The  Mineral  Mosaic      

   

Quartz   (SiO2)   is   one   of   the   most   common   and   most   important   rock   forming  

minerals  of  the  silica  group.  Quartz  is  typically  colourless  or  white,  like  the  samples  used  

in  the  display  to  depict  the  snow  on  the  mountains.  The  white  color   is  caused  by  fluid  

inclusions  of  gas  and/or   liquid   that  gets   trapped  during  crystal   formation.  Some  other  

varieties   of   quartz   that   appear   in   the   display   are   agate,   tiger’s   eye,   aventurine,   and  

jasper.  

 

 

 

 

Agate   is  a  banded  form  of  chalcedony  and  a  cryptocrystalline  variety  of  quartz.  

Cryptocrystalline   means   the   crystal   structure   is   only   visible   under   magnification.   The  

banding   associated   with   agate   is   due   to   deposition   in   cavities   and   is   typically   found  

parallel  to  the  walls  of  the  cavity.  Natural  agate  shows  colourful  banding  and  can  also  be  

artificially  dyed.  This  is  easy  to  do  because  chalcedony  is  slightly  porous  and  allows  the  

dye  to  be  absorbed.  Agate   is  used   in   the  display  as  part  of   the  base  of   the  volcano  as  

well  as  the  ash  plume  being  ejected  from  the  volcano.        

 

 

 

 

 

 

 

Tiger’s  eye   is  formed  when  blue  asbestos  

starts   to   get   replaced   by   quartz.   The   tiger’s   eye  

can   retain   the   asbestos’   blue   colour   as   seen   in  

the   mountains   of   the   mosaic,   but   is   more  

commonly  seen  as  a  golden  brown  colour.  This  is  

from   the   oxidization   of   iron   and   can   be   seen   as  

part  of  the  base  of  the  volcano  in  the  mosaic.    

 

  Jasper   is   another   type   of   chalcedony  

or   cryptocrystalline   variety  of  quartz.   It   is   an  

opaque  variety  and  is  generally  red,  yellow  or  

brown.   The   variety   in  our  display   is   red,   and  

this   colour   is   due   to   included   colloidal  

particles  of  iron  oxides.      

 

  The  last  variety  of  quartz  used  in  the  mosaic  represents  grass  and  is  aventurine.  

It  is  characterized  by  its  translucency  and  a  shimmering  or  glistening  characteristic  called  

aventurescence.  Aventurine   is  most  commonly  green,  and  can  have  fuchsite   inclusions  

in   it.   Fuchsite   is   a   chromium-­‐rich   variety  of  muscovite   and   gives   aventurine   a   banded  

appearance.  This  can  cause  aventurine  to  be  classified  as  a  rock  sometimes.    

 

The   last   mineral   used   in   the   mosaic   is  

sodalite   (Na8(AlSiO4)6Cl2).   It   is   a   commonly   blue  

tectosilicate   mineral   with   a   translucent  

characteristic.  It  is  most  commonly  found  in  both  

volcanic   and   plutonic   rocks   of   the   nepheline  

syenite   family,   and   is   generally   associated   with  

nepheline.    

           

References    Berry,  L.  G,  Brian  Harold  Mason,  and  Richard  Vincent  Dietrich.  Mineralogy,  Concepts,  Descriptions,  Determinations.  San  Francisco:  Freeman,  1983.  Print.    All  photos  courtesy  of  The  University  of  Waterloo  Earth  Sciences  Museum.