6o 



REPORT ON SOME CRYSTALLINE COMPONENTS OF 

 THE WEDDELL SEA DEPOSITS 



By F. a. bannister, M.A. 



With chemical analyses by M. H. HEY, M.A., B.Sc. 



Assistant Keepers in the Mineral Department of the British Museum (Natural History) 



(Plates II, 11^) 



DURING his Study of the Foraminifera present in oceanic bottom samples brought back 

 by the Scotia Expedition (1902-4) from the Weddell Sea, Mr A. Earland picked out 

 a number of minute crystals and crystalline nodules which he separated by external 

 characters into three groups. A brief description of these crystalline components and 

 an account of their examination by optical and X-ray methods seemed desirable chiefly 

 because the substances identified have not hitherto been recorded from ocean bottom 

 deposits. 



HYDRATED CALCIUM OXALATE, CaCA-sH^O 



The first group consists of minute "envelope" crystals recorded from a number of 

 stations in the very deep water of the central Weddell Sea, from depths of 2425-2739 

 fathoms (4434-5008 m.) (Plate II, fig. i). These crystals are transparent, colourless, 

 tetragonal bipyramids varying in size from 0-2 xo-i to 0-3 x 0-15 mm. When immersed 

 in a liquid and viewed under the microscope they present sharp, square outlines with 

 well-marked diagonals showing the intersection of the four uppermost pyramid faces. 

 Intergrowths of two or more individuals are not uncommon but the grouping appears 

 to be accidental and not in conformance with any twin-law. The "envelope" crystals 

 are sparsely distributed in the deep-sea deposits. A gram of unpicked material from 

 St. 286 yielded about fifty crystals weighing only half a milligram, the mineral residue 

 consisting principally of fragments and rounded pebbles of clear, colourless quartz. 

 Fragments of green hornblende, pink almandine, orange hessonite, brown biotite and 

 glauconite were also detected. 



When examined with convergent polarized light each "envelope" crystal gives a 

 positive uniaxial figure consisting of a black cross surrounded by two interference rings ; 

 approximate birefringence = 0-02. If monochlorobenzene be used as the immersion 

 liquid the crystal outline completely vanishes; hence the refractive index a> = 1-523 

 ± 0-005. The specific gravity could not be determined by balancing in a mixture of 

 bromoform and benzene owing to their minute size and lack of colour. Several, how- 

 ever, were mounted on thin glass fibres and each in turn accurately centred in a parallel 

 beam of light restricted in diameter by an iris diaphragm and pin-hole collimator. By 

 such means it was possible to measure the angle between pyramid faces. The average 

 value for the angle between a pyramid face r and the basal plane c (001) is cr = 30° 35'. 



At this stage the tetragonal symmetry of a crystal was confirmed by a Laue photo- 

 graph taken with the X-ray beam passing along the c axis [001]. Rotation photographs 



