The Silicates

These silicate minerals are grouped into sub-categories based on their chemical structure and abundance of silica (oxygen and silicon).  These sub groups are:
 

Olivine

Pyroxene Group

Amphibole Group (hornblende)

Micas  (biotite)

Feldspars    (very abundant ... 50% of the crust)

Quartz    (pure silica SiO₂)

There are several reasons for these sub-divisions.  First, there are so many silicates that subdivision becomes necessary.  The divisions themselves are based on the amount of silica in the mineral and the ways the chemical bonds differ.  That is, all micas have a similar chemical structure.  I tend to think that knowing these differences are not very important in a class like this ... but it is very important to understand that these chemical differences effect the minerals overall properties.  That is, differences in composition and arrangement of atoms affects things like density, melting point, how it looks, how hard it is, how its crystals are shaped, etc.

Physical Properties of the Silicates

This chart is the key to understanding the major physical properties of the silicates.  They are:

 

Silicate	Density (solid)	Melting Point	Silica Content	Viscosity
Olivine	High	High 1000-1200 C	Low (around 50%)	Low (melted butter)
Pyroxene Group
Amphibole Group
Micas
Feldspars
Quartz	Low	Low 600-800 C	High (around 70%)	High (thick like honey)

Viscosity refers to the ability to flow when it is in the liquid state.  A high viscosity means it will be very sticky (when melted) and tends to flow like honey.  A low viscosity means it flows quite easily.  The reason for these differences is directly related to the percent of silica in the mineral.  Much of the silicon and oxygen in these minerals form tetrahedrons (3 sided pyramids)  in which one silicon atom is surrounded by 4 oxygen atoms.  These tetrahedrons tend to link to other similar tetrahedrons in groups or long chains.   Lots of silica means lots of tetrahedron chains.  Even when the mineral appears molten, many of these tetrahedral chains are still intact ... increasing the viscosity of the melt.

Basalt and Granite

There is another common rock you should know -  Andesite is a common rock which comes out of many volcanoes.  It is somewhere between the two extremes - granite and basalt.  It is named for the Andes mountain chain in South America (which was formed from volcanoes).  Because of its high silica content, it has a rather high viscosity.

Now the question becomes:  How did the earth acquire these properties?

The rock type you get depends on several factors, but the main things are:
 

• Minerals available at the time of formation
  (You can only make an angle food cake if you have the right ingredients)
   
• Conditions at the Time of Formation
  
  
  • Intrusive (forming deep underground) vs. Extrusive (forming at or near surface).  This will directly effect the time available for crystals to form.  As molten rock cools, chemical bonding occurs.  It is kind of like choosing a mate.  If you do it in a hurry, you tend to match up with whoever is nearby.  If you have lots of time, you can be choosy and pick the best one available.  Atoms play the same game.  As a melt cools slowly, the atoms tend to become more choosy .... many times bonding with similar atoms.  This forms larger crystals (course grains).  This will happen if the melt is cooling deep underground.  If the melt is cooling closer to the surface, it tends to cool faster ... forming smaller crystals (fine grain) ... all because the atoms are in a hurry finding a partner.  Below are some common examples.  All have the same chemical composition but form in different conditions ...slowest to fastest.
     
    • Granite,  Rhyolite, Obsidian
    • Diamond vs. Graphite
    • Quartz vs. Glass
      
      
  • Presence of liquid water (important when considering subducted crust)
    • if wet (water is present), melting point decreases.
    • if dry (water not present), melting point rises.
      
      
  There are many scenarios which occur in nature to make different kinds of minerals....can you think of some?  We will cover several in the next section.  The most important one is in explaining why the continents are made mostly of granite and the oceanic crust is mostly basaltic rock.
   
  ©Jim Mihal 2004, 2006 - all rights reserved