230Pyrite and calcite

face is characterized by the development of striations since layer growth does not occur on the face. A {210} face showing striations only is a crystal face of this kind. In such a crystal, the larger the normal growth rate, or the smaller the step separation, the more {hk0} or {210} faces develop, and the larger the degree of change from cubic to pentagonal dodecahedral Tracht as growth proceeds. A larger driving force condition corresponds to this situation. The observed difference in Tracht variation depending on grain size between the central portion and outer region of a deposit appears to reflect this difference. When pyrite grows under favorable conditions, such as high sulfur fugacity, pentagonal dodecahedral Tracht appears, whereas under less favorable conditions pyrite crystals maintain a cubic form. It is probably for this reason that pyrite crystals occurring further away from an ore deposit do not show distinct changes depending on grain size, and that the Trachts are different between ore deposit and the country rock, crystals formed in country rock taking exclusively simple cubic form.

Although crystallographically equivalent, the {210} faces of pyrite from a particular locality are characterized by striations perpendicular to the edge with {100}. These striations are different from the striations described above and appear due to narrow growth layers on the {210} face, indicating that, under this condition, {210} behaves as an F face. It is still to be discovered why {210} faces behave differently depending on the growth conditions.

As compared to the relation between {210} and {100}, the appearance of {111} is slightly different. It is known that octahedral crystals contain more arsenic (As) than cubic crystals in the same locality, and there is a possibility that {111} faces develop due to an impurity adsorption of As. It is observed that the internal texture of this type of crystal shows a drastic change from cubic to octahedral in association with precipitation of minerals containing As.

11.1.4 Polycrystalline aggregates

In sedimentary rocks, pyrite often occurs as nodular, spherical, irregular, massive, framboidal polycrystalline aggregates of minute crystals. It often occurs in fossils, without modifying the external fossil form, and a bacterial origin has been suggested for pyrite aggregates of this type. Among such polycrystalline aggregates, framboidal pyrites have attracted particular interest. They are spherical, they measure a few tens of micrometers across, and one framboid consists of close-packed, equal-sized idiomorphic pyrite crystals with the same Tracht. Cubic, octahedral, pentagonal dodecahedral Trachts, etc. are known. One framboid consists of a unique Tracht.

Framboidal pyrite occurs, for example, in sedimentary rocks, muddy sediments, and precipitates in hot springs; two controversial origins have been suggested, one bacterial and the other relating to agitation in hydrothermal solution. Framboidal