13.3 Muscovite formed by regional metamorphism 257

(1)The content of Cr in corundum in the dendritic portion is lower than that in both the core portion and the growth sectors, and is nearly constant from the root to the tip of the dendrite.

(2)Within the growth sector portion, the variation in the content of Cr and Ti is noticed alongside the external form, and the content is highest at the boundary with the dendritic portion, namely where the growth mechanism transforms from two-phase co-precipitation to the growth of a single phase, decreasing as growth proceeds, and gradually diminishing as a whole, although there is a slight fluctuation. At the final stage the contents increase slightly, and again decrease. The Ti content also shows a similar decreasing tendency as growth proceeds [3].

The characteristics of the element partitioning indicate that the dendritic portion was formed rapidly and over a short period; this element partitioning is principally controlled by the thermodynamics, i.e. the temperature and pressure, whereas the growth of the core portion and the growth sectors are by the layer growth mechanism, and the element partitioning is principally controlled by the kinetics.

When a crystal is formed and becomes large in the mother sedimentary rocks metamorphosed, by contact metasomatism, crystals with similar textures are often formed. Chiastolite, a form of andalusite (Al2SiO5) with a particular texture, may be such an example.

13.3Muscovite formed by regional metamorphism

Muscovite, KAl2(AlSi3O10)(OH)2, is a rock-forming mineral formed under a wide range of conditions. In granite, particularly in pegmatite, it occurs as a large idiomorphic crystal, and takes platy to tabular Habitus with well developed {001}. Step patterns of growth spirals are observed only on {001} surfaces, whereas prism faces are characterized by striations. Muscovite also occurs as hexagonal or lathshape thin platy crystals of micrometer size in hydrothermal veins or hydrothermal metasomatic deposits. Muscovite crystals of this size are generally called sericite. On their {001} surfaces, typical growth spiral step patterns are revealed by the decoration method. In contrast, muscovite crystals grown from the supercritical vapor phase or hydrothermal solutions typically show growth spirals. We will investigate in this section the characteristics exhibited by muscovite crystals formed by regional metamorphism.

Regional metamorphism proceeds in a wide area, and characteristic mineral paragenesis appears in zones depending on the temperature and pressure conditions that were confronted by the original rock. These are called metamorphic

258Metasomatism and metamorphism

zones (belts), and it is possible to distinguish between metamorphic facies, in successive zones, based on the grade of metamorphism, through geological and petrological investigations. Muscovite occurs as a major constituent mineral in all metamorphic facies, but the crystal size increases as the metamorphic grade progresses. In the zone with the lowest metamorphic grade, the size is of micrometer order, corresponding to sericite, but as the metamorphic grade increases, the size increases and the crystals are called white mica. On further increasing the metamorphic grade, the crystals become up to millimeter order across, and are called porphyroblastic (porphyritic large crystals formed by metamorphism) white mica.

Muscovite found in the metamorphic belt in Shikoku, Japan, whose metamorphic facies are well established by geological and petrological investigations, was investigated by the decoration method. The results indicated that spiral growth step patterns were not observed on these crystals, in contrast to the commonly observed spirals on muscovite crystals grown in pegmatite and by hydrothermal metasomatism. Instead of spiral patterns, the surface microtopographs of these muscovite crystals are characterized by closed-loop step patterns [5]. Also noticeable is the coexistence of sawtooth steps due to etching and smooth step patterns (Fig. 13.5); step patterns of this sort are most frequently encountered near the boundary of successive metamorphic facies.

The neighboring metamorphic facies correspond to the boundary from which the paragenesis of new minerals takes place; dehydration, which is associated with this reaction, is seen to occur when crystals of different sizes coexist under the same driving force conditions, the smaller crystals dissolve due to the effect of the surface energy, and the dissolved component is supplied to the neighboring larger crystals, which results in the larger crystals continuing to grow larger at the expense of smaller crystals.

This phenomenon, namely dissolution and growth occurring simultaneously under the same condition simply due to the size difference of crystals, with the larger crystals growing even larger, is called Ostwald ripening (see ref. [5], Chapter 3). This is one of the empirical rules which, together with Ostwald’s step rule described in Section 3.4, is qualitative, yet pin-points the essentials of crystal growth. The Ostwald ripening rule has been observed in various crystal growth taking place on Earth, as well as in the synthesis of large single crystals. The origin of single crystals of NaCl with edge length attaining tens of centimeters occurring in salt deposits, or of a large single crystal of ice in the North and South Polar regions, may also be due to Ostwald ripening. As the metamorphic grade in regional metamorphic rocks is increased, the size of the muscovite increases, and the observed changes from sericite to white mica to porphyroblastic muscovite may also be due to Ostwald ripening [5]. The observation of the coexistence of step patterns due to growth and dissolution also support this argument.

13.3 Muscovite formed by regional metamorphism 259

Figure 13.5. White mica in a regional metamorphic rock using the decoration

method [5].

260 Metasomatism and metamorphism

Compared with this, sericite (muscovite) crystals formed in hydrothermal solution and hydrothermal metasomatism exclusively show typical spiral step patterns; the {001} faces of muscovite in regional metamorphic rocks do not show spiral step patterns, but are characterized by step patterns of closed loop. The closed loop is not due to screw dislocations. The most likely step sources are contact points with other solid particles. We may understand this step pattern if we assume that the growth of muscovite takes place in thin interstices of other mineral grains. Observations of muscovite indicate that, even for crystal growth taking place in regional metamorphic rocks, the crystal growth is not a simple solid state crystallization but that growth from the solution phase plays an essential role. Similar step patterns are universally observed on crystals of a wide range of species formed by a sintering process with a solvent component.

Large idiomorphic porphyroblast of garnet crystals attaining sizes of more than 10 cm across occur in regional metamorphic rocks. Textures suggesting rotation of the crystals during growth are often observed in this type of garnet crystal. Many investigations have been carried out on these crystals [6], but there are still many problems left to be solved.

References

1I. Sunagawa and Y. Koshino, Growth spirals on kaolin group minerals, Am. Min., 60, 1975, 407–12

2I. Sunagawa, Y. Koshino, M. Asakura, and T. Yamamoto, Growth mechanism of some clay minerals, Fortschr. Min., 52, 1975, 515–20

3I. Sunagawa, H-J. Berhardt, and K. Schmetzer, Texture formation and element partitioning in trapiche ruby, J. Crystal Growth, 206, 1999, 322–30

4K. Nassau and K. A. Jackson, Trapiche emeralds from Chivor and Muzo, Colombia,

Am. Min., 55, 1970, 416–27

5S. Tomura, M. Kitamura, and I. Sunagawa, Surface microtopography of metamorphic white micas, Phys. Chem. Min., 5, 1979, 65–81

6T. Masuda, To rotate or not to rotate, this is the question; controversial opinions on the origin of snowball structures in garnet porphyroblasts, J. Geograph., 108, 1999, 94–109 (in Japanese)