All fibres are parallel and the fibrous shape is not related to the crystallographic habit or lattice orientation of the mineral. Common in calcite, but can also occur in other minerals such as quartz.
Crystals get wider in the growth direction as some crystals 'lose' in the growth competition.
Common in crack-seal veins. Syntaxial vein - Vein with one growth surface in the middle of the vein. The fibres or elongate crystals started growing at the edge of the vein and are in continuity with the wall rock. Antitaxial vein - Vein with two growth surfaces on the edge of the vein. The vein-forming fibres are usually of another mineral than the wall rock and there is no continuity with the wall rock.