and some Na, Ca atoms inter-diffuse to yield a quasi-homogeneous structure with a diffraction pattern which qualitatively approaches that of high albite. Upon prolonged heating at high temperature, Si, Al atoms inter-diffuse producing nonquenchable changes to the high-albite structure.At Na-rich bulk compositions, some domains of low albite grow into large lamellae while others remain small in contact with anorthite domains producing alternate lamellae of intermediate structure type; hence the peristerite intergrowth. A similar but opposite process could cause an intergrowth of lamellae of anorthite structure interposed with an intermediate type structure.A unique low plagioclase series is not expected. Plagioclases of intermediate composition trend towards slightly different endproducts depending on the details of the cooling history. Breaks and bends in plots of physical properties, and intergrowths for certain specimens, depend on special compositional, growth and annealing factors.The intergrowth responsible for iridescence of intermediate plagioclase is ascribed to Na, K segregation prior to development of the complex domain structure. Prolonged annealing at high temperature in a dry environment is suggested.It is futile to attempt to describe low entropy plagioclases in terms of classical thermodynamics: only a kinetic interpretation based on atomic and sub-microscopic textural factors can be viable.

Chemistry of micas from kimberlites and xenoliths—I. Micaceous kimberlites

Micaceous kimberlites from South Africa and Canada contain two types of groundmass mica less than 1 mm across. Very rare Type I micas are relatively iron-rich with mg [ = Mg/(Mg + Fe)] 0.45–0.65, TiO2 3–6 wt%, Al2O3 14–16wt%, no Fe3+ required in tetrahedral sites, low NiO (~0.02 wt%), and relatively high na [Na2O/(Na2O + K2O)] 0.02–0.03. The much more abundant Type II micas are variable in composition, but relative to Type I micas are more magnesium (mg 0.80-0.93), lower in TiO2 (0.7–4.0 wt%) and Al2O3 (6.8–14.2 wt%), have substantial Fe3+ in tetrahedral sites, and have relatively low na. Both types may have rims with compositions indicative of mica-‘serpentine’ mixtures resulting from reaction with a highly aqueous fluid. The petrographically-determined ‘serpentine’ is chemically of two types: Fe-rich serpentine and Fe-rich talc. Associated phases in the ground-mass vary from one kimberlite to another: calcite, dolomite, diopside, chromite, Mg-ilmenite, perovskite, barite, pyrite, pentlandite, millerite?, heazlewoodite?, quartz. Inter-grain variations in composition of Type II micas may result from establishment of local reservoirs on a mm scale, consequent upon mechanical mixing and competition of other phases for minor elements (e.g. chromite for Cr, serpentine for Ni). Type I micas may result from an intrusive precursor (carbonatitic?) to kimberlite, perhaps genetically related, which was incorporated into a later pulse of kimberlite from which the Type II micas crystallized.