EXPLANATION OF PLATE XXI. A (one nicol) and B (x-nicols). Colloform gel banding; microcrystalline quartz and chalcedony; zonal growth lines in comb quartz; parallelism of comb quartz individuals. Gold Road, Ariz. (X 35.) C (one nicol) and D (x-nicols). Fibrous chalcedony; crustification banding flamboyant quartz (at left of upper band, -marked 'x'). White Rock, Australia. (X 35.) E (one nicol) and F (x-nicols). Micro-botryoildal gel structure, its preservation by inclusions in flamboyant quartz crystals; these crystals are unrelated to the structure of the original gel. Talisman Mine, New Zealand. (X 35.) not passing completely through the crystal, but running along a certain distance and being intersected by others crossing them." Whether this cracking is due to the rarely observed rhombehedral cleavage of quartz or to shear planes which may develop in any rigid body under pressure, is uncertain, as the paragraph goes on to say. Since the grid of inclusions is a property of the individual crystal, and not a pattern extending over a number of crystals, it is probable that the rhombehedral cleavage is the controlling factor in this orientation. However, the development of this inclusion pattern is probably not by actual fracturing and subsequent healing, an origin assumed by Ferguson39 for inclusions in the Allegheny district quartz. Judd has shown that rows of bubbles may be developed along directions of shearing in minerals without, or prior to, faulting. In the slides examined there is nothing to indicate that the mineral has fractured. The latticed structure shown by inclusions may sometimes be seen under crossed nicols (Pl. XXVI., F). The features brought out in this photograph are: (1) each single apparent veinlet consists of a thread of quartz which disagrees in extinction position with the parent crystal; (2) the crystallographic orientation of the quartz in parallel threads, the traces of roughly parallel planes, is the same; (3) the orientation in one set of planes is not the same as in the other set. Since the quartz in these planes is not parallel to that in the parent crystal, they cannot be a result of fracturing and later filling such as causes "phantom" veinlets. Judd11 has shown that a lamellar twinning parallel to (r) and (z) may be developed in quartz by pressure. It is possible that the phenomenon pictured is related to this gliding described by Judd. Typical microscopic brecciation needs no description, for its features are miniatures of those of large scale brecciation. The following "shredding" of quartz, which is somewhat analogous 39 U. S. G. S. Bull. 580, p. 161. 40 Mineralogical Magazine, Vol. VII., 1886, p. 82, 83. 41 Mineralogical Magazine, Vol. VIII., 1888, p. 1. to "slicing," and recrystallization by strain, are, however, phenomena which are worth separate mention. Shredded Quartz. A rather unusual effect of shearing is shown in Pl. XXVII., A. Large quartz crystals have been fractured into aggregates of roughly columnar fragments, which have distinct but not widely separated extinction positions. Extreme shearing produces a shredded or fibrous texture. Quartz which has this appearance under the microscope is usually platy in the hand specimen. Quartz shredded by strain is easily confused with other rather rare phenomena which have the same appearance; e.g., a similar structure has been observed: (1) in quartz pseudomorphic after shredded siderite; (2) in the rearrangement quartz shown in Pl. XXVIII., D; (3) in slightly fractured combs of slender quartz prisms; and (4) in the variety of replacement quartz shown in Pl. XXVI., C. Recrystallization of Quartz. The word "recrystallization" has been used to denote a number of varied, though associated phenomena. As a factor in the development of metamorphic rocks and rock cleavage the process has been thoroughly discussed by Leith12 and Van Hise. 43 The various ways in which the term is used with respect to individual crystals may be summarized from the discussions by Van Hise, who describes three major processes of recrystallization. These are: (1) the solution of mineral material at points of greatest pressure, and the simultaneous deposition of mineral material at points of least pressure; (2) the coalescence of very small mineral particles to produce larger particles; (3) the breakdown of larger crystals to form an interlocking aggregate of smaller crystals. Evidence of the first process has not been observed in vein 42 U. S. G. S. Bull. 239, 1905. 43 U. S. G. S. Mon. 47, 1904. quartz. Recrystallization by coalescence is mentioned later under the head of "rearrangement," As it was not observed as a pressure phenomenon, this process has not been included under this heading. The third phenomenon, recrystallization by the breakdown of large crystals to small ones, is usually the result of strain, and is often accompanied by distortion and granulation. This process in pegmatitic quartz has been pictured by Bastin.44 The phenomenon is often a definite characteristic of certain deepseated quartz veins. Recrystallization of quartz by the breakdown of large crystals to an interlocking aggregate of smaller individuals is developed prominently in quartz ores from the gold fields of West Australia; from the Mother Lode of California; and from the Juneau gold belt in Alaska. Thin sections cut from unstrained specimens from these districts show that the vein material is an aggregate of coarsely granular subhedral quartz crystals. Before recrystallization all of the quartz contained inclusions of gas, minute mineral matter, and occasionally liquid with gas bubbles. The first step in the process is a disturbance of the optical continuity of a crystal, producing strain shadows, of wavy or undulatory extinction. Incipient recrystallization closely follows initial strain. It appears as a more pronounced undulatory extinction in which small, rather definite, areas may be seen to extinguish at once, giving the crystal a rough and mottled appearance. Incipient recrystallization can only be detected by this mottling of high lights when the parent crystal is at the maximum extinction position, thus furnishing a dark background for contrast. At the position of maximum transmission for the main crystal, the variance in interference color is so slight as to be imperceptible. As recrystallization progresses the divergence in optical orientation between original and the recrystallized portions increases. Pl. XXVII., B, shows a single strained quartz crystal, with partial recrystallization. The final stage is an interlocking aggregate of rather equidimensional quartz. Van Hise15 Geology of the Pegmatites and Associated Rocks of Maine," U. S. G. S. Bull. 445, 1911, Plate VI. 44 45 Bull. Geol. Soc. Am., Vol. I., 1890, p. 217. |