A new type of interpenetration twin of labradorite from East Greenland

(010) have the axis [001] in common. The (010) planes of the two fourlings form an angle of 38° ± 1° in the example illustrated. Two rectangular planes of symmetry between the two systems intersect in [001], but are not developed as morphological boundary planes. These symmetry planes bring the twin axes of the Albite, Carlsbad and Albite-Carlsbad laws of one fourling into the position of those of the second fourling. The same mirror reflection brings the vibration directions na, nO, ny of each individual of the first fourling into the position of corresponding indicatrix axes of an individual of the second fourling. As a sec­ ond interpenetration with common [001] but with an angle of 52° between the planes (010) has been found, we conclude that the coinciding axis [001] is the characteristic feature of this new type of intergrowth. A geological implication from this study is the proof for the occurrence of basalt in the bed-rock below the ice cap west of Daugaard-Jensen Gletscher, 150 km north of the northernmost extension of the thick pile of plateau basalts of the Brito-Arctic province along the East Coast of Greenland.