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«Dissertation ETH No. 14488 PRESSURE- AND TEMPERATURE-BEHAVIOUR OF HYDROXIDES AND HYDROUS MINERALS: STRUCTURE, COMPRESSIBILITY AND PHASE TRANSITIONS ...»

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Dissertation ETH No. 14488

PRESSURE- AND TEMPERATURE-BEHAVIOUR OF

HYDROXIDES AND HYDROUS MINERALS:

STRUCTURE, COMPRESSIBILITY AND PHASE TRANSITIONS

Dissertation

submitted to the

Swiss Federal Institute of Technology

Zurich

for the degree of Doctor of Natural Sciences

Presented by

Alexandra Friedrich

Mag. rer. nat. (University of Vienna)

born 24 November, 1974 in Austria Accepted on the recommendation of Dr. Martin Kunz Prof. Dr. Walter Steurer Prof. Dr. Thomas Armbruster I Abstract Only a very limited region of the Earth’s interior is accessible for direct observations. Therefore, the thermodynamic conditions in the depth are reconstructed by experiments in order to investigate the stability and properties of minerals, which make up the inner Earth. Fluid-bearing phases have a large influence on processes within the Earth’s mantle. In order to understand possible transport mechanisms of fluids into the depth it is necessary to gain information on the stability of fluid-bearing phases. The aim of this thesis was to extend the knowledge on OH-bearing minerals as transportation and storage sites for water in the Earth’s mantle.

Garnets and olivine are important minerals that make up the Earth’s lower crust and upper mantle. Garnets are known to form solid-solution series with hydrogarnets, such as grossular, Ca3 Al2(SiO4)3, and katoite, Ca3 Al2(O4H4)3. The O4H 4 ´ SiO4 substitution shows an important mechanism of structural water incorporation in silicates. Structurally bound water might be transported within garnets to the depth through subduction zones. In this context the high-pressure behaviour of the silicatehydrogarnet henritermierite, Ca3Mn2(SiO4)2(O4H4), was studied up to 8.6 GPa by single-crystal X-ray diffraction. High-precision compressibility data yielded a strong dependence of the bulk modulus on O4H 4 incorporation if compared to other garnets.

Axial anisotropic compressibility was attributed to a reduction of Jahn-Teller distortion of the Mn3+O6 polyhedron with pressure.

Previous studies showed that nominally anhydrous minerals might contain a few weight percent of water. Olivine (Mg2SiO4), the major component of the Earth’s mantle, exhibited monolayers of clinohumite in high-resolution transmission electron microscopic studies. Clinohumite, Mg9(SiO4)4(F,OH)2, and chondrodite, Mg5(SiO4)2(F,OH)2, members of the humite-group minerals, show a strong structural relationship to olivine, and belong to the hydrous dense magnesium silicates. They are candidate minerals for the transportation of water into depth. The humite-group minerals show a large variability in chemical composition due to different substitution mechanisms. In this study, we concentrated on the influence of the F(OH)-1 substitution on the structural high-pressure behaviour. High-pressure compressibility studies were performed for natural F-bearing chondrodite and clinohumite. The results showed only slightly higher bulk moduli if compared to the OH-endmembers. High-pressure singlecrystal X-ray intensity data were collected up to 9.6 GPa for F-bearing chondrodite and synthetic hydroxylchondrodite. The data of F-bearing chondrodite gave insight into the overall compressional behaviour of the polyhedral framework (non-hydrogen atoms).

These results were complemented by high-pressure neutron powder diffraction data of synthetic (OD,F)-chondrodite, which gave structural information on the O-D◊◊◊O/F geometry with pressure.

A complementary study dealt with the low-temperature behaviour of natural titanian hydroxylclinohumite, F-bearing clinohumite and F-bearing chondrodite. Singlecrystal neutron diffraction data were collected between room temperature and 10 K in order to investigate the effect of F(OH)-1 substitution on the hydrogen-atom environment on the one hand, and the importance of symmetry-forbidden reflections on the space-group symmetry and their origin, as can be derived from the behaviour at low temperature, on the other hand. H-H repulsion between H sites otherwise related by a II centre of symmetry in humites requires a site occupation of £ 50 % and a concomitant disordered distribution of the H atoms on the symmetry-related sites. Therefore, the centre of symmetry is locally violated by H atoms. Depending on the size of locally ordered domains relative to the coherence length of the X-rays, symmetry-forbidden reflections can occur. Hardly any OH-endmember is found in nature, as long as the geochemical environment provides sufficient F and/or Ti. If not, a second H site is formed at higher H content.

