Richterite

Abstract: The stability field of pargasitic amphibole in a model mantle composition (MORB pyrolite) has been experimentally determined for a fixed water content. A solidus for a pargasite-bearing lherzolite has been defined at pressures below the limit of amphibole stability of 30 kbar at T = 925 °C. The maximum temperature for pargasitic amphibole in MORB pyrolite occurs at 1075 °C between P = 18 and 25 kbar. This maximum lies between that determined for a fertile peridotite composition (Hawaiian pyrolite) and a depleted peridotite composition (Tinaquillo lherzolite). A comparison of the new results with those from earlier studies suggests that the stability for a particular bulk H2O content is mostly controlled by alkali content of the lherzolite composition. The systematic compositional variation of pargasitic amphibole as a function of pressure and temperature can be represented as an increase of the richterite component with increase in both pressure and temperature. For a given pressure the tschermakite component increases with increasing temperature. The compositions of coexisting clinopyroxenes also show a systematic variation with pressure and temperature. The phase relationships in MORB pyrolite combined with the modal abundance of coexisting phases show that the breakdown reactions of pargasitic amphibole occur continuously throughout the subsolidus region studied. The temperature stability limit of pargasitic amphibole coincides with the water-undersaturated solidus (amphibole-dehydration solidus) at pressures below 30 kbar. The experimental results are applicable to pargasitic amphibole-bearing natural peridotites. Cooling and decompression paths and heating events observed in natural peridotites can be interpreted from changes in the composition of pargasitic amphibole. The data are also applicable to a model for peridotite melting and hydration process in the subduction environment.

Abstract: Thirty samples of amphibole-rich rock from the largest mined vermiculite deposit in the world in the Rainy Creek alkaline-ultramafic complex near Libby, Montana, were collected and analyzed. The amphibole-rich rock is the suspected cause of an abnormally high number of asbestos-related diseases reported in the residents of Libby, and in former mine and mill workers. The amphibole-rich samples were analyzed to determine composition and morphology of both fibrous and non-fibrous amphiboles. Sampling was carried out across the accessible portions of the deposit to obtain as complete a representation of the distribution of amphibole types as possible. The range of amphibole compositions, determined from electron probe microanalysis and X-ray diffraction analysis, indicates the presence of winchite, richterite, tremolite, and magnesioriebeckite. The amphiboles from Vermiculite Mountain show nearly complete solid solution between these end-member compositions. Magnesio-arfvedsonite and edenite may also be present in low abundance. An evaluation of the textural characteristics of the amphiboles shows the material to include a complete range of morphologies from prismatic crystals to asbestiform fibers. The morphology of the majority of the material is intermediate between these two varieties. All of the amphiboles, with the possible exception of magnesioriebeckite, can occur in fibrous or asbestiform habit. The Vermiculite Mountain amphiboles, even when originally present as massive material, can produce abundant, extremely fine fibers by gentle abrasion or crushing.

Abstract: On the basis of both natural samples and experimental studies, clinopyroxene is a potential reservoir for potassium in the Earth9s mantle. The amount of K partitioning into clinopyroxene depends on the phase assemblage present, the bulk composition, pressure, and temperature. To investigate some of these dependencies, subsolidus and melting phase relations in the system phlogopite-diopside have been studied to 17 GPa. In this system, phlogopite becomes unstable with increasing pressure, breaking down to potassium richterite, which in turn breaks down to another K-bearing hydrous phase (phase X), such that a K-rich phase coexists with clinopyroxene to 17 GPa. Clinopyroxenes contain 2 O in assemblages of phlogopite + clinopyroxene + or - olivine + or - liquid at 3-5 GPa, phlogopite + clinopyroxene + garnet + or - olivine + or - liquid at 7-9 GPa, clinopyroxene + garnet + olivine + or - potassium richterite + or - liquid at 11 GPa, and clinopyroxene + olivine + garnet + phase X at 14 and 17 GPa. In these assemblages, K is partitioned into hydrous phases or liquid, rather than into the clinopyroxene. By inference, phlogopite (or its higher-pressure breakdown products) is the primary host for K in the mantle (if H 2 O is present), and any coexisting clinopyroxene has very low concentrations of K. Conversely, the natural occurrence of clinopyroxene with >>1 wt% K 2 O requires that phlogopite, potassium richterite, or phase X is not stable in the local source environment of such samples.