Magnetite twinning

Magnetite Octahedra Intergrown octahedra (twins), rhombic dodecahedra, cubes, spheres, bullets... 

Natural and synthetic magnetite occurs most commonly as octahedral crystals bounded by 111 planes and as rhombo-dodecahedra (Fig. 4.24). Twinning occurs on the 111 plane (Tab. 4.1). STM examination of the (100) face of a natural sample... 

Fig. 4.24 Crystal forms of magnetite, a, c) octahedron b) rhom-bodecahedron d) twin. (Kostov, 1968 with permission).

Twinning in crystals results front the intergrowth of two or more individuals in such a way as to y ield parallelism in the ease of certain parts of the different individuals und. at the same time, other parts of the different individuals are in reverse positions in respect to each other. For example, an octahedral crystal of magnetite is twinned when one-half of the crystal is rotated ISO parallel to an octahedral facial plane. This type of twinning is known as spinel rn inirini . owing to its common occurrence in the spinel group of minerals. 

Fic. 7. Morphological forms of bacterial magnetite observed in projection, (a) Hexagonal (cubo-octahedral) (b) rectangular [note the twinned ciystal (arrow)] (c) cubic and (d) bullet shaped. In each case the crystals are oriented with the (111) faces perpendicular to the chain axis. Bars = 50 nm in all micrographs. 

The Funiushan copper deposit is a skam deposit on the inverted northern limb of the western Tang-Lun anticline. It occurs within the contact zone of a quartz diorite and comprises chalcopyrite, subordinate pyrite and sphalerite, as well as minor amounts of magnetite and martite. The ore body lies beneath sedimentary formations and 20-30 m of eluvium and alluvium. Thermally-released Hg was determined along three soil traverses. As the traverse illustrated in Fig. 13-4 shows, the Hg, pattern picks out the mineralisation better than the Hg, pattern. The shape of the Hg, anomaly is characterized by twin peaks over the margins of the ore body. A large thrust fault seems to have a role in anomaly development. 

Comprising mainly magnetite, sphalerite, galena and a little chalcopyrite, the Xiaomaoshan polymetallic skam deposit occurs at the contact of porphyritic granite and limestone, at a depth of 110 m. The surface of the area is covered by exotic overburden, on which rice is cultivated. Soil samples taken at 20 m intervals along a traverse yielded twin peaks of Hg, above the hangingwall and footwall contacts of the mineralisation (Fig. 13-7). 

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