Carbonate Content, Carbonate Mineralogy, and Bulk Mineralogy of DSDP Leg 42A Samples [chapter]

J. Muller
1978 Initial Reports of the Deep Sea Drilling Project  
For the determination of total carbonate and for x-ray diffraction analysis samples were dried (40°C) and ground to a size of less than 40 µm. Inorganic carbon was analyzed volumetrically with a LECO-analyzer. Accuracy of this method is ±1% (See Sigl, this volume). X-ray analysis was made with unoriented powder specimens with a scanning speed of 1° 20/min; when Mg-calcite (>4 mol-% MgCO 3 ) was present, scanning speed was reduced to 1/2° 20/min to obtain better resolution. The ratios of
more » ... e ratios of aragonite, calcite, and Mgcalcite were determined by measuring peak areas of the different carbonate minerals and calculated using calibration curves published elsewhere (Müller and Müller, 1967; Müller, 1969) . The calcite-dolomite ratio was determined after the method described by Tennant and Berger (1957) . The proportions of carbonate minerals were calculated using the inorganic carbon determined by the LECO-method and by the ratios obtained by X-ray diffraction. The precision of this method is ± 5% by weight. The MgCO 3 -content of the Mg-calcite and the composition of the dolomite were determined by the shift of the (104) peak of calcite and of dolomite, respectively (Goldsmith and Graf, 1958; Füchtbauer and Goldschmidt, 1965) using the position of the (101) peak of quartz as an internal standard. Many samples contain both stoichiometric dolomite (Ca 5 oMg 5O ) and Ca-dolomite (usually Ca^Mg^), the latter being more frequent and abundant. The proportions of quartz, K-feldspar, plagioclase, gypsum, anhydrite, and pyrite were estimated by their peak heights assigning arbitrary symbols for their abundance. The respective symbols cover the following ranges (Table I 1 With the exception of the samples from the "evaporites" of Site 374 (Core 17 to Core 21), halite has not been listed though present since in most cases it 1 A to abbreviations used in Table 1 appear on its last section. formed during the drying of samples from the entrapped interstitial water. In some instances ("bomb" samples) bassanite (CaSO 4 × 1/2 H 2 O) was noted. It is assumed that it originated by drying "bomb" samples containing gypsum at elevated temperatures (>60°C) on shipboard. Bassanite was therefore listed as gypsum. Since no oriented powder specimens were run, no attempt has been made to quantify clay minerals, they were only recorded (with the exception of mixed-layers and smectite) when distinct reflections were noted on the diffractogram. In the sections between Cores 12 and 15 of Site 374 occur small white nodules which were identified as lüneburgite [Mg 3 (PO 4 ) 2 B 2 O (OH) 4 × 6 H 2 O], they are described in more detail by Müller and Fabricius (this volume). X-ray diffractograms from Samples 374-11-2, 120-121 cm, and 374-17-1, 67-68 cm, contain a number of reflections which were tentatively ascribed to bischoffite (MgCl 2 × 6 H 2 O), the latter sample contains additional peaks which could not be identified, they are probably related to MgSO 4 . ACKNOWLEDGMENTS
doi:10.2973/dsdp.proc.42-1.app8.1978 fatcat:fphjruhsrzc5rcv22aot4njun4