﻿376 
  Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  

  

  The 
  a-/3 
  Quartz 
  Inversion 
  . 
  

  

  This 
  inversion 
  has 
  been 
  the 
  subject 
  of 
  considerable 
  investi- 
  

   gation 
  since 
  the 
  phenomenon 
  was 
  first 
  observed 
  by 
  Le 
  Chatelier 
  

   in 
  1890. 
  The 
  attendant 
  changes 
  in 
  expansion 
  coefficients, 
  cir- 
  

   cular 
  polarization, 
  and 
  birefringence 
  have 
  all 
  been 
  studied, 
  

   and 
  O. 
  Miigge* 
  has 
  been 
  able 
  to 
  show 
  by 
  means 
  of 
  etch-figures 
  

   combined 
  with 
  crystallographic 
  reasoning 
  that 
  the 
  inversion 
  

   signifies 
  a 
  change 
  from 
  tetartohedral 
  hexagonal 
  to 
  hemihedral 
  

   hexagonal 
  symmetry, 
  the 
  axial 
  ratios 
  of 
  the 
  two 
  forms 
  being 
  

  

  Fig. 
  9. 
  

  

  

  

  

  

  

  

  

  

  

  

  

  '/■ 
  

  

  

  

  

  

  

  ?~ 
  .. 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

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  / 
  

  

  

  

  

  

  

  

  ----- 
  

  

  ~~°^ 
  

  

  

  \ 
  

  

  

  

  

  

  

  

  

  / 
  

  

  

  

  

  

  

  

  

  

  >>. 
  

  

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  * 
  

  

  - 
  - 
  

  

  

  

  

  

  

  

  

  Fig. 
  9. 
  Heating 
  and 
  cooling 
  curve 
  of 
  the 
  a- 
  (3 
  quartz 
  inversion. 
  Tempera- 
  

   ture-differences 
  plotted 
  against 
  temperatures. 
  Horizontal 
  scale 
  : 
  1 
  square= 
  

   200 
  microvolts 
  (approximately 
  20°) 
  ; 
  vertical 
  scale 
  : 
  1 
  square 
  = 
  6 
  microvolts 
  

   (approximately 
  O^ 
  ). 
  

  

  nearly 
  identical. 
  Quite 
  recently 
  Wright 
  and 
  Larsenf 
  have 
  

   extended 
  Miiggc's 
  work 
  and 
  have 
  shown 
  that 
  the 
  possibility 
  

   of 
  distinguishing 
  between 
  a 
  and 
  /3 
  forms 
  gives 
  to 
  quartz 
  con- 
  

   siderable 
  value 
  as 
  a 
  geologic 
  thermometer. 
  The 
  temperature 
  

   of 
  inversion 
  as 
  determined 
  in 
  a 
  thermal 
  microscope 
  is 
  given 
  as 
  

   575° 
  ± 
  2°. 
  

  

  *0. 
  Miigge, 
  Neues 
  Jahrb., 
  Festband, 
  1907, 
  181-196. 
  

  

  f 
  F. 
  E. 
  Wright 
  and 
  E. 
  S. 
  Larsen, 
  this 
  Journal, 
  (4), 
  xxvii, 
  162, 
  421-447, 
  

   lyuy. 
  

  

  