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 well-formed interpenetration twins up to 4 x 1 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 least squares refinement based on an X-ray diffractogram are (trigonal, space group R3 or R˜3) a= 10.418(1) Å, c= 20.928(2) Å, V= 1967.1(3) Å3. Chemical analysis yields the structural formula Pb1.13(Ti13.66Fe3+3.44Fe2+1.22Zn1.87Mn0.55Y0.26Nb0.15)?21.5O38 which is in close agreement with the general formula AM21O38. Crystallization 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.