The Varuträsk pegmatite consists of a middle seam, chiefly composed of microcline perthite, cleavelandite, quartz and different lithium minerals, constituting the main part of the pegmatite body, surrounded by quartz-muscovite pegmatite.1 The middle seam has been formed by hydrothermal solutions replacing the quartz-muscovite pegmatite, which was deposited from solutions of epimagmatie character. The minerals of the pegmatite may be divided into several groups according to their genesis. The microcline perthite group, also containing pollucite, petalite, spodumene, amblygonite, beryl, and quartz, was the first to form during the hydrothermal period. This group was succeded by another containing cleavelandite, coloured tourmaline, lepidolite, and quartz as main minerals. The microcline perthite is described. Especial attention is drawn to the intergrowths of poikilitic albite and their importance for the interpretation of the genesis of the perthite. The types of perthites observed are vein perthite, patch perthite, film perthite, and string perthite.2 The two latter are found only locally and in insignificant amounts. Evidence is given supporting the theory that the vein and patch perthite have been formed by replacement of microeline by albite solutions preceding the cleavelandite phase in the evolution of the pegmatite. The rubidium and caesium content of the microcline perthite is discussed and compared with that of microcline from other localities. The cleavelandite is described and its genesis discussed. A contribution to the discussion on the genesis of different perthite types is given.