Minerals - Station 1     Minerals in igneous rocks: The Feldspars
Wednesday, May 09, 2012
8:39 AM
Station 1:  The feldspars - orthoclase and plagioclase feldspar 
The most common minerals in igneous rocks and the Earth's crust
To learn more 
Did you know that major streets in Pacific Beach, California are named after minerals?  
• Can you find the "mineral" streets on a map?  

• What street is between Diamond and Felspar? 

• Do these streets run mostly North-South or East-West? 

• How old are these street names?
Article for advanced readers
Pacific Beach and Its Street Names 
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Plagioclase feldspars from the Moon
Astronauts collected rocks from the Moon during the first  Apollo mission  landing and field trip to the Moon in 1969. When sliced very thin and viewed under a microscope using polarized light, the minerals appear in bright colors and you can see the bold striations of lunar plagioclase feldspars! 
Images
Moon Rocks Through the Microscope
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Feldspars as gemstones
The gemstone called "Moonstone" is a variety of orthoclase feldspar,
Image
Moonstone from the Smithsonian Instituaion
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More info
Feldspar   From Wikipedia
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Note:
"Orthoclase feldspar" can also be called "potassium feldspar."  
Look at the orthoclase and plagioclase feldspars. 
These minerals  are all feldspars.   
They are the most common mineral in the Earth's crust.
Be able to tell apart plagioclase and orthoclase feldspar. 
You need this skill  to identify your mystery rock. 
What color is orthoclase feldspar?
What color is plagioclase feldspar?
Which feldspar comes in a pink color?
Expert tip: - Not always, but most of the time when you find a mineral that has a salmon pink in color, it will be orthoclase feldspar.  This mineral is common. Salmon pink is a little more orangey than just pink.  The color can very light and nearly whitish pink to much darker red.
Can you tell apart orthoclase and plagioclase by color when they are both white?
So how can you tell plagioclase apart from orthoclase feldspar and other similar-looking minerals?
Look for the striations found on the crystals of plagioclase feldspar.
Striations are rows of very tiny lines that run across certain faces of plagioclase specimens.
Are the striations on the specimen perfectly straight or wiggly and only sort of straight?
To find the striations, turn the crystal around until light catches on them.  You may need to use a hand lens.