A partly destabilised Na-richterite has been found in an olivine-nephelinite from Morocco. The riehterite crystal (600 × 420 μm) is surrounded by a reaction zone (400-700 μm) of K- and Si-rich glass containing small (<50 μm) olivine (Fo80-83%) and endiopside crystals. Outwards, another zone is formed of normal magmatic minerals and circumscribes the original crystal, indicating that the destabilisation event took place at the end of the crystallisation sequence. Estimated ascent time of about 100 hours would have completely decomposed an isolated richterite crystal, which suggests that the amphibole was originally included in a xenolith. A mass-balance calculation shows that the fichterite isovolumic decomposition was accompanied by exchanges with the magma. The loss of Na from the reaction zone and the gain of AI from the magma allowed the precipitation of an analcime-rich zone observed around the destabilised amphibole and the concentration of K in the reaction zone glass. Compositional variations, Fe and Ti increase and Mg, Ca and F decrease at the richterite edge are interpreted as the result of a diffusion process. No alkali gradients are observed. The diffusion phenomenon lasted less than 100 hours and ceased to be operative at a temperature of 900-950°C i.e. just below the solidus temperature. Diffusion coefficients for the amphibole are proposed: e.g 10−9 cm2 s−1 for K2O and 10−10 for FeO at 900°C

Two specimens of tremolite and one of richterite, all with nephrite jade texture, have been examined by transmission electron microscopy using ion-thinning for specimen preparation. The specimens contain clusters of very small lath-like crystallites with z-axes approximately parallel but in a range of azimuthal orientations. It is suggested that these clusters which are themselves in varied orientations are the result of post-tectonic recrystallization of strained amphibole crystals, the new crystals inheriting the z-axis orientations of the old. The extreme toughness of nephrite jade is attributed to a number of the sub-microscopic features observed, including the sizes, habits, and orientations of its crystallites, and the nature of its grain boundaries.

Three natural amphibole samples collected from the former vermiculite mine near Libby, Montana. USA, have been analysed by Rietveld X-ray powder diffraction (XRPD) refinement and Fourier transform infrared spectroscopy (FTIR) in the OH-stretching region. The same materials have been analysed previously by electron microprobe analysis (EMPA), Mössbauer spectroscopy and structure refinement (SREF) single crystal X-ray diffraction (SC-XRD), which revealed that these amphiboles have a crystal chemical formula very close to an intermediate composition between winchite and richterite, i.e. AA0.5BNaCaCMg4.5M3+T0.5Si8O22(OH)2 (A = Na and/or K; M3+ = Fe3+ and/or Al). The Rietveld analysis showed the powder samples used for the experiments here to be composed only of amphibole. This in turn allowed us to use FTIR OH-stretching data to derive cation ordering on these powder samples. The three FTIR spectra are quite similar and up to four components can be fitted to the patterns. The two lower-frequency components (labelled A and B) can be attributed to a local O(3)-H dipole surrounded by M(1)M(3)Mg3 and M(1)M(3)Mg2Fe2+; (respectively), an empty A site and rSi8 environments; on the contrary, the higher-frequency C and D bands indicate the presence of an occupied A site. The FTIR OH-stretching data alone allow us to calculate the site occupancy of the A, M(1)–M(3) and T sites with confidence, as compared with previously published data. By contrast M(4)- and M(2)-site occupancies are more difficult to evaluate. This study takes advantage of the large database of well characterized synthetic amphiboles, built over the last two decades. The comparison of vibrational spectroscopy data with micro-chemical and crystallographic data reported in this study demonstrate that the FTIR OH-stretching method alone is a valuable and rapid method to derive or at least sensibly constrain site occupancy for natural amphiboles. A much more detailed cation site occupancy can be obtained by combining micro-chemical and FTIR OH-stretching data.

The distribution of Mg and Ni over the octahedrally-coordinated sites M(l,2,3) in Ni-substituted potassic richterite solid solution, AKB(NaCa)c(Mg2.5Ni2.5)Si8O22(OH)2, synthesized at 0.1 GPa/750°C (25 days), has been studied in situ to 700°C by neutron powder diffraction with Rietveld structure refinement. Using a 2.7 g sample it was possible to make short data collections (3 h) at each temperature at intervals of 50° from 50 to700°C and locate the onset of cation exchange. Above 700°C the amphibole decomposes rapidly and so only an on-heating dataset was collected. Unit-cell parameters increase smoothly upon heating, with no discontinuities evident. Site occupancies of M(l), M(2) and M(3) sites were refined from site-scattering values. The initial XNi values of M(l), M(2) and M(3) sites, corresponding to the synthesis temperature of 750°C, are 0.57(1), 0.34(1) and 0.68(2), respectively. Above 400°C, there is initial ordering whereby thermal annealing allows the disordered Mg-Ni distribution of the quenched synthesis product to adjust to be closer to those appropriate for lower temperatures of the heating sequence. This ordering involves the exchange of Mg and Ni between M(l) and M(2), with no significant change in M(3) occupancy. At 700°C, XNi values of the M(l), M(2) and M(3) sites are 0.64(1), 0.27(1) and 0.69(2), respectively. No disordering was observed, due to the short duration of the high-temperature data collections and decomposition at 750°C. The results of this study indicate that divalent cations exchange between octahedral sites in amphiboles in a matter of hours.

A small 33 ± 0.8 Ma lamproite pluton is exposed in the midst of a 23–26 Ma basalt-rhyolite province in Middle Park, NW Colorado. It contains abundant phlogopite phenocrysts in a fine-grained groundmass of analcime pseudomorphs after leucite, biotite, potassic richterite, apatite, ilmenite and accessory diopside. The phlogopite phenocryst cores contain ∼4 wt.% TiO2, 1% Cr2O3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ∼2% Al2O3, ∼8% TiO2 and F <1.5%. Apart from those elements affected by leucite alteration, both the elemental and isotopic composition of this lamproite are close to those of the Leucite Hills, Wyoming. Its Nd-isotopic model age (TDM = 1.6 Ga) is outside the Leucite Hills range but within that of other Tertiary strongly potassic magmatism in the region underlain by the Wyoming craton. Evidence from both teleseismic tomography and the mantle xenoliths within other western USA mafic ultrapotassic igneous suites shows that the total lithospheric thickness beneath NW Colorado was probably ∼150–200 km at 33 Ma, when the Middle Park lamproite was emplaced. This is an important constraint on tectonomagmatic models for the Cenozoic evolution of this northernmost part of the Rio Grande rift system.

The crystal structure of strontian potassicrichterite (Mg4.871Fe0.1263+ Mn0.052)Σ5.049(Na1.425Ca0.348Sr0.297)σ2.07K0.873Si8.048O22(OH)2.15 – an unusual amphibole containing up to 3.7 wt.% SrO from the Murun alkaline massif, Eastern Siberia has been refined from X-ray powder diffraction data in monoclinic space group C2/m, with a = 10.0325(1) Å, b = 17.977(1) Å, c = 5.2698(1) Å, and β = 104.70(1)°. According to Mössbauer spectroscopy, nearly all iron occurs as Fe3+ in octahedral coordination. Rietveld refinement shows the Fe3+ to be ordered onto the M(2) site. Sr is completely ordered onto M(4).