﻿506 
  Is^EW 
  YOEK 
  STATE 
  MUSEUM 
  

  

  • 
  

  

  most 
  commonlj 
  present 
  and 
  we 
  may 
  also 
  find 
  calcite, 
  gypsum, 
  mica, 
  

   siderite 
  or 
  carbonate 
  of 
  iron, 
  pjrite, 
  dolomite, 
  iron 
  oxid, 
  etc. 
  

  

  Quartz. 
  This 
  mineral 
  is 
  present 
  in 
  sedimentary 
  clays 
  mostly 
  in 
  

   tbe 
  form 
  of 
  fine 
  grains, 
  or 
  sometimes 
  in 
  crystals, 
  while 
  in 
  resid- 
  

   ual 
  clays 
  the 
  particles 
  ai-e 
  usually 
  angular. 
  It 
  may 
  be 
  colorless, 
  but 
  

   the 
  grains 
  may 
  be 
  often 
  superficially 
  colored 
  either 
  red 
  or 
  yellow 
  

   by 
  iron 
  oxid. 
  It 
  is 
  a 
  very 
  hard 
  mineral 
  and 
  scratches 
  glass 
  easily. 
  

   Feldspar 
  might 
  be 
  mistaken 
  for 
  it, 
  but 
  feldspar 
  will 
  not 
  scratch 
  

   glass. 
  

  

  Flint 
  or 
  amorphous 
  silica 
  is 
  sometimes 
  present 
  in 
  clays. 
  It 
  

   usually 
  has 
  a 
  muddy 
  color, 
  and 
  a 
  conchoidal 
  fracture. 
  It 
  might 
  

   be 
  found 
  in 
  either 
  residual 
  or 
  sedimentary 
  clays. 
  

  

  Quartz 
  and 
  flint 
  are 
  infusible 
  except 
  at 
  very 
  high 
  temperatures; 
  

   but 
  the 
  presence 
  of 
  other 
  minerals 
  in 
  the 
  clay 
  acting 
  as 
  fluxes 
  often 
  

   causes 
  them 
  to 
  soften 
  at 
  a 
  much 
  lower 
  temperature. 
  In 
  addition 
  

   quartz 
  serves 
  to 
  diminish 
  the 
  shrinkage 
  of 
  a 
  clay, 
  and, 
  if 
  not 
  natur- 
  

   ally 
  present 
  in 
  sufficient 
  quantity, 
  has 
  to 
  be 
  added 
  during 
  the 
  pro- 
  

   cess 
  of 
  manufacture. 
  The 
  admixture 
  of 
  quartz 
  also' 
  tends 
  to 
  de- 
  

   crease 
  the 
  plasticity, 
  the 
  more 
  so, 
  the 
  coarser 
  the 
  grain. 
  The 
  size 
  

   of 
  the 
  quartz 
  grains 
  affects 
  the 
  ease 
  vnth 
  which 
  they 
  can 
  be 
  fluxed; 
  

   for, 
  as 
  fusion 
  begins 
  on 
  the 
  outside 
  of 
  a 
  quartz 
  grain, 
  the 
  larger 
  

   the 
  grain 
  the 
  longer 
  it 
  will 
  take 
  to 
  reach 
  the 
  center. 
  Therefore 
  if 
  

   the 
  heat 
  is 
  not 
  continued 
  long 
  enough, 
  it 
  may 
  happen 
  that 
  the 
  

   outside 
  of 
  the 
  grain 
  has 
  been 
  softened 
  and 
  the 
  center 
  is 
  unaffected. 
  

  

  Feldspar. 
  Since 
  kaolinite 
  is 
  formed 
  by 
  the 
  decomposition 
  of 
  

   feldspar, 
  it 
  seems 
  but 
  natural 
  that 
  we 
  should 
  fijid 
  some 
  undecom- 
  

   posed 
  gi^ains 
  of 
  the 
  latter 
  in 
  almost 
  every 
  clay. 
  The 
  fragments 
  

   would 
  be 
  scaly 
  or 
  rhombohedral 
  in 
  form. 
  Feldspar 
  is 
  slightly 
  

   softer 
  than 
  quartz, 
  and 
  while 
  the 
  latter 
  scratches 
  glass, 
  the 
  former 
  

   will 
  not. 
  It 
  is 
  commonly 
  pink, 
  red, 
  yellow 
  or 
  even 
  white. 
  Few 
  

   fragments 
  fail 
  to 
  show 
  a 
  white 
  coating 
  on 
  the 
  surface 
  of 
  the 
  grains, 
  

   or 
  lining 
  the 
  cracks 
  and 
  cleavage 
  planes 
  of 
  the 
  mineral, 
  indicating 
  

   the 
  presence 
  of 
  some 
  kaolinite. 
  

  

  Calcite. 
  This 
  mineral 
  may 
  occur 
  in 
  clays 
  in 
  the 
  form 
  of 
  little 
  

   rhombohedral 
  grains, 
  soft 
  enough 
  to 
  be 
  scratched 
  with 
  a 
  knife. 
  

  

  