﻿524 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  silicates 
  are 
  probably 
  the 
  most 
  important 
  form 
  of 
  its 
  occurrence 
  in 
  

   clay, 
  and 
  are 
  represented 
  by 
  the 
  minerals, 
  mica, 
  hornblende, 
  chlorite 
  

   and 
  pyroxene. 
  These 
  are 
  scaly 
  minerals 
  containing 
  from 
  15^-25^ 
  

   of 
  magnesia. 
  Mica 
  is 
  a 
  very 
  common 
  constituent 
  of 
  many 
  clays, 
  

   and 
  its 
  shining 
  scales 
  easily 
  render 
  it 
  recognizable. 
  Chlorite 
  scales 
  

   may 
  be 
  present 
  in 
  many 
  clays, 
  and 
  if 
  in 
  abundance 
  color 
  the 
  clay 
  

   green. 
  Hornblende 
  also 
  is 
  not 
  an 
  uncommon 
  constituent, 
  and 
  

   specially 
  present 
  in 
  clays 
  derived 
  from 
  rocks 
  of 
  very 
  basic 
  composi- 
  

   tion, 
  that 
  is, 
  those 
  with 
  a 
  low 
  silica 
  percentage. 
  Indeed 
  the 
  de- 
  

   composition 
  of 
  hornblende 
  may 
  give 
  rise 
  to 
  a 
  hydrous 
  aluminum 
  

   silicate, 
  which 
  is 
  highly 
  colored 
  by 
  iron, 
  the 
  product 
  therefore 
  being 
  

   a 
  ferruginous 
  clay. 
  (G. 
  P. 
  Merrill's 
  Rocks, 
  rock-weathering 
  and 
  

   soils, 
  p. 
  21) 
  

  

  Dolomite, 
  the 
  double 
  carbonate 
  of 
  lime 
  and 
  magnesia, 
  may 
  be 
  

   a 
  source 
  of 
  magnesia 
  as 
  well 
  as 
  of 
  lime 
  in 
  clay. 
  

  

  Magnesium 
  sulfate, 
  or 
  Epsom 
  salts, 
  occurs 
  sparingly 
  in 
  clays, 
  but 
  

   when 
  present 
  may 
  give 
  rise 
  to 
  the 
  formation 
  of 
  a 
  white 
  coating 
  on 
  

   the 
  surface 
  of 
  the 
  ware. 
  It 
  is 
  commonly 
  found 
  in 
  those 
  clays 
  where 
  

   sulfuric 
  acid, 
  set 
  free 
  by 
  the 
  decomposition 
  of 
  pyrite, 
  has 
  attacked 
  

   magnesium 
  carbonates. 
  The 
  presence 
  of 
  this 
  salt 
  can 
  frequently 
  be 
  

   detected 
  by 
  the 
  bitter 
  taste 
  which 
  it 
  imparts 
  to 
  the 
  clay. 
  

  

  The 
  chemical 
  effects 
  of 
  magnesia 
  in 
  clays 
  are 
  probably 
  similar 
  to 
  

   those 
  produced 
  by 
  lime. 
  This 
  is 
  not 
  to 
  be 
  taken 
  as 
  absolutely 
  cei^ 
  

   tain, 
  for 
  magnesia, 
  is 
  present 
  in 
  most 
  clays 
  in 
  such 
  small 
  amounts 
  

   as 
  to 
  make 
  its 
  exact 
  action 
  uncertain. 
  

  

  The 
  range 
  of 
  the 
  percentage 
  of 
  magnesia 
  in 
  the 
  different 
  clays, 
  

   deduced 
  from 
  the 
  analyses 
  given 
  at 
  the 
  end 
  of 
  this 
  report, 
  is 
  as 
  

   follows 
  : 
  

  

  Quality 
  Min. 
  Max. 
  Aver. 
  

  

  Brick 
  clays 
  02 
  11.03 
  2.66 
  

  

  Pottery 
  clays 
  .05 
  4.80 
  .85 
  

  

  Fireclays 
  02 
  6.25 
  .513 
  

  

  KaoHns 
  tr 
  2.42 
  .223 
  

  

  