Spiral hillock

The prismatic faces of natural rock-crystal are characterized by the development of striations parallel to the edges between m, r, and z faces (perpendicular to the c-axis). Natural rock-crystal showing no distinct striations is almost exceptional. In industrially mass-produced synthetic quartz using NaOH or KOH as mineralizers, no striations are observable on 1010 faces. As shown in Fig. 10.5(a), five-sided growth spiral hillocks are generally observed. However, if quartz crystals are synthesized in hydrothermal solution with NaCl as the mineralizer, the prismatic faces exhibit similar striations to those observed on natural crystals [5]. 

When the step separation is wide enough, typical spiral step patterns observable by optical microscopy may appear, but if the separation becomes narrower than the resolution power of the optical microscope, the spirals appear in the forms of polygonal pyramids or conical growth hillocks. Even if spiral patterns are not directly observable, we may assume that these growth hillocks are formed by the spiral growth mechanism. Examples representing the two cases are compared in Fig. 5.8. 

Figure 5.8. (a) Typical spiral pattern (phase contrast photomicrograph of (0001) face of Sic grown from the vapor phase), and spiral growth hillocks which appear as (b) polygonal and (c) conical pyramids due to narrow step separation. Part (b) is a differential interference photomicrograph, (1010), and part (c) is a reflection photomicrograph, (1011), of hydrothermally synthesized quartz. 

K. Maiwa, K. Tsukamoto. and 1. Sunagawa. Activities of spiral growth hillocks on the (111) faces of barium nitrate crystals growing in an aqueous solution,/. Crystal Growth,... 

Figure 10.5. (a) Polygonal spiral growth hillocks universally observed on 1010 faces of synthetic quartz. (The arrows indicate the summits of the growth hillocks.) (b) Striation patterns commonly seen on the 1010 faces of natural and synthetic quartz grown in NaCl aqueous solution. 

Figure 6 (left), (a) Differential interference contrast (DIC) photomicrograph of a trigonal growth hillock on a 100 face of apatite from the Golconda Mine, Minas Gerais Brazil. The three vicinal faces of the hillock exhibit macrosteps. The horizontal steps run in the [001] direction. The three step orientations are parallel to the three dominant step orientations of the spiral in Figure 5. Image is approximately 666 pm across, (b) Schematic of the face symmetry with respect to the hillock in a. Steps on the basal vicinal face parallel [001]. Lines within each vicinal face represent the orientations of growth steps. Arrows indicate the directions of advancement of steps during growth. [Modified after Rakov an and Reeder (1994)].

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