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‘Irite’ (Hermann, 1836/1841) from the Urals

Geochemistry-Mineralogy, Université Libre de Bruxelles, CP 160/02, 50, Avenue F.D. Roosevelt, B-1050 Brussels, Belgium

View larger version (90K): [in a new window]   FIG. 1. Isolated Ir and Os lamellae MNHN* [*See explanations of abbreviations on p. 394]. Hand mt. (a) General view of isolated iridium and osmium lamellae, threads and blades. SEM(SM). 1400 µm. (b) Dendrite-like parallel stems of Os-Ru iridium. SEM(SM). 450 µm. (c) Stem of Os-Pt-iridium with split-off lamellae. SEM(SM). 450 µm. (d) Hollow Ir-Os foils with crystalline outlines. SEM(SM). 1200 µm. (e) Leaf and blade of Ir-osmium. SEM(SM). 600 µm. (f) Group of partly split-off, Ir-Ru-Os blades. The dirt particles on the surfaces are mixed Ir-, Os- and Pt-chlorocompounds. SEM(BS). 160 µm.

View larger version (96K): [in a new window]   FIG. 2. Grouped Ir and Os lamellae. (a). ENSM-6774. Hollow skeleton of Os-Ru-iridium lamellae. Pol. sect. SEM(BS). 325 µm. (b) ENSM-6774. Skeleton of iridium and osmium lamellae partly filled with ‘irite’. Pol. sect. SEM(BS). 300 µm. (c) MNHN. Group of loose iridium lamellae filled with lumps of micrograined ‘irite’. Pol. sect. SEM(SM). 140 µm. (d) ENSM-6774. Dense agglomerate of Ir-osmium lamellae, partly corrugated and bent. Pol. sect. SEM(BS). 260 µm. (e) MNHN. Disordered agglomerate of iridium and osmium lamellae (white straight lines) and of laurite (grey irregular masses). Partly pol. sect. SEM(SM). 140 µm. (f) MNHN. Compact grain composed of Ir-Ru-osmium (large diagonal and fine intercrossed lines), laurite (grey irregular masses) in a matrix of unresolved Ru-Os-Ir-Fe-oxide(s). Pol. sect. SEM(BS). 160 µm.

View larger version (146K): [in a new window]   FIG. 3. Altered symplectic (eutectoid) grains ENSM. (a) Os-Pt-Ru iridium grain with eutectic texture. Metal residues (white) display parallel, curved and crooked lamellar shapes (Table 1, col. 1). Pol. sect. SEM(BS). 260 µm. (b) Border of the same grain, showing the residual metal (white), rimmed by incipient oxide (smooth grey), embedded in an oxidic matrix (fibrous grey), and a border of well-developed Ir-oxide crystals (light grey). The nearly complete disappearance of the metal in the border layer is notable. SEM(BS). 65 µm. (c) Border of the particle, showing the local development of larger fibrous/tabular Ir-oxide crystals. (Table 1, col. 2). SEM(BS). 10 µm. (d) Another field of the same border under RLM, showing the large difference in reflectance of the crystals and the metal. 12 µm.

View larger version (112K): [in a new window]   FIG. 4. Altered primary minerals. (a) ENSM-6774. Fibrous slab of alteration Ir-Ru-Pt-Os oxide (dark grey, bronze colour under reflected light; Table 1, col. 4) still containing residual rutheniridosmine (white; Table 1, col. 3). Pol. sect. SEM(BS). 160 µm. (b) NHML. Particle of Ir-osmium (inner white residue) surrounded by rhythmic layers of alteration Ir-Os-Ru-Fe-Ti-Cu oxides (various shades of grey). Pol. sect. SEM(BS). 125 µm. (c) ENSM-6774. Altered Ir-Ru-Pt osmium (white; Table 1, col. 5) with thick crust of alteration Ru-Os-Pt-Ir oxide(s) (spotted grey). Pol. sect. SEM(BS).160 µm. (d) ENSM-6774. Enlargement of Fig. 4c showing fibrous microcrystals of Ir-Fe oxide, composing the crust and resulting from the alteration of the metal (Table 1, col. 6). SEM(BS). 10 µm. (e) ENSM-6774. Fragment of a large particle composed of laurite (light grey), rutheniridosmine (white, segmented) and Ru-Ir-Pt-Fe oxide (dark grey alterations of laurite). SEM(BS). 16 µm. (f) ENSM-6774. Particle of Os-Pt-Ru-iridium (white) containing hongshiite inclusions (grey), surrounded by a thick alteration crust of Ru-Os-Pt-Ir oxide(s) (spotted grey). Pol. sect. SEM(BS). 160 µm.

