Introductory Geology

Project - Volcanoes

Part 2 : Mineral Properties

Although there are more than 3,000 recognized minerals, a relatively small number account for a large volume of the Earth's crust and mantle. (For now, we will not be concerned with the core except to note that there is an outer liquid core and an inner solid core. The core consists predominantly of an Iron- and Nickel-rich alloy.)

The "simplicity" of the mineralogy of the crust and mantle reflects the fact that eight chemical elements -- oxygen, silicon, aluminum, iron, calcium, magnesium, sodium and potassium -- account for about 99% of the weight of the crust and of the mantle.

The crust is enriched in oxygen, silicon, aluminum, potassium and sodium. The mantle is enriched in iron, calcium and magnesium. The oceanic crust has a composition intermediate between the continental crust and the mantle.

The most common minerals in the crust are silicates -- combinations of oxygen, silicon, and (usually) other positively charged ions.

The Mineral Gallery offers a full text search ability which you should use to complete this exercise.

• Critical Concepts:
  1. Solid Solutions: Most minerals do not have a fixed chemical composition (quartz is a notable exception). Plagioclase feldspars are a mineral group in which composition varies between two extremes (albite is the sodium-rich end member and anorthite is the calcium-rich end member). Plagioclase is concentrated in igneous rocks from the oceanic crust. Alkali feldspars are members of a solid solution series between a potassium-rich end and a sodium-rich end and are concentrated in the continental crust. About 65% of the weight of the crust (both continenal and oceanic) is contributed by feldspar and about 15% by quartz.

     Olivine and pyroxene are both solid solution series and are common silicate minerals concentrated in the mantle.

     A few minerals form primarily near the surface and are concentrated in sedimentary rocks -- halite (salt), calcite (calcium carbonate) and dolomite (calcium, magnesium carbonates).

      

  2. Polymorphism: Minerals which have the same composition but different structures are polymorphs. Diamond and graphite, for example, both are made up of 100% carbon and calcite and aragonite are both made up of calcium carbonate -- CaCO₃

      

Questions:

Physical Properties at the Mineral Galleries contains good discussions of the physical properties of minerals. Read the section on Specific Gravity.

1. Define, in your own words, specific gravity. How does this property relate to density?
2. Search Mineral Galleries for the specific gravities of :
   1. quartz _______
   2. olivine (use the Magnesium-rich end member ___________ and the Iron-rich end member ________)
   3. pyroxene (use augite) _________
   4. plagioclase [(use albite ________ and use anorthite _______)
   5. alkali feldspar (use microcline) ___________

       

Prepare a table for the above minerals with:

Mineral Name	Specific Gravity	Location

For location, identify where the mineral or mineral group is concentrated : (1) continental crust, (2) oceanic crust, or (3) mantle. In general, high pressures favor higher densities and high temperatures favor lower densities. Within the outer part of the Earth, the pressure effect tends to dominate.

From your table, generalize about the relationship between specific gravity and the concentration of these common minerals.

In general, the viscosity of a melt (a measure of its resistence to flow) is inversely proportional to its temperature. At high temperatures the melt tends to flow more readily than at low temperatures. Water can drastically modify the viscosity of a melt; the higher the water content, the lower the viscosity.

Questions:

1. Relate the type (quiet versus explosive) of eruption to the viscosity of a melt.
2. In your own words, compare and contrast rhyolites, andesites and basalts. Which of these, other things being equal, tends to have the highest viscosity?
3. At what types of plate settings do you tend to find these three lava types?

    

Answers to parts 1 and 2 are to be handed in on or before, October 15, 1998.

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