11.2 Calcite 231
pyrite may also be synthesized by hydrothermal experiments. The genesis of framboidal pyrite remains a stimulating problem. It remains for us to resolve the problem of why the close-packed aggregation of equal-sized polyhedral crystals exists with the same Tracht.
11.2Calcite
11.2.1Habitus
The main polymorphs of CaCO3 are calcite (trigonal system, 3m (lowpressure phase)), and aragonite (orthorhombic system, mmm), but vaterite (hexagonal system, 6/mmm) is also known. In biological activity, both aragonite and calcite are formed as major minerals constituting shells and exo-skeletons under 1 atmospheric pressure, but, after some time of precipitation and deposition, aragonite transforms into calcite, the stable phase that exists below 5000 Pa. As a result, limestone, which is a sedimentary rock of biological origin, consists only of calcite. When a granitic magma intrudes into limestone, grain growth occurs and marble is formed by a contact metamorphic (metasomatic) process. Since there is free space in druses or veins formed during this process, idiomorphic crystals of calcite are formed. Idiomorphic crystals of calcite also occur in a wide temperature range, from hydrothermal veins to limestone eroded by underground water. They also occur in druses of volcanic rocks. The Habitus and Tracht show the greatest variations among mineral crystals. The Habitus may be thin platy, thick platy, short-prismatic, long-prismatic, rhombohedral, dog-tooth (scalenohedral), nail-head, etc. Many crystal faces are reported, but the only one corresponding to an F face is {1011}, which is a cleavage face. If {1011} is the only F face, the representative Habitus of calcite should be rhombohedral, but real crystals show astonishingly variable
Habitus. On the other hand, when calcite crystals are synthesized from pure hydrothermal solution, only rhombohedral Habitus is obtainable, whereas when polymer is added to the solution as an impurity component, dog-tooth Habitus is obtained. In natural calcite crystals, those formed under relatively high-tempera- ture conditions, such as those occurring in contact metasomatic deposits, exhibit platy or nail-head Habitus, and, on decreasing the temperature, prismatic or dogtooth Habitus occur. That Habitus changes with decreasing temperature has been shown to be a general tendency.
Figure 11.4 shows the various Habitus of calcite crystals observed in nature. To demonstrate Habitus changes at different orders of crystallization, or those depending on crystallization temperatures, three examples of intergrowths of two different Habitus of calcite are shown in Fig. 11.5 [3], see also Fig. 7.10. Figure 11.5(a) shows that later-grown calcite crystals on earlier-formed prismatic crystals of calcite show a different prismatic to a dog-tooth Habitus from the host crystal.
232 Pyrite and calcite
Figure 11.4. Various Habitus observed for natural calcite crystals: C {0001}, M {1010},
¯
R {1011}, –R {0kkl}, S {hkı¯l}, and A{1120} faces [3].
Figure 11.5(b) shows that the later-formed calcite crystal has a different Habitus from the earlier-formed prismatic crystal around which it grows, and Fig. 11.5(c) is an example of the growth of a crystal with dog-tooth Habitus on an earlier-formed crystal with rhombohedral Habitus. (See also Fig. 7.10.) By compiling these relations, it is possible to trace systematically how the Habitus of calcite changes from earlier to later stages or as the temperature decreases in the case of contact metasomatism [2], [3]. The Habitus variation with decreasing temperature summarized above is a general trend based on data of this type.
11.2.2Surface microtopography
Although calcite crystals exhibit a great variety of Habitus change and many crystal faces are known, the crystal faces may be grouped as follows.
11.2 Calcite 233
Figure 11.5. Three examples showing different Habitus of later-grown calcite crystals preferentially nucleated on the edges and corners of earlier-formed calcite crystal. Changes in Habitus depending on crystallization stages or growth temperatures are indicated [3]. (a) Earlier-formed hexagonal prism (A) and later-formed scalenohedral crystal (B). (b) Earlier-formed hexagonal prism (b) and later-formed thicker crystal (a). (c) The shaded area shows an earlier-formed rhombohedral crystal, and the remaining area represents later-formed scalenohedral crystals.
(1){1011}. This is the unique F face from PBC analysis, and the face appearing through the spiral growth mechanism. Spiral growth hillocks of rhombic form are observed, and the step advancing rates have anisotropy following the symmetry element of the face. Since element partitioning will alter according to the growth step advancing rate, which affects optical properties, intra-sectorial sectors are formed (see Sections 3.14 and 6.2) [4], [5]. These were confirmed by atomic force microscopy (AFM) and X-ray fluorescence (XRF).
(2){0kkl}. These are minus rhombohedral faces (–R), and in most cases they appear as {0112} faces with nail-head Habitus. Growth step patterns are not observed on these faces, which are characterized by the development of striations parallel to the edges with {1011}.
(3){hkıl}. This is a group of habit-controlling faces of dog-tooth Habitus, which commonly occur at low-temperature conditions. Many high-index faces, such as {2131} and {65111}, are reported to grow as large as
Habitus-controlling faces, but they are exclusively characterized by striations; step patterns due to layer growth are not observed.
(4){1010}, {1120}. Among these prism faces, {1120} appears as an extreme of dog-tooth Habitus, and is characterized similarly to {hkıl} by striations parallel to the edge with {1011}. No reliable observations have so far been
234 Pyrite and calcite
Figure 11.6. Characteristics of surface microtopography of crystal faces of calcite.
reported on the surface microtopography of {1010}, but step patterns are not reported.
(5){0001}. This shows a rugged surface. No reports are available so far on step patterns on this face.
The characteristics of the surface microtopography of these faces are schematically illustrated in Fig. 11.6.
From the observations of the surface microtopographs of crystal faces of calcite, it is confirmed that the only face which behaves as a smooth interface under any conditions and grows by the spiral growth mechanism is {1011}. The –R and {1010} faces determining a nail-head or prismatic Habitus and the {hkı¯l} faces determining a dog-tooth Habitus are all characterized by striations parallel to the edge with {1011}, in spite of the fact that they develop as large as Habitus-controlling faces. These may be regarded to have appeared through the piling up of the edges of growth layers on the {1011} face. Unless we assume that these faces become large in order to determine the Habitus, in spite of their nature as S faces, by stabilizing vicinal faces appearing due to the piling up of steps of growth layers on {1011} faces, it is not possible to understand their appearance and development. It is possible that the vicinal faces are stabilized owing to the impurity adsorption along the steps forming these striations. It is necessary that the faces can adsorb the impurities, and that the temperature is set appropriately, so that adsorbed impurities can play such a role. The observations that calcite crystals formed under lowtemperature conditions tend to take on a dog-tooth Habitus, and that dog-tooth
Habitus can be produced by hydrothermal synthesis only when polymers are added