Deprecated: Assigning the return value of new by reference is deprecated in D:\nadm\history\ on line 75 地質名人館
  • 館藏編號:NTUG010-T300047
  • 題名:Igneous Rocks
  • 作者:阮維周
  • 藏品形式:稿本
  • 尺寸:210X297(mm)
  • 簡述:P.45: Granularity
  • 內容:

    P.45: Holocrystalline wholly crystalline Percrystalline, crystal / glass > 7 / 1, extremely crystalline with some glass. Docrystalline, crystal / glass < 7 / 1 > 5 / 3, dominantly crystalline. Hyalocrystalline, crystal / glass <5 / 3 > 3 / 5, crystals and glass equal or near so. Dohyaline, crystal / glass < 3 / 5 > 1 / 7, dominantly glassy. Perhyaline, crystal / glass < 1 / 7, extremely glassy with some crystals. Holyhyaline sholly glassy Granularity. The influence of viscosity on granularity depends upon the chemical composition of magmas. Difference in temperature between the intrusive body and country rock produces a relatively rapid cooling near the contact parts. Thus the resulting rock is usually fine-grained. The size of crystals is also dependent on the molecular concentration, the greater the amount of a suostance in solution the greater the amount of material crystallized in a given time. Degree of supersaturation together with molecular concentration enables certain crystal to grow very big. The size of crystal developed in rock magma depends on viscosity of solution, rate of cooling and molecular concentration. a) Effect of viscosity - The more viscous magma yields smaller size of crystal. Hence magma of viscous, highly siliceous character yield rocks often glassy with microscopically minute crystals of quartz and potash feldspar. Since viscosity is a specific character of substance, it varies .vith tne chemical composition. Consequently the size of crystal separating from magma is to a certain extent dependent on the chemical composition of the liquid magma. - 18 -