Humite-group Minerals Research Articles

Electron microscope study of the reaction olivine + H2O + TiO2→ titanian clinohumite + Titanian chondrodite synthesized at 8 GPa, 1300 K

The nucleation and growth processes of titanian clinohumite (Ti-Cl) and titanian chondrodite (Ti-Ch) during hydrous alteration of olivine at 8 GPa, 1300 K are described in detail. We observed two different nucleation sites and orientation relationships for Ti-Cl: (1) heterogeneous nucleation at olivine grain or phase boundaries, yielding semicoherent (001) lamellae with (001)ol//(001)Ti-Cl and (010)ol//(010)Ti-Cl; (2) formation of coherent nanometer-sized nuclei inside olivine with the orientation relationship (100)ol//(001)Ti-Cl and (011)ol//(020)Ti-Cl. Only the former, for which the interface with olivine facilitates the transport of OH–and Ti necessary for the formation of the humite-group minerals, survives to large sizes. These results suggest that Ti-Cl should form readily in a Ti- and H-rich environment at mantle temperatures, independent of F content, from shallow environments to those at depths exceeding 250 km, and could play a role in recycling water back into the deep mantle.

An investigation of matrix effects in the analysis of fluorine in humite-group minerals by EMPA, SIMS, and SREF

Accurate determination of F in minerals is a difficult task even when high F concentrations are present. Fluorine usually is determined by means of electron micro-probe analysis (EMPA) standardized on non-silicate-matrix compounds (e.g., fluorite), and some previous work has revealed the difficulties in determining F at high concentrations such as found in the humite-group minerals. Moreover, when both single-crystal structure refinement (SREF) and EMPA are available for the same crystal, the two estimates do not always agree. On the other hand, the secondary ion mass spectrometry (SIMS) technique is not easily applied at high F concentrations due to the existence of matrix effects related to the chemical composition and structure of the sample as well as to the concentration of the element itself. We tested the agreement among these analytical techniques in the estimation of high F contents and propose an analytical procedure for the analysis of fluorine. Our results indicate that careful selection of working conditions for EMPA of F together with appropriate correction, can yield accurate fluorine concentrations in minerals. Fluorine data extracted from refined site occupancies are systematically overestimated. New accurate working curves have been worked out for SIMS analysis of F taking Si and Mg, in turn, as the reference element for the matrix. Humite-group minerals show SIMS matrix effects on the order of ~10%. In analyzing fluoborite in the most unfavorable cases, the difference in Ion Yield (F/Mg) between “disoriented” humite-group minerals and “oriented” fluoborite samples can reach ~27%. Finally, a lower than expected IY(F/Si) from the F/Si working curve (made with humite minerals) is shown by topaz, which can be ascribed to chemical matrix effects, as well as to the covalent-type bonding between F and the major element in the matrix (Al).

Single-crystal elastic properties of chondrodite: implications for water in the upper mantle

Chondrodite, a member of the humite group of minerals, forms by hydration of olivine and is stable over a range of temperatures and pressures that includes a portion of the uppermost mantle. We have measured the single crystal elastic properties of a natural chondrodite specimen at ambient conditions using Brillouin spectroscopy. The isotropic aggregate bulk (K) and shear (µ) moduli calculated from the single-crystal elastic moduli, Cij, are: KS=118.4(16) GPa and µ=75.6(7) GPa. A comparison of the structures and elasticity of olivine and chondrodite indicate that the replacement of O with (OH,F) in M2+O6 octahedra has a small effect on the elasticity of humite-group minerals. The slightly diminished elastic moduli of humite-group minerals (as compared to olivine) are likely caused by a smaller ratio of strong structural elements (SiO4 tetrahedra) to weaker octahedra, and perhaps a more flexible geometry of edge-sharing MO4(O,OH,F)2 octahedra. In contrast to the humite-olivine group minerals, the incorporation of water into garnets and spineloids leads to a more substantial decrease in the elastic properties of these minerals. This contrasting behavior is due to formation of O4H4 tetrahedra and vacant hydroxyl-bearing octahedra in the garnets and spineloids, respectively. Therefore, the mechanism of incorporation of H/OH into mineral phases, not only degree of hydration, should be taken into account when estimating the effect of water on the elastic properties of minerals. The bulk elastic wave velocities of chondrodite and olivine are very similar. If humite-like incorporation of OH is predominant in the upper mantle, then the reaction of OH with olivine will have a minor or possibly no detectable effect on seismic velocities. Thus, it may be difficult to distinguish chondrodite-bearing rocks from “anhydrous” mantle on the basis of seismically determined velocities for the Earth.

Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land, Antarctica: monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20 °C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with X NaCl in the range 0.15–0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650 °C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800 °C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650 °C) assemblages. Biotite in all but two syenite samples crystallized at log(f H 2 O/f HF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f H 2 O/f HF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f H 2 O/f HCl) derived from biotite covers the range 0.5–2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510–2790 bars for H2O, 1.3–5.3 bars for HF and 7–600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(f H 2 O /f HF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(f H 2 O /f HF) increases significantly. Chlorine preferably partitions into the fluid and hence log(f H 2 O /f HCl) does not change markedly during fluid-rock interaction.

Parageneses and compositional variations of Sb oxyminerals from Långban-type deposits in Värmland, Sweden

AbstractThe Långban, Nordmark and Jakobsberg Mn-Fe deposits contain the only known occurrences of filipstadite and manganostibite (ideal formulae (Mn, Mg)2(Sb0.55+Fe0.53+)O4 and Mn72+SbAsO12, respectively). Filipstadite from Nordmark is newly recognized, and occurs in assemblages with svabite-johnbaumite, calcite, tephroite-forsterite, phlogopite-kinoshitalite, tilasite, ±jacobsite, ±plumbia roméite, ±adelite, ±hedyphane. Manganostibite from Långban and Jakobsberg is reported for the first time, and the mineral is generally associated with katoptrite, tephroite, humite-group minerals, calcite, svabite, allactite, manganosite, hausmannite, jacobsite, spinel s.s., etc. Whereas filipstadite is clearly secondary relative to the major part of the matrix components, manganostibite is believed to have formed coevally with the principal ore and skarn minerals at these deposits.The previously known compositional ranges are extended. Based on electron-microprobe analyses, Nordmark filipstadite contains 4.1–7.3 MgO, 0.0–0.5 Al2O3, 30.5–45.3 MnO, 17.0–40.1 Fe2O3, 0.2–0.9 ZnO, 19.9–29.9 Sb2O5 (all in wt.%), corresponding to 58–100 mol.% of a pure filipstadite component. Associated jacobsites show Sb2O5 contents of up to c. 5 wt.%. Manganostibites (all three deposits considered) contain 1.0–2.9 MgO, 2.8–3.8 SiO2, 57.4–60.3 MnO, 0.2–3.5 Mn2O3, 0.3–2.0 Fe2O3, 0.0–2.4 ZnO, 21.5–23.0 Sb2O5, 7.7–10.0 As2O5 (all in wt.%). Si and trivalent cations are incorporated via a (Mn3+,Fe3+) + Si4+=Mn2+ + As5+ exchange mechanism, which improves the local charge balance at tetrahedral structural sites dominated by As.

Sound velocities of clinohumite, and implications for water in Earth's upper mantle

The velocities of longitudinal and shear acoustic waves were measured for clinohumite, a possible host for water in the upper mantle. Brillouin scattering measurements on a natural crystal under ambient conditions indicate that the hydration of olivine to clinohumite lowers most of the single‐crystal elastic moduli, as well as the bulk and shear moduli of a polycrystalline aggregate. However, the velocities of mantle rocks in which all olivine is hydrated to clinohumite are only 1–1.5% lower than those of the equivalent dry rocks under ambient conditions. These results suggest that in shallow, cold portions of the uppermost mantle (for example the seismic lid), water may be difficult to identify seismologically if all of it is bound up in humite‐group minerals. Negative velocity anomalies in the shallow mantle may require a different explanation than the presence of hydrous minerals.

Geochemistry and origin of the Bald Knob manganese deposit, North Carolina

Geochemical and textural evidence for the submarine hydrothermal origin of the Bald Knob Mn deposit, North Carolina, is preserved in a mineralogically diverse suite of rocks that have experienced amphibolite facies metamorphism. Manganese- and Mn-Fe-rich lithologies have high Mn/Fe ratios and low concentrations of Ni, Cu, Zn, U, Th, and rare earth elements that are similar to concentrations reported from other ancient manganiferous deposits and from crusts and sediments which are forming in hydrothermally active regions of the present sea floor. Manganese- and Ca-rich carbonate minerals, a variety of Mn-rich silicates (including tephroite, the Mn humite-group minerals, spessartine, rhodonite, and pyroxmangite), and the oxides galaxite and jacobsite form fine laminations that represent different bulk compositions. No chemical or textural evidence for replacement of Mn-poor by Mn-rich minerals was found and, in some samples, the same lithologic units are repeated on the scale of a thin section, consistent with a primary sedimentary origin for the layering. Chert, which is one of the country rocks at Bald Knob, and Mn- and Ca-rich carbonate rocks represent original sedimentary protoliths. More than one detrital component may be present in the Mn-rich layers. Biogenic detritus was the probable source of at least some of the siliceous material that allowed for the formation of the Mn-rich silicates. An aluminous detrital component, which probably was derived from a continental or andesitic source, is reflected in spessartine-rich layers. Minor Mn and Co sulfide mineralization (alabandite, cattierite, cobaltite) was contemporaneous with deposition of the Mn-rich sediments. The association of the manganiferous lithologies with chert and meta-igneous rocks (amphibolite) is consistent with deposition of the sediments in a rifting environment, such as a back-arc basin, or possibly, a midocean ridge.

