The twin-boundary formation of Japanese twins in low-temperature quartz was studied by means of optical microscopy, electron backscatter diffraction on polished sections and transmission electron microscopy (TEM). It was found that the twin junction consists of two units, which are related to different growth stages. In the interior part the crystals exhibit an ~100 μm straight twin junction, a so-called twin boundary, which continues as an undulating boundary towards the crystal surface. Moreover, TEM results showed that this crystallographically defined straight twin boundary can only be found in the first 400 μm of the crystal, where it is attached to the bedrock. The selected-area electron-diffraction pattern, acquired at the twin boundary, showed that reflections common to both twin individuals correspond to the {112̄2} family of planes, which is in accordance with the Japan twin law. The combined spatially resolved EDX and EELS analyses confirmed no compositional changes across the twin boundary. It is concluded that the formation of a straight twin boundary in quartz is related to the earliest stage of crystal growth. The subsequent growth of Japanese twinned crystals is characterised by the formation of an undulating twin boundary, resulting in a random intergrowth of both twin individuals.

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