Growth defects in quartz druses, <a> pseudo-basal dislocations

Growth defects were studied by X-ray topography in a succession of plates cut parallel to the basal planes of two quartz crystals from an Italian druse (Rocca dei Cristalli, Val Malenco, Italy). Growth bands, dislocations and precipitates are the dominant defects. Defect configurations, Burgers vectors and orientations of line defects were found to be similar in both crystals. Distinctive growth marks characterize the growth history of the druse: in particular, characterization was made of unreported dislocations with <a> Burgers vectors, whose line orientations deviate from the basal plane by an angle of about 10°. These dislocations, referred to here as pseudo-basal dislocations (p.b.d.), were found to be perpendicular to the x {511} faces.