11
Pyrite and calcite
Pyrite and calcite are mineral crystals that represent a wide Habitus and
Tracht variation. Pyrite is the most persistent mineral among sulfide minerals, occurring in a wide range of modes, including inorganic processes and bacterial action, and it can also be synthesized by hydrothermal or chemical vapor transport methods.
Pyrite crystals exhibit a wide range of Tracht and Habitus, and also occur in unusual forms of polycrystalline aggregate, such as framboidal pyrite. Although numerous crystal faces have been reported, the most important ones are {100}, {111}, and {210}. Calcite also exhibits a variety of Tracht and Habitus, such as platy, nail-head, prismatic, or dog-tooth forms, but {1011} is the only F face. In this chapter, we focus our attention on the factors controlling the observed variations in Tracht and Habitus of pyrite and calcite.
11.1Pyrite
11.1.1Tracht and Habitus
Pyrite is the most common mineral among sulfides. It occurs not only as a major mineral of sulfide ore deposits of base metals, such as Cu, Pb, Zn, in veintype, massive-replacement type, kuroko-type* deposits, etc., but also sporadically as an accessory mineral in volcanic, sedimentary, and metamorphic rocks. It also occurs as a precipitate in hot springs, and it may be formed by bacterial action. Pyrite itself is not an ore of Fe, though it contains iron, and at best may have economic value as an ore to obtain sulfuric acid. However, due to its occurrence in and
*Kuroko deposits are massive-type ore deposits formed by the deposition of various sulfide minerals around submarine fumaroles. They are major ore deposits of Cu, Pb, and Zn in Japan.
226 Pyrite and calcite
Figure 11.1. Crystal structure of pyrite.
around economically viable ore deposits, many investigations on its Tracht and
Habitus were made, particularly in the former Soviet regions, in the hope that the results would assist the ore prospecting process. Single crystal synthesis has been carried out by CVT and hydrothermal methods, to utilize the semiconductor properties of pyrite, such as thermoelectromotive force.
Reflecting its wide mode of occurrence, pyrite is observed to be extremely rich in variations of Tracht and Habitus, and more than 460 crystal faces have been reported. In addition to pyrite’s equi-dimensional polyhedral Habitus, such as cubic, octahedral, or pentagonal dodecahedral, Habitus such as remarkably flattened, malformed or kinked whiskers are reported. Crystal sizes range from sub-micrometer order to more than 10 cm along an edge. Polycrystalline aggregate, in the forms of framboidal pyrite (see Section 8.4), nodules, spherulitic, and irregular and granular aggregates have been reported. Although so many crystal faces and a wide variety of Habitus and Tracht are known, the three major crystal faces determining the morphology of pyrite crystals are {100}, {111}, and {210}, and the
Tracht variation seen in Fig. 2.5 appears as combinations of these three faces. Although many crystal faces , such as {hhl}, {hkl}, and {hk0}, are known, {hhl} and {hkl} appear due to the growth of {111} faces, and {hk0} appear due to that of {100} and {210}. The reason why {210} appears as the major crystal face among {hk0} faces is due to the presence of a glide plane in the symmetry elements involved (see Section 4.2).
In the crystal structure of pyrite (Fig. 11.1), S2 molecules with dumbbell form and
11.1 Pyrite 227
Figure 11.2. Characteristics of surface microtopographs of three major faces: {100}
((a), (b), (d), (e)); {111} ((d), (e)); {210} ((b), (c), (f)).
Fe are arranged alternately. Since the dumbbells are aslant in opposite orientations, pyrite belongs to the hemihedral crystal group m3, and the space group is Pa3. The reticular density of {110} in the Bravais–Friedel law should be calculated on the basis of {220} in the Donnay–Harker law, and the order of morphological importance becomes {210} {110}. According to PBC analysis on Hartman–Perdok theory, {100}, {111}, and {210} are F faces.
11.1.2Characteristics of surface microtopographs
The surface microtopographs of the three major faces of natural pyrite crystals have the following characteristics (see Fig. 11.2 for a schematic representation).
