The weathering of a trachytic pumice within a pyroclastic flow underlying an andic-brown soil on the volcano Vico has been studied. The main mineral formed is a spherical 10 Å halloysite which has been shown by SEM and in situ microprobe analysis to have formed directly from the glass. The major mineralogical characteristics as determined by XRD, IR, DTA, TEM and microdiffraction are typical of 10 Å halloysite. However, some minor mineralogical properties and the high Fe and K contents, suggest that it is an interstratification of 74% halloysite and 26% illite-smectite. The calculated formula of the hypothetical 2:1 minerals reveals an Fe- and K-rich clay, with high tetrahedral substitution, like an Fe-rich vermiculite, but the detailed structure of this mineral remains uncertain.

L'argile blanche en provenance du sol ferrallitique dérivé de matériaux pyroclastiques de Vaté (Vanuatu) possède un rapport moléculaire SiO2/(Al2O3 + Fe2O3) proche de 2, est riche en fer (∼7% Fe2O3) et se présente sous forme de fines lamelles irrégulières et parfois froissées. Sous l'effet de tout traitement de déshydratation, l'espacement basal de cette argile passe progressivement de 10 Å vers 7 Å. Toutes ces caractéristiques permettraient son identification comme halloysite (10 Å) ferrifère hydratée. Cependant, les spectres de diffraction des rayons X de cette argile déshydratée puis déshydroxylée ne correspondent pas à ceux que l'on obtient pour une halloysite ‘normale’. Ils révèlent plutôt la présence d'un édifice interstratifié 1:1/2:1 où le minéral 2:1, présent à raison de 20% environ, est dioctaédrique. En outre, la CEC de ce matériau (∼44 mEq/100 g à pH 4) est anormalement élevée pour un minéral 1:1. Par spectroscopie Mössbauer, on établit que la quasi-totalité du fer présent dans cet échantillon doit être localisée en position octaédrique au sein d'un phyllosilicate. Comme ce fer est ferrique, son inclusion dans la couche octaédrique d'un minéral 1:1 ne pourrait justifier l'importante charge négative permanente que porte cette argile. En réalité, toutes les propriétés reconnues pour ce matériau justifient son identification comme minéral interstratififé de 80% environ d'halloysite et de 20% de smectite ferrifère. Certaines particularités des spectres d'infrarouge, notamment l'absence des bandes à 3560 et 820 cm−1 caractéristiques des vibrations d'élongation et de déformation des hydroxyles de la nontronite, suggèrent que la smectite ferrifère inclue dans l'édifice interstratifié pourrait être une hisingérite dioctaédrique.

Optimal antiretroviral strategies for HIV-infected patients still need to be established. To this end a decision tree including different antiretroviral strategies that could be adopted for HIV-infected patients was built. A 10-year follow-up was simulated by using transitional probabilities estimated from a large cohort using a time-homogeneous Markov model. The desired outcome was for patients to maintain a CD4 cell count of >500 cells/mm3 without experiencing AIDS or death. For patients with a baseline HIV viral load ⩾5 log10 copies/ml, boosted protease inhibitor-based immediate highly active antiretroviral therapy (HAART) allowed them to spend 12% more time with CD4 ⩾500/mm3 than did delayed HAART (6·40 vs. 5·69 and 5·57 vs. 4·90 years for baseline CD4 ⩾500 and 350–499/mm3, respectively). In patients with a baseline HIV viral load ⩽3·5 log10 copies/ml, delayed HAART performed better than immediate HAART (6·43 vs. 6·26 and 5·95 vs. 5·18 for baseline CD4 ⩾500 and 350–499/mm3, respectively). Immediate HAART is beneficial in patients with a baseline HIV viral load ⩾5 log10 copies/ml, whereas deferred HAART appears to be the best option for patients with CD4 ⩾350/mm3 and baseline HIV viral load <3·5 log10 copies/ml.

The aim was to investigate the impact of the main prognostic factors on HIV evolution. A multi-state Markov model was applied in a cohort of 2126 patients to estimate impact of these factors on patients' clinical and immunological evolutions. Clinical progression and immunological deterioration shared most of their prognostic factors: male gender, intravenous drug use, weight loss, low haemoglobin level (<110 g/l), CD8 cell count (<500/mm3) and HIV viral load (>5 log10 copies/ml). Highly active retroviral therapy reduced the risks of clinical progression and immune deterioration whatever patients' CD4 cell count. Risk reductions were 41–60% for protease inhibitor-based and 27–68% for non-nucleoside reverse transcriptase inhibitor-based regimens. Three-year transition probabilities showed that only patients with a CD4 cell count ⩾350 CD4/mm3 could in most cases maintain their immunity. This model provides ‘real life’ transition probabilities from one immunological stage to another, allowing decision analyses that could help determine the beneficial therapeutic strategies for HIV-infected patients.