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine

Widely dissociated dislocations have been observed in mantle olivine from the Erro-Tobbio peridotite in N.W. Italy. Analysis of diffraction contrast in transmission electron microscopy (TEM) indicates that the dissociation reaction involves the climb dissociation on (001) and {021} planes of b=[001] unit dislocations into partial dislocations with Burgers vectors approximately equal to 1/x 〈011〉. In the most extreme case a unit dislocation dissociates into four partials which bound three planar defects. The unusually wide dissociation and the greater extent of dissociation in olivine from amphibole-bearing rocks suggests that the dissociation is related to hydration. The occurrence of fluid inclusions along the dislocations confirms that the partials and planar defects are saturated with volatiles. Analysis of possible planar defect structures in Fo90 shows that; (i) the most likely partial Burgers vectors are b=〈0 3/11 1/4〉; (ii) two of the planar defects are cation-deficient and can be stabilised by segregation of H+ to produce (Mg, Fe) (OH)2 layers which are iso-structural with the OH-rich interlayer of the humite group minerals; (iii) the central planar defect is formed by removing a stoichiometric olivine (002) layer so does not produce any local chemical changes. The climb dissociation provides a possible mechanism for the transformation of olivine to a humite group mineral. OH-rich interlayers may nucleate on dislocations and extend into the crystal by climb resulting in a gradual increase of (Mg, Fe) (OH)2 content. The only addition of material required is hydrogen which can rapidly diffuse into olivine. If the dissociation is stable and occurs at high temperatures and pressures it may significantly influence the nature and kinetics of deformation mechanisms and the olivine — spinel shear transformation mechanism in hydrated olivine.

Titanian chondrodite and clinohumite in marbles from the �tztal crystalline basement

Marble lenses within the polymetamorphic Otztal crystalline complex frequently contain pockets of clinohumite and chondrodite and their titanium-rich varieties. Chondrodites contain up to 9.1 wt% TiO2 and clinohumites about half that amount. The maximum amounts of Ti in both minerals do not exeed values of 0.4 in the formula. XTi and XF are correlated by a slope of −1, indicating the efficiency of the TiO2Mg−1F−2 substitution at constant OH (Evans andTrommsdorff, 1983). Clinohumites sometimes contain lamellae which have nonstoichiometric cation ratios reflecting a composition between clinohumite and olivine. In many cases Ti-rich clinohumites and chondrodites coexist with Mg-ilmenite. Ti-rich clinohumites and chondrodites often break down to a symplectite of Ti-poor clinohumite (or olivine) and Mg-ilmenite. Parts of the marbles, which are barren of Mg-ilmenite contain F-rich clinohumites. The latter do not show any signs of breakdown features. We conclude therefore that fluorine stabilizes the humite group minerals to a wide range of metamorphic conditions. Ti-rich humite-group minerals are not stable till amphibolite facies conditions, and below F-rich humite-group minerals and olivine are the more stable phases in a H2O-rich, SiO2-poor carbonate system.

The natural occurrence of hydroxide in olivine

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/10^(6)Si, whereas olivines from basalts contain the least at 3 H/10^(6)Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H_2 in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm^(−1) are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H_2O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.

Mineralogy and geochemistry of the Read magnetite deposit, southwestern Stevens County, Washington

The Read magnetite deposit of contact metasomatic origin has been formed along the contact of probable Cambrian limestone with granite of Mesozoic age. Notable among the various minerals formed is ludwigite, which appears to occupy a hanging wall position, or is generally farther from the granite than other minerals in the zone, and which appears to be a selective replacement of forsterite. Other nonmetallic contact metasomatic minerals include abundant diopside; lesser amounts of almandite, andradite, and hedenbergite; and minor amounts of spinel, epidote, tremolite, quartz, sanidine, labradorite, orthoclase, apatite, humite group minerals, serpentine, calcite, and dolomite. Metallic minerals include abundant magnetite; lesser amounts of pyrite, chalcopyrite, and pyrrhotite; and very minor amounts of scheelite and spalerite. Accessory metallic elements include Sn, Ti, Mn and probably Mo. The minerals are believed to have formed mainly in response to a decline in temperature inward from the contact. Magnetite, sphalerite, and possibly other metallic minerals appear to have formed in the pneumatolytic phase. Greatest loss of material was in carbon dioxide and lime; greatest gain was in silica, iron oxides, magnesia, B, and alumina. Ultimate sources of B probably were in Cambrian and Precambrian metamorphic rocks that were intruded and assimilated by the granite.