a Carbonates b Silicates c Phosphates d Oxides e Native elements

Polymer crystals whose Habitus is characteristically thin platy exclusively take spherulitic form under high driving force conditions. In crystals having this type of Habitus, crystals are often bent or twisted, and often sub-parallel growth results as growth proceeds, due to the strain induced into the growing crystal or introduced through the effect of precipitation of a foreign substance on the growing surface. As a result, instead of the formation of a perfect spherulite on the spherical substrate surface, various incomplete spherulites, such as those shown in Fig. 8.5(c), for example sheaf-like, bow-tie, and two-eye forms, appear.

Spherulite formation by geometrical selection may rarely be seen on crystals with isotropic Habitus. Native arsenic, As, occurs in a confeito-like form, and is a type of spherulite grown through the geometrical selection of rhombohedral crystals. Spherical aggregation of calcite crystals with nail-head Habitus is also observed. Semi-spherical aggregates of platy barite crystals known as desert rose are shown in Fig. 8.6.

8.4Framboidal polycrystalline aggregation

There is a type of polycrystalline aggregate of pyrite crystals showing a framboidal appearance, known as framboidal pyrite. It occurs in sedimentary

158 Forms and textures of polycrystalline aggregates

Figure 8.5. Modes of formation of spherulites [2], [3]. Spherulites formed (a) on a spherical particle of foreign material, (b) on a polycrystalline aggregate of the same species, and (c) by split growth.

rocks or as hot spring precipitates, and forms a framboidal aggregate of one micrometer to a few tens of micrometers order, consisting of close-packed idiomorphic crystals of less than 1 m. Figure 8.7 shows a scanning electron microscope (SEM) photograph of a framboidal pyrite. Characteristic features are that pyrite crystals of framboidal appearance are idiomorphic crystals of equal size and the same Habitus. Various structures, such as cubes, octahedra, and pentagonal dodecahedra are known. Judging from the fact that these crystals are arranged in a closepacked manner, we may assume that geometrical selection does not operate in their formation.

Two different opinions have been put forward to explain the mechanism of formation of minute idiomorphic crystalline aggregation with equal size and the same Habitus. The first considers the aggregation of minute crystallites due to the stirring of an inorganic hydrothermal solution, and the second assumes it is due to living organisms such as bacteria. Hydrothermal synthetic experiments have demonstrated that the polycrystalline aggregate of pyrite, with the same texture

8.5 Texture formation in multi-component systems 159

Figure 8.6. Semi-spherical aggregate of the platy barite crystals known as desert rose.

and structure as the natural crystal, could be reproduced, and it has been demonstrated that the formation of framboidal pyrite by an inorganic process, without the help of bacterial activity, is possible. However, the second argument is based on the observation of the modes of occurrence of this type of pyrite.

8.5Texture formation in multi-component systems

The mechanisms operating in the formation of textures seen in polycrystalline aggregates of the same species have been discussed in Sections 8.1–8.4. This may correspond to the analysis of a mechanism controlling the so-called selforganization or self-assemblage. Other mechanisms are possible; for example, tiny spherical particles are assembled and a close-packed structure is formed due to surface tension. The formation of opal consisting of a close-packed structure of minute amorphous silica spheres may be such a case.

In this section, we shall analyze the factors controlling the formation of the texture and structure of polycrystalline aggregates of multiple phases seen in

160 Forms and textures of polycrystalline aggregates

Figure 8.7. Framboidal pyrite; SEM image.

rocks, ores, or ceramics when crystal growth takes place under entirely uncontrolled conditions. Since the population density and morphology of constituent particles determine the texture and structure of this type of system, various factors affect their formation. The texture and structure of the resulting form are determined through an inter-relation of the static and dynamic factors, such as temperature, pressure, driving force, chemical composition of the system, crystallization differentiation, the initial state, and movement.

Magma, which is a high-temperature solution, solidifies to form igneous rock. If magma cools down slowly in the depths of the Earth, the nucleation density is low and the crystal can grow until it comes into contact with others, after which a plutonic rock with holocrystalline texture appears. If magma containing growing crystals that nucleated earlier solidifies at an intermediate depth, a rock with porphyritic texture, consisting of large idiomorphic crystals (phenocrysts) and groundmass consisting of minute idiomorphic crystals, will result.

