Natrocarbonatite lavas erupted from hornitos T37B and T49B at Oldoinyo Lengai (Tanzania) during 23–30 July, 2000 are unusual in containing sylvite and fluorite microcrysts together with fluorite-nyerereite intergrowths. The latter are relatively coarse grained and exhibit granular textures indicative of slow crystallization rates relative to those of their host subaerial lavas. Fluorite microcrysts are considered to be derived by the fragmentation of the fluorite-nyerereite clasts. Sylvite microcrysts contain inclusions of ferroan alabandite [(Mn0.67-0.71Fe0.33-0.29)S] and are poor in Na (1.9–7.7 wt.% Na; 6.1–23.4 mol.% NaCl). Intergrowth and microcrystal fluorite contains 1–3.5 wt.% Sr. Intergrowth nyerereite has a composition similar to that occurring as bona fide phenocrysts. The groundmass of lava erupted from hornito T37B contains nyerereite microphenocrysts (4–8 wt.% BaO) that are epitaxially mantled by barian nyerereite (12–20 wt.% BaO). The latter are compositionally and texturally distinct from the groundmass phase X, which is considered to be a burbankite-group mineral. The fluorite-nyerereite clasts are considered to be derived from the magma chamber underlying hornitos T37B and T49B, and thus representative of some of the products of crystallization of natrocarbonatite magma under hypabyssal conditions. The origins of the sylvite microcrysts cannot, as yet, be determined.

Mavlyanovite, ideally Mn5Si3, is a new mineral from a lamproite diatreme close to the upper reaches of the Koshmansay river, Chatkal ridge, Uzbekistan. It occurs together with unnamed manganese siliciphosphide and manganese silicicarbide minerals in round to ovoid segregations, up to 10 cm in diameter, in volcanic glass. Segregations of hexagonal prismatic mavlyanovite up to 1–2 mm occur in interstices in the matrix and tiny inclusions (1–2 μm) of alabandite and khamrabaevite occur within mavlyanovite. It is opaque with a metallic lustre, has a dark-grey streak, is brittle with a conchoidal fracture and a near-perfect basal cleavage. VHN100 is 1029–1098 kg/mm2 (Mohs hardness ~7). In plane-polarized reflected light, mavlyanovite is a pale-brownish-grey against the accompanying unnamed manganese silicicarbide (white). Reflectance values and colour data are tabulated. Average results of 19 electronmicroprobe analyses give Mn70.84, Fe 6.12, Si 22.57, Ti 0.15, P 0.18, total 99.86 wt.% leading to an empirical formula of (Mn4.66Fe0.40)5.06(Si2.91Ti0.01P0.02)2.94 based on8 a.p.f.u. The calculated density is 6.06 g/cm3, (on the basis of the empirical formula and unit-cell parameters from the structure determination). Mavlyanovite is hexagonal (P63/mcm) with a 6.8971(7), c 4.8075(4) Å, V 198.05(3) Å3 and Z = 2. The structure has been determined and refined to R1 = 0.017, wR2 = 0.044, GoF = 1.16. Mavlyanovite is the naturally-occurring analogue of synthetic Mn5Si3 which is the parent aristotype structure of the Nowotny intermetallic phases studied extensively by the material-science community. It is also the Mn-dominant analogue of xifengite Fe5Si3. The mineral name honours Academician Gani Arifkhanovich Mavlyanov (1910–1988), for his contributions to the understanding of the geology of Uzbekistan.

The Miocene, epithermal, Ag-rich polymetallic Santo Toribio deposit is hosted by the volcanics of the Quiruvilva-Pierina subbelt, Northern Peru, which also comprises the world-class, high sulphidation Pierina deposit. The Ag-rich, alabandite-bearing veins of the Santo Toribio deposit formed during two major stages. The early stage is characterized by deposition of arsenopyrite, pyrite, quartz, Mn- sphalerite, stannite, alabandite and minor miargyrite. Sphalerite associated with this stage is exceptionally enriched in Mn (up to 14.5 wt.%) and alabandite is optically and mineralogically zoned. Its brown zones have greater Fe+Sb and smaller Mn contents than the green zones and Fe+Sb replaces Mn in its structure. During this early stage, fs2 must have been high to allow the stabilization of alabandite relative to rhodochrosite. In the second stage, the physicochemical conditions changed and the CO2/S ratio increased, causing dissolution of alabandite and the deposition of abundant rhodochrosite and a second generation of arsenopyrite, pyrite and quartz, sphalerite with chalcopyrite inclusions, mirargyrite, ramdohrite, and finally stibnite. This polymetallic ore probably formed due to an abrupt decrease in H+ and/or Cl- concentration caused by boiling or dilution of the high-salinity hydrothermal fluids and constitutes an example of “intermediate sulfidation-state” epithermal deposits.