Computer Science
Scientific paper
May 1961
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1961gecoa..23..233m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 23, Issue 3-4, pp.233-246
Computer Science
12
Scientific paper
The synthesized garnets at relatively low hydrostatic pressures belong to limited pyralspite compositions (field 1 on Abb. 14). The experiments started at H 2 O-pressures (or p H 2 O + CO 2 , P Ar ) of between 50 and 1500 atm. and showed that pressures below 200 atm. had an increasingly unfavourable influence on the reaction of these garnets. For the pure spessartite composition the lower reaction limit at pressures between about 200 and 1500 atm. was at 410°C. For spessartite-almandite mixed crystals the limit rose with increasing almandite content from 410°C (spess 90 alm 10 ) to 500°C (spess 50 alm 10 ). Spessartite with a low content of pyrope already was obtainable from a reaction mixture (Mn 90 Mg 10 ) at temperatures between 390 and 400°C. Some attempts have been made to demonstrate, that these lower reaction limits correspond to the lower stability limits. At a temperature of t~800°C and under the pressures employed, the upper limit of reaction temperature (or breakdown temperature) had not yet been reached for any of the synthesized garnets. The only pure garnet component in the pyralspite system which could be crystallized was spessartite and neither almandite nor pyrope were obtained. Mixed crystals of spessartite with almandite were obtained up to a maximum almandite content corresponding to spess 40-30 alm 60-70 . These mixed crystals (as also the pure spessartite component) were able to take up a maximum of about 15 per cent of the pyrope component and of about 20 per cent of the grossularite component. These limits correspond approximately to those found in natural mixed crystals. Garnets obtained at temperatures below 700°C possess a water content which increases in the lower temperature ranges and is present as the hydro-spessartite component without any clear dependence to the amount of water present. The interpretation of some natural occurrences of the accentuated spessartite-almandite garnets in relation to the given experimental results is tried. Finally the formation of accentuated almandite garnets as a constituent of homfelses in a thermal aureole and the relations between spessartite-hydrospessartite are discussed in the light of the given experimental observations.
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