8.5 Texture formation in multi-component systems 161

Depending on the solidification conditions, igneous rock with vitreous groundmass will be formed. If magma containing no phenocrysts solidifies rapidly, glassy igneous rocks are formed. When magma intrudes into cracks in a stratum forming a dyke, a quenched phase consisting of minute crystallites will appear because the surrounding stratum is quenched along the contact zone. Due to the uplifting movement of the magma, phenocrysts originally present in the magma are broken, and minute crystalline particles become detached; these act as secondary nucleation sites, resulting in the modification of the state of tiny crystallites in the groundmass. In laboratory experiments, in which cooling is performed in two stages (trying to grow phenocrysts in the first stage and tiny crystallites in groundmass in the second stage), it was impossible to reproduce the hyalopyritic texture commonly observed in natural basalt, which consists of phenocrysts and microcrystalline groundmass. The texture reproduced by the two-stage experiments consists of phenocrysts and the surrounding dendritic crystals, which is a result of the morphological variation due to the difference in supercooling. However, if the system is stirred, hyalopyritic textures may be successfully reproduced in the laboratory (see ref. [4], Chapter 3) because the nucleation density increases drastically by an order of magnitude as minute particles, detached from phenocrysts by stirring, act as secondary nucleation sites.

By consideration of these examples, we see that the knowledge accumulated in the industrial crystallization field is important and useful in understanding the crystallization process in magma. Since, in industrial crystallization, the nucleation, growth, and size distribution or morphological change of crystals proceed in a closed reaction vessel, it is necessary to evaluate the kinetics based on the final products. Also useful in the analysis of magmatic crystallization is the concept of crystal size distribution (CSD), which may be used to evaluate the nucleation rate and the growth rate from the resulting size distribution.

All apparatus used in industrial crystallization is designed to obtain crystal particles of uniform size, and so the CSD is chosen to facilitate this outcome. The evaluation of the peak in the CSD is made by using an average value of the CSD, or using the value of the dispersion coefficient using the central value (CV) and the standard deviation. When particle sizes are completely uniform, the CV equals zero, and in complete mixing CV 0.5. In complete mixing, the density function of the CSD, n(L) ( m 1 m 3), or the logarithm (m 4) and particle size L ( m) plot, is linear, and from its gradient the growth rate R ( m/s) and the stay period ( (s)) are obtained. The section of L–0 is uniquely determined by the ratio of the nucleation rate to the growth rate. The distribution formula is

n(L) (B/R) exp (–L/R ).

Examples are shown in Figs. 8.8(a) and (b), in which the above relations obtained in the industrial crystallization field are applied to crystals in igneous rocks formed

Figure 8.8. Crystal size distribution (CSD) plots of (a) plagioclase in igneous rock and

(b) garnet porphyroblast in contact metamorphic rock [4].

by the solidification of magma, and those in metamorphic rocks formed by contact metasomatism. Figure 8.8(a) shows an example of the measurement on plagioclase in lava, and Fig. 8.8(b) illustrates garnet in contact metamorphic rocks. The horizontal axis is particle size, and the vertical axis is particle density/cm3 [4]. From these results and geological data, the growth rate of plagioclase in the lava shown in Fig. 8.8(a) is estimated to be 10 10 10 11/(cm s); the nucleation rate is 10 2 10 3/(cm3 s); the growth time is 10 100 years. In Fig. 8.8(b), the growth time is less than 100 years at T 10 °C , and the nucleation rate is 10 4 10 1/(cm3 s). The difference in the curves in Figs. 8.8(a) and (b) is interpreted as due to the annealing effect involved in contact metasomatism.

Sedimentary rocks are composed of epigenetic minerals that have survived weathering and authigenic minerals formed in situ from low-temperature aqueous solution. Since authigenic minerals are formed from aqueous solution at low temperature, they are either amorphous or minute crystalline, and so the texture of sedimentary rocks is principally determined by the sizes and forms of epigenetic minerals.

In regional metamorphic rocks and contact metamorphic (metasomatic) rocks, new mineral crystals grow in solid rocks in which there was a change in conditions. However, the process is not the same as straightforward solid state growth or recrystallization. Since volatile components such as H2O and CO2, which were originally present in the rock, are involved, it is better to assume a