View larger version (125K): [in a new window]   FIG. 5. Peripheral crusts and alterations of ilmenite and chromite. (a) ENSM-6774. Altered chromite (dark grey with convoluted cracks) bearing a fine layer of ‘irite’. Pol. sect. SEM(BS). 260 µm. (b) MNHN. Altered ilmenite (various shades of dark grey) bearing a thick layer of ‘irite’ (spotted white). Pol. sect. SEM(BS). 140 µm. (c) ENSM. Chromite (dark grey with orthogonal cracks; Table 2, col. 1) rimmed with a lighter border. Pol. sect. SEM(BS). 160 µm. (d) ENSM. Enlargement of Fig. 5c, showing the lighter border as a diffuse reaction zone, enriched in unresolvable Fe-Ir-Os-Pt-Ru-Cl-K which are absent from the inner chromite (Table 2, col. 2), and in fine metallic inclusions. Pol. sect. SEM(VS). 50 µm.

View larger version (148K): [in a new window]   FIG. 6. Agglomerates of ‘irite’. (a). NHMV. ‘Irite’ agglomerate composed of chromite and silica fragments (various shades of grey), iridium and osmium grains, and lamellae bound together by ‘irite’. The smaller particles at the right of the main body were detached during mounting. Hand mt. SEM(SM). 550 µm. (b) NHMV. Fluffy ‘irite’ agglomerate composed of chromite, silica, Ir-Fe oxides and metal grains and lamellae. Pol. sect. SEM(BS). 700 µm. (c) ENSM-6774. Compact particle composed of ‘irite’ (large light grey spot to the right, and unresolved web), chromite and silica fragments (grey particles) and metals (white spots). Pol. sect. SEM(SM). 150 µm. (d) ENSM-6774. Blowup of the same particle, showing the mixed character of the ‘irite’ field (white metallic grains, dark grey silica and light grey ‘irite’. This ‘irite’ displays a dark bronze colour under reflected light) 1300 µm.

View larger version (105K): [in a new window]   FIG. 7. Pt-Ir-Cl-K-containing silica. (a) MNHN. Al-Si-Fe-Mg-Ca-O fragment (light grey) encrusted with zoned silica layers (dark grey; Table 4, col. 1). Pol. sect. SEM(BS). 160 µm. (b) MNHM Enlargement of part of Fig. 7a. Zones of lighter grey shades correspond to increased concentrations of Pt, Ir and Fe (Table 4, col. 2). SEM(BS). 25 µm. (c) MNHM. Silica grain (dark grey; Table 4, col. 3) showing lighter central spots (Table 4, col. 4), corresponding to increased concentrations of Pt. Light grey inclusions: chromite. Irite is concentrated along the right border (white). Pol. sect. SEM(BS). 140 µm. (d) ENSM. Al-Fe- and Mg-Ca-silicate particles (smooth grey) cemented by fluidal Pt-Fe-Cl-K-containing silica (dark grey; Table 3, col. 5). Pol. sect. SEM(BS). 260 µm. (e) Enlargement of the right hand side of Fig. 6b. Pt-Ir-containing silica. Pol. sect. SEM(BS). 50 µm. (f) Enlargement of a portion of Fig. 7e showing concentric zones of Pt-Ir-enriched silica. SEM(BS). 12 µm. (Table 3, cols 6 and 7).

View larger version (145K): [in a new window]   FIG. 8. Pt-Ir-(K)-chlorocompounds. (a) NHMV. Fe-hydroxide particle showing blistered parts to the right, and a metallic dusting (white). Hand mt. SEM(BS). 400 µm. (b) NHMV. Enlargement of Fig. 8a, showing a particle of Pt-K-Cl (white) and an incrusted lamella of iridium (left). Hand mt. SEM(BS). 35 µm. (c) NHMV. Surface of an altered chromite (dark grey), dusted with various chlorocompounds. SEM(BS). 130 µm. (d) NHMV. Enlargement of Fig. 8c. showing a concentration of fibrous Ir-chlorocompounds (white, elongate), and one crystal of Ca carbonate (upper bipyramidal prism). SEM(BS). 12 µm. (e) NHMV. Chromite crystal. Hand mnt. SEM(SM). 650 µm. (f) NHMV. Enlargement of Fig. 8e, showing a triangular hollow in the chromite with 3–4 crystals of K-chloroplatinate (light grey), and a heavy dusting of the surface by various Ru-Os-Ir-Pt-K-containing particles (white). SEM(BS). 50 µm. (g) NHMV. Enlargement of Fig. 8f, showing two Pt-K-Cl crystals with bipyramidal habit. SEM(BS). 10 µm.