13

Crystals formed by metasomatism and metamorphism

When a hydrothermal solution permeates into a pre-existing rock, or heat and vapor components are supplied by the intrusion of magma, hydrothermal or contact metasomatism occurs, and mineral phases are newly formed. When deposited strata are placed at higher temperature and pressure conditions due to subduction owing to plate movement, metamorphism proceeds in a huge area, leading to the paragenesis of new minerals and regional metamorphic rocks. In this chapter, we shall analyze which aspects of crystal growth taking place in solid rocks are the same as or differ from those occurring in the ordinary vapor or solution phases. As representative examples, the kaolin group of minerals and trapiche emerald are selected for hydrothermal metasomatism; trapiche ruby is considered for contact metasomatism; and white mica is selected for regional metamorphism.

13.1Kaolin group minerals formed by hydrothermal replacement (metasomatism)

Kaolin group minerals are sheet silicates, and they belong to a group of clay minerals, with 1: 1 stacking of the unit layer with a chemical composition of Al2Si2O5(OH)2. Depending on the type of stacking, three polytypes, kaolinite, dickite, and nacrite, are distinguished. In addition, halloysite, containing (OH) and H2O, belongs to this group, but this will not be discussed in this chapter.

Kaolin group minerals have been used since ancient times as raw materials for china and porcelain production. They occur by the weathering of igneous rocks, such as granite, that contain a quantity of feldspar, by hydrothermal replacement (metasomatism) of pre-existing rocks, or in hydrothermal veins. Both in weathering and hydrothermal metasomatic processes, kaolin group minerals grow from minerals constituting solid rocks, e.g. feldspar. How these processes proceed is

252Metasomatism and metamorphism

analyzed based on the observation of surface microtopographs of these crystals. Crystals of kaolin group minerals are 0.5 1.0 m in size, and their surface micro-

topographs may only be investigated using electron microscopy. If gold evaporation is applied in a vacuum on well dispersed samples, Au grains selectively nucleate along the steps; thus, after carbon coating and dissolving samples, it is possible to view replica samples showing the surface microtopograph under an electron microscope. This method is called the decoration method.

Step patterns due to elemental spiral growth steps, with a step height of 1 nm and a step separation of 102 103 nm order, are universally observed on the surface

of the {001} faces of kaolinite, dickite, and nacrite.

Commonly, one crystal face is covered by steps originating from one spiral center, and it is rare to see a case with a large number of growth centers on one surface. Interlaced step patterns (see Section 5.6) due to polytypes of kaolinite, dickite, and nacrite are discernible. This observation indicates that kaolin group minerals grow by the spiral growth mechanism in a dilute aqueous solution phase formed by the dissolution of pre-existing solid phases in hydrothermal metasomatism, and that the crystals grow in an environment in which growing crystals cannot move freely. Namely, hydrothermal metasomatism proceeds through the process of dissolution–precipitation and not through a direct phase transition from the solid state, and the dissolution front proceeds gradually in pre-existing rock, behind which crystallization of kaolin group minerals occurs. Two examples of surface microtopographs of kaolinite formed in this manner are shown in Fig. 13.1 [1].

In contrast to the above, aggregates of a few crystals of kaolinite, sericite (muscovite of micrometer size), or chlorite (a clay mineral) occurring in vein-type deposits are more frequently encountered than in hydrothermal metasomatic deposits. On individual crystal surfaces, growth spirals are observable, but one characteristic of this mode of occurrence is the aggregation of a number of crystals (Fig. 13.2), which suggests that the crystals moved freely during growth and grew in a moving environment [2].

13.2 Trapiche emerald and trapiche ruby

In the Muzo and Chivor mines, in Colombia, calcite veins, in which emerald crystals occur, develop, and they fill the cracks of sedimentary slate rock. Compared with emeralds occurring in basic metamorphic rocks, like those in the Urals, South Africa, or India, Colombian emerald crystals have a higher perfection, fewer inclusions, and attract higher evaluations as gemstones, since crystals grew freely in a hydrothermal solution.

In the Muzo and Chivor mines, emerald single crystals with a particular texture

13.2 Trapiche emerald and trapiche ruby 253

(a)

(b)

Figure 13.1. Two examples of surface microtopographs of kaolinite using the

decoration method [1].

254 Metasomatism and metamorphism

Figure 13.2. Characteristics of chlorite crystals in hydrothermal veins using the

decoration method [3].

occur in sedimentary rocks around calcite veins. As schematically shown in Fig. 13.3(a), these crystals are hexagonal prismatic consisting of a hexagonal core and six growth sectors (arms) of trapezoid form. The boundaries between the hexagonal core and the trapezoid sectors, and those of the trapezoid, i.e. boundaries in six directions starting from the corners (edges) of the core hexagon, consist of two minerals, emerald and albite. Since the texture appears to resemble the gears (trapiche in Spanish) of a sugar cane compressor, they are named “trapiche emeralds” [4].

Ruby crystals with a similar texture are found at a contact metasomatic deposit in Myanmar, and are called trapiche rubies. Sapphires with similar textures are also known. Trapiche ruby crystals occur sporadically in marble formed by contact metasomatism due to the intrusion of a granitic magma into Mg-containing limestone. Trapiche sapphire is also formed by the contact metasomatism of Al-rich sedimentary rocks.

Since the terms used to describe the parts of the texture are different for trapiche emerald and trapiche ruby, the crystals are compared in Figs. 13.3(a) and

(b). A photograph of trapiche ruby is shown in Fig. 13.3(c). The shared characteristics of the texture in both emerald and ruby are: the presence of a clear hexagonal or tapered-prismatic core portion; the dendritic portion starting from and stretching in six directions from the edges of the core prism; a similar dendritic portion

13.2 Trapiche emerald and trapiche ruby 255

(c)

Figure 13.3. Textures of (a) trapiche emerald and (b) trapiche ruby [3]. (c) Photograph of

trapiche ruby, taken by K. Schmetzer.

covering the surface of the core; and six clear trapezoid growth sectors surrounding the core and filling up dendrite interstices. In trapiche emerald, the dendritic portion consists of beryl and albite; in trapiche ruby, it consists of corundum, calcite, and a silicate mineral; and in trapiche sapphire, albite and rutile are present, without corundum [3], [4]. However, the core and the growth sector portions consist, respectively, of emerald, ruby, or sapphire, containing no other phases. From these textures, we understand that the growth process of these crystals should be as follows.

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Figure 13.4. Element partitioning in trapiche ruby. Analyses are by micro-area XRF. The dendritic growth portions are denoted by “arm” and “branch”; “core” represents the core portion; and A, B, 1, 2, and 3 denote scanned lines [3].

(1)The core portion consists of an idiomorphic crystal of a single phase, either emerald, ruby, or sapphire.

(2)Rapid dendritic growth occurs through co-precipitation of two or more phases on the substrate of the core portion. In this stage, the phase of the core portion may be included as a co-precipitating phase (emerald or ruby), or may not be present (sapphire). The important point is that the growth of two or more phases is controlled by the structure of the core portion, and the dendritic growth proceeds very rapidly, so determining the size and the skeleton of the present crystal.

(3)The portions corresponding to the growth sectors are formed by filling up the interstices of the earlier-formed skeleton. The growth in this stage is single phase and slow.

The above process is well recorded in element partitioning in trapiche ruby. Figure 13.4 shows the summarized results of micro-area XRF (X-ray fluorescence spectroscopic analysis) [3]. The partitioning of Cr, Fe, and Ti in corundum in the core portion, the dendritic portion, and the growth sectors is indicated. The following points should be noted.