Only orthoclase can be pink or blue, and only plagioclase can be dark gray to black. Unfortunately, both can be light in color, and since most feldspars are nearly white we're often still faced with a nasty identification problem. If it's white and has striations, I call it plagioclase.
If it's white but I can't see any striations, it may be either plagioclase or orthoclase. In this case, I just call it feldspar and look for other clues to come up with a name for the rock. The following graphic summarizes the field identification of feldspar:. Each plagioclase grain is made up of stacks of thin crystal with their molecules arranged in opposite directions. This causes the cleavage planes to have fine parallel line across them called striations.
Another diagnostic feature is the feldspars colour range. In this case, the colour typically ranges from white to gray. The main feldspars in this grouping include:. Plagioclase Feldspar and Hornblende. Road cut on highway East of Quadeville, Ontario.
Unlike plagioclase, orthoclase does not have striations across its cleavage plane. Cleavage faces will appear smooth. Orthoclase also typically has a colour range between white and pink, which is different than that of plagioclase. Feldspars which are in this grouping include:. Orthoclase Crystals. Goodsprings, Nevada. The feