Clue 1 - Minerals - Station 4     Minerals in igneous rocks:  Dark minerals
Wednesday, May 09, 2012
8:39 AM
Station 4:  Amphibole, pyroxene and olivine 
The dark minerals
Birthstones
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Green sand beach, Hawaii
A rare place where the olivine crystals that were in basalt accumulate into piles of green sand.
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Olivine in meteorites
Rare meteorites that contain large crystals of olivine in a background of iron-rich metal.  These meteorites are thought to be bits of the insides of planets.  Olivine and iron occur near our core-mantle boundary.  When planetoids collide, the rocks go flying across the solar system as meteorites.\
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Peridot gemstone
A rare olivine gem cut from a single crystal of olivine that was unusually clear, flawless and very green.
Smithsonian
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More about olivine
Olivine
Wikipedia
Amhibole, pyroxene and olivine -  They are considered the dark minerals because they are generally green, greenish-gray, various shades of gray, and black.   
Learn how to identify the mineral olivine.  
Because olivine is usually found deep in the Earth's mantle, rocks made up entirely of olivine are less common at the surface.  Exceptionally large, clear and brightly colored crystals of olivine are called "peridot" when used as gemstones .  Is peridot your birthstone?
What color is olivine?  ● 
Olivine usually forms clumps of sandy-looking grains.  Individual crystals can be transparent to cloudy.
Learn how to identify the minerals amphibole and pyroxene.  
Look at specimens of amphibole and pyroxene.  Both are commonly black to dark green and difficult to tell apart, particularly when the individual crystals within a rock are tiny.   If you come across a very dark mineral, it could be mica, amphibole or pyroxene.  Mica tends to flake while the other two will not.  
Observe the flat planes that run through the crystals of amphibole and pyroxene. Those flat planes are called "cleavage planes."  Those planes are areas within each crystal where the atoms inside aren't held together or bonded strongly.  If you strike these minerals, they will tend to break or cleave along these planes.   
The planes of flakiness in mica are cleavage planes.  The cleavage planes in mica are more resistant to splitting and cut through crystals of amphibole and pyroxene at different but specific angles.  Geologists use these cleavage angles to distinguish amphibole from pyroxene.  But many times, amphibole and pyroxene cannot be distinguished by just look and special methods and equipment are used by geologists.
What color is amphibole?   ● 
What color is pyroxene? ● 
Is the color of amphibole or pyroxene diagnostic?
The word diagnostic describes a characteristic of a mineral that identifies its type. For example, we might say that extreme hardness is a "diagnostic" characteristic of diamond because only diamonds have that degree of hardness and we can use that characteristic to identify diamond.
Are cleavage planes very flat or wavy?
Use the handlens to take a good look. 