Senaite from syenite pegmatite at Tvedalen in the southern part of the Oslo region, Norway

Senaite, the plumboan member of the crichtonite group, has been found in a syenite pegmatite in a larvikite quarry at Bjørndalen in the Tvedalen area in the southem part of the Oslo Region. Senaite occurs as weU-formed interpenetration twins up to 4 x l mm large with c as the twin axis. The colour is black with a submetallic adamantine lustre. The streak is greenish black. The density is 4.96(1) g/cm3 (measured) and 4.93 g/cm3 (calculated). The unit cell dimensions found by !east squares refinement based on an X-ray diffractogram are (trigonal, space group R3 or RJ) a= 10.418(1) Å, c= 20.928(2) Å, V= 1967.1(3) Å3. Chemical analysis yields the structural formula .Pbt.n(Tit3.66FeJ:t.Fei.�Zn�.87Mno." Y o.26Nb0.u·h:21.1,038 which is in dose agreement with the general form ula AM21 038• Crystalliza tion of senaite, which contains the chalcophile elements Pb and Zn, requires a hydrothermal fluid with an extremely low H2S fugacity, typically found in syenites, alkali granites and certain hydrothermal veins.