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  • 館藏編號:NTUG010-T300036
  • 題名:Igneous Rocks
  • 作者:阮維周
  • 藏品形式:稿本
  • 尺寸:210X297(mm)
  • 簡述:P.34
  • 內容:

    P.34 of the same metals, and with other metasilicates, orthosilicates and polysilicates, and sometimes with quartz. In most cases magnesium preponderates over iron in the compound, indicating the stronger base-forming character of magnesium. Calcium does not enter this orthosilicate in large amounts, although it is known to form with magnesium and iron the ortho-silicate monticellite, as a result of metamorphism. CaSiO4 is unstable at low temperature. Fayalite occurs in strongly alkalic rocks low in silica and almost free from magnesium. The formation of olivine is subsequent to the satisfication of the stronger base-forming elements. Garnets R3R2(SiO4)3, in which R = Ca, Mg, Fe, (Mn), R = Al, Fe, (Cr, Ti) and in which Si may be partly replaced by Ti. The compounds in this group form an isomorphous series. Ca3Fe2(SiO4)3 ˙˙˙˙˙˙˙˙˙andradite(melanite) (Ca, Mg)3Fe2(SiO4)3 Ca3(Fe, Ti)2((Si, Ti)O4)3 schorlomite Fe3Al2(SiO4)3 almandite Mn3Al2(SiO4)3 spessartite Mn3Al2(SiO4)3 pyrope Ca3Al2(SiO4)3 grossularite The compound Ca(Mg, Fe)2Al2(SiO4)3 corresponds to a mixture of anorthite, CaAl2(SiO4)2, and olivine, (Mg, Fe)2(SiO4). And in a sense garnet bears the same relation toward anorthite feldspar and olivine that biotite bears toward orthoclase feldspar and olivine. There is no hydrogen or alkalk metal in garnet. The principal garnet that forms in highly siliceous magmas is almandite, Fe3Al2(SiO4)3. In magmas rich in alkalies and low in silica forms andradite. The garnet sometimes formed in magmas rich in magnesium and low in silica is pyrope. Pyrogenetic or primary garnets are not often seen.