12.3 Hematite and phlogopite in druses of volcanic rocks 247

T

T

Figure 12.9. Spiral step pattern observed on both sides of a twin boundary on a {0001} face of a hematite crystal [7]. T.B. is the twin boundary, and indicate orientations. Phase contrast photomicrograph.

12.3Hematite and phlogopite in druses of volcanic rocks

In volcanic rocks formed by the solidification of magma near the Earth’s surface, small cavities of millimeter to centimeter order are often present, in which idiomorphic crystals of phlogopite and hematite occur. All these crystals grown from the vapor phase show typical spiral patterns. However, they grow in a much-reduced free space as compared to the case of pegmatite, and this characteristic is well represented on their surface microtopographs.

Figure 12.12 shows a step pattern on the {001} face of phlogopite; the step height is 1 nm, and the step separation is of order 10 m. The characteristic form of the spiral is five-sided, i.e. hexagonal with one edge truncated [10]. This characteristic form is due to the fact that the stacking of the unit layer, which has a hexagonal form, is in one direction. Since an interlacing pattern is not observed in this step pattern, this crystal is identified as a 1M polytype. On the surface of the (001) face shown in Fig. 12.12, a five-sided step pattern rotated by 120° and 180° appears in an island form on one surface. This indicates that several crystals agglutinate in rotated orientation, and that crystals attaining a certain size move around in the cavity and come together to form the crystal [11].

12.3 Hematite and phlogopite in druses of volcanic rocks 249

0.1 mm

Figure 12.12. Five-sided spiral steps observed on {001} face of phlogopite [10].

Another example showing the formation of many tiny crystallites in a small cavity, which move around and agglutinate, is found in hematite occurring in a similar way. Figure 12.13 is a surface microtopograph of the {0001} face of such a hematite crystal, and it indicates numerous hexagonal island-like portions on the terraces of step patterns. Around these islands, the step patterns of the host crystal are disturbed. This observation indicates that, as for phlogopite crystals, a large number of crystals with varying sizes are formed within a cavity, within which they move around and agglutinate.

250 Minerals formed by vapor growth

Figure 12.13. Surface microtopograph of hematite from a druse of ryolite. A large number of minute hematite crystals adhere and affect the advancement of the spiral growth layers on the host crystal.

References

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2M. D. Barton, Phase equilibria and thermodynamic properties of minerals in the BeO- Al2O3-SiO2-H2O system with petrologic applications, Am. Min., 71, 1986, 277–300

3 I. Sunagawa, Growth history of hematite, Am. Min., 45, 1959, 566–75

4I. Sunagawa, Mechanism of crystal growth, etching and twin formation of hematite,

Mineral. J., 3, 1960, 50–89

5 I. Sunagawa, Mechanism of growth of hematite, Am. Min., 47, 1962, 1138–55

6I. Sunagawa, Mechanism of natural etching of hematite crystals, Am. Min., 47, 1962, 1332–45

7Lu Taijing and I. Sunagawa, Origin of undulated growth steps on hematite crystals from Sasazawa, Japan, Mineral. J., 13, 1987, 409–23

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33, 1962, 1–8

9I. Sunagawa, Growth and etch features of hematite crystals from the Azores Islands, Portugal, Surv. Geol. Portugal, Mem., 6, 1960, 1–44

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11I. Sunagawa and S. Tomura, Twinning in phlogopite, Am. Min., 61, 1976, 939–43