In addition to the studies on hydrogen-bearing silicates, simple fluid-bearing materials were investigated in order to contribute to a theoretical basis of the crystalchemical behaviour of structurally bound water as a function of pressure, temperature and chemical composition. This basis should serve as analogue for the interpretation of the behaviour of complex minerals, such as fluid-bearing silicates. Hydroxides of the earth-alkaline elements were investigated, because they are simple compounds and allow the distinction of the relative influences of the different thermodynamic parameters from chemical effects more easily than complex minerals. Previous studies dealt with the high-pressure behaviour of Mg(OH)2 and Ca(OH)2. Our investigations were extended on Sr(OH)2 and Ba(OH)2, the cations of which are higher coordinated and larger in size. These compounds could simulate the behaviour of the hydroxides of smaller cations at much higher pressures.





Various synthesis experiments of hydroxides were carried out. Powder of pure deuterated and non-deuterated barium hydroxide was successfully produced. Singlecrystal synthesis of Ba(OH)2 and Sr(OH)2 did not succeed by now due to difficulties in handling these sensitive materials.

The structural behaviour of both b- and a-Ba(OD)2 was first investigated as a function of temperature. Neutron powder diffraction data of the deuterated samples were collected between 10 and 552 K. Although the b phase remained stable at low temperature, the high-temperature a phase, which is metastable at low temperature, showed an unusual reversible phase transition towards another metastable a m phase between 100 and 150 K. This phase transition is driven by an order-disorder mechanism of one of the deuterium atoms.

A high-pressure study showed results quite different to the behaviour at low temperature. Synchrotron X-ray powder diffraction data were measured for b- and aBa(OH)2, as well as for Sr(OH)2 up to 13 GPa. b-Ba(OH)2 showed up to three phase transitions in the investigated pressure range. The first phase transition is described by a doubling of the unit-cell volume and the loss of symmetry elements. At higher pressures the structure becomes too complicated to be solved from present high-pressure powder data, as the symmetry is again reduced and the unit-cell becomes even larger. The results at lower pressures showed increasing coordination numbers of the barium atoms.

a-Ba(OH)2 was not stable either during loading or at increasing pressure, hence transforming back to the stable b phase. The data of Sr(OH)2 showed either a mixture with an unknown phase, or an unresolved phase transition below 1.2 GPa. Above 13 GPa the sample amorphised.

III On the whole, the behaviour of hydroxides at high pressure seems to become more complex with increasing cation/anion size ratio. Furthermore, none of these studies showed decomposition of the hydroxides to their oxide components under pressure.

IV Zusammenfassung Ein nur sehr limitierter Bereich des Erdinnern ist unseren direkten Beobachtungen zugänglich. Deshalb sollen die thermodynamischen Bedingungen, wie sie in der Tiefe herrschen, mittels Experimenten nachvollzogen werden, um die Stabilität und die Eigenschaften der Minerale, welche das Innere der Erde aufbauen, zu untersuchen.

Fluidhaltige Phasen haben einen grossen Einfluss auf die Prozesse innerhalb des Erdmantels. Um nun mögliche Transportmechanismen von Fluids in die Tiefe zu verstehen, ist es notwendig, Informationen über die Stabilität von fluidhaltigen Phasen zu gewinnen. Das Ziel dieser Doktorarbeit war, das Wissen über OH-haltige Minerale als Transport- und Speichermedien für Wasser im Erdmantel zu erweitern.

Granat und Olivin tragen zu einem erheblichen Anteil zum Aufbau der Erdkruste und des oberen Erdmantels bei. Es ist bekannt, dass Granate eine Mischkristallreihe mit Hydrogranaten bilden, wie z.B. Grossular, Ca3Al2(SiO4)3, und Katoit, Ca3Al2(O4H4)3.

Die O4H4 ´ SiO4 Substitution ist ein wichtiger Mechanismus, um Wasser strukturell in Silikate einzubauen. Strukturell gebundenes Wasser könnte in Granaten über Subduktionszonen in die Tiefe transportiert werden. In diesem Zusammenhang wurde das Hochdruckverhalten des Silikathydrogranats Henritermierit, Ca3Mn2(SiO4)2(O4H4), mittels Einkristallröntgenbeugung bis 8.6 GPa untersucht. Hochpräzise Kompressibilitätsdaten zeigten im Vergleich mit den Daten anderer Granate eine starke Abhängigkeit der Kompressibilität vom Einbau von O4H4 Gruppen. Die axiale anisotrope Kompressibilität wurde auf eine Reduktion der Jahn-Teller Verzerrung des Mn3+O6 Oktaeders mit Druck zurückgeführt.

Frühere Untersuchungen zeigten, dass nominell wasserfreie Minerale ein paar Gewichtsprozent Wasser enthalten können. Olivin als Hauptbestandteil des Erdmantels zeigte in hochauflösenden Transmissionselektronenmikroskopaufnahmen Zwischenschichten von Clinohumit. Clinohumit, M g 9 (SiO4)4(F,OH)2, und Chondrodit, Mg5(SiO4)2(F,OH)2, gehören der Humitgruppe an, weisen eine starke strukturelle Ähnlichkeit zu Olivin auf, und werden den wasserhaltigen dichten Magnesiumsilikaten zugesprochen. Sie sind wahrscheinliche Kandidaten für den Transport von Wasser in die Tiefe. Die Humitminerale weisen eine grosse chemische Variabilität und somit unterschiedliche Substitutionsmechanismen auf. In diesen Untersuchungen konzentrierten wir uns auf den Einfluss der F(OH) -1 Substitution auf das strukturelle Hochdruckverhalten. Es wurden Hochdruckkompressibilitätsmessungen an F-haltigem Chondrodit und Clinohumit durchgeführt. Im Vergleich zu den OH-Endgliedern weisen sie eine leicht geringere Kompressibilität auf. Einkristallröntgenbeugungsintensitätsdaten wurden von F-haltigem Chondrodit und synthetischem Hydroxylchondrodit unter Drücken bis zu 9.6 GPa gesammelt. Die Daten des F-haltigen Chondrodits gaben einen Einblick in das gesamte kompressive Verhalten des Polyedergerüsts (ohne die Wasserstoffatome). Diese Resultate wurden durch Hochdruckpulverneutronenbeugungsdaten von synthetischem (OD,F)-Chondrodit erweitert. Diese gaben uns strukturelle Informationen über die O-D◊◊◊O/F Geometrie als Funktion von Druck.

Eine komplementäre Untersuchung beschäftigte sich mit dem Tieftemperaturverhalten von natürlichem Titanhydroxylclinohumit, F-haltigem Clinohumit und Fhaltigem Chondrodit. Es wurden Einkristallneutronenbeugungsdaten zwischen V Raumtemperatur und 10 K gesammelt, sowohl um den Effekt der F(OH)-1 Substitution auf die Umgebung des Wasserstoffatoms, als auch die Bedeutung von symmetrieverbotenen Reflexen auf die Raumgruppe und deren Herkunft aus dem Verhalten bei tiefer Temperatur zu untersuchen. H-H Abstossung zwischen an und für sich zentrosymmetrisch gekoppelten Wasserstoffpositionen in Humiten macht eine Besetzungsdichte dieser Position von über 50 % unmöglich. Die Verteilung der Wasserstoffatome auf die teilbesetzte Position ist ungeordnet. Dadurch wird das Symmetriezentrum lokal durch die Wasserstoffatome verletzt. Je nach Grösse der lokal geordneten Domänen relativ zu der Kohärenzlänge der Röntgenstrahlung können symmetrieverbotene Reflexe auftreten. Solange das geochemische Umfeld genügend F und/oder Ti zur Verfügung stellt, werden kaum OH-Endglieder gefunden. Wenn nicht, dann wird bei höherem Wasserstoffgehalt eine zweite Wasserstoffposition gebildet.

Zusätzlich zu den Untersuchungen an wasserstoffhaltigen Silikaten wurden einfache fluidhaltige Substanzen untersucht. Dies sollte dazu beitragen, die theoretische Grundlage für das kristallchemische Verhalten von strukturell gebundenem Wasser als Funktion von Druck, Temperatur und chemischer Zusammensetzung zu erweitern.

Diese Grundlagen sind als Analogon für die Interpretation des Verhaltens komplexerer Minerale (z.B. fluidtragender Silikate) von Bedeutung. Im besonderen sollten die Hydroxide der Erdalkalien betrachtet werden, weil sie sehr einfache Substanzen sind, mit denen die relativen Einflüsse der verschiedenen thermodynamischen Parameter besser von chemischen Effekten unterschieden werden können. Vorangegangene Untersuchungen haben sich mit dem Hochdruckverhalten von Mg(OH)2 und Ca(OH)2 beschäftigt. Unsere Untersuchungen wurden auf die Hydroxide von Kationen höherer Ordnungszahl, wie Sr(OH)2 und Ba(OH)2, welche höher koordinierte und grössere Kationen aufweisen, ausgeweitet. Diese Substanzen könnten das Verhalten von Hydroxiden kleinerer Kationen unter viel höheren Drücken simulieren.

Es wurden mehrere Syntheseexperimente von Hydroxiden durchgeführt. Reines Pulver von deuteriertem und nicht-deuteriertem Bariumhydroxid wurde erfolgreich hergestellt. Die Einkristallsynthese von Sr(OH)2 und Ba(OH)2 war aufgrund der Schwierigkeiten beim Umgang mit den sensiblen Materialien noch nicht erfolgreich.

Das strukturelle Verhalten von b- und a-Ba(OD)2 wurde zuerst als Funktion der Temperatur untersucht. Neutronenbeugungsdaten wurden an deuterierten Pulverproben zwischen 10 und 552 K gemessen. Obwohl die b Phase bei tiefen Temperaturen stabil blieb zeigte die Hochtemperatur a Phase, welche bei tiefen Temperaturen metastabil ist, zwischen 100 und 150 K einen ungewöhnlichen, reversiblen Phasenübergang zu einer anderen metastabilen am Phase. Dieser Phasenübergang wird durch einen OrdnungsUnordnungsmechanismus eines der Deuteriumatome ausgelöst.



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