228Pyrite and calcite
(1){100} faces are characterized by step patterns with elongated rectangular form in one axis, or elongated rectangular form with truncated corners parallel to the edge with {111}. Such step patterns are universally observed on {100} faces of pyrite crystals formed under any conditions, indicating that this face always grows either by two-dimensional nucleation or the spiral growth mechanism. The direction of elongation of the rectangular form follows the symmetry of point group m3, and is thus perpendicularly oriented on the neighboring {100} faces.
(2)Triangular step patterns are observed on {111} faces. {111} faces also grow either by two-dimensional nucleation or the spiral growth mechanism. However, triangular step patterns are oppositely oriented and inclined to the triangle of the {111} face. It should be noted that the orientation and inclination of triangular growth layers on the {111} face of pyrite differ from those of regular triangular growth hillocks observed on the {111} faces of diamond, which belongs to the holohedral crystal group. For diamond, the step patterns on the {111} face are triangular with the same orientation as the triangle of the face.
(3)Two entirely different surface microtopographs are observed on {210} faces depending on the locality where the crystal grew [1]. The most commonly observed has striations parallel to the edge with the neighboring {100} faces, showing no step patterns. This characteristic corresponds to that of an S face in PBC analysis. The {hk0} faces of crystals showing characteristics of this type are usually associated with other {hk0} faces, such as {430}, {410}, and {310} in addition to {210}. On the other hand, in the {210} faces of pyrite crystals from a specific locality, such as Elba, the observed striations are perpendicular to the edge with the neighboring {100} face, and the striations represent the edges of step patterns of narrow, elongated growth layers on the {210} face. In this case, {210} faces are grown as F faces. Similar situations have been observed on the {1010} face of quartz (see Chapter 10), and in both cases the same crystal face behaves either as an F face or an S face, depending on the growth conditions.
From the surface |
microtopographic characteristics summarized above, it |
is concluded that |
the order of morphological importance of pyrite is |
{100} {111} {210}. This order is in agreement with the results of PBC analysis.
11.1.3Growth conditions and Tracht
If a statistical analysis is performed on the appearance of different Tracht dependent on grain size of a few hundred pyrite crystallites occurring in a handful of clay in a kuroko ore deposit, it is universally observed that, as crystal sizes
11.1 Pyrite 229
Figure 11.3. Difference in Tracht changes of pyrite crystals from the Kanbe Mine depending on grain size between ore deposits (
), weakly altered zone (
), and in country rock (
) (see ref. [2], Chapter 2).
decrease, crystals mostly exhibit cubic Tracht bounded by {100}, and, as the grain sizes increase, {210} faces start to appear, and the frequency of appearance of pentagonal dodecahedral Tracht bounded by {210} faces increases. It was explained that this variation is due to Tracht variation associated with growth (see Section 4.4). The degree of change from cubic to pentagonal dodecahedral or to octahedral within the same grain size range varies from the center of an ore deposit to the outer region.
The change is most remarkable at the center of an ore deposit; as the region outside the deposit is approached, practically no Tracht change is observable. In other words, Tracht variation associated with growth is most remarkable in the region with a high grade of sulfide mineralization, which includes pyrite, where
Trachts exhibiting well developed {210} or {111} faces appear, whereas pyrite crystals formed in a weakly mineralized zone, for example outside an ore deposit, do not show distinct change and remain as cubic even if their sizes increase. In Fig. 11.3, the statistical results obtained within ore deposits and outside are compared.
A similar tendency is observed between vein and country rock of vein deposits; in country rocks, pyrite generally takes cubic Tracht. This tendency is recognized in many observations on typomorphism reported in the former Soviet countries [2].
If growth layers spreading on {100} faces either by two-dimensional nucleation or the spiral growth mechanism reach the edge of the face, the Tracht remains as simple cubic. If steps bunch together to form macro-steps as they spread, {hk0} faces consisting of striations will appear as vicinal faces through the piling up of macro-steps. Faces having higher stability than others will increase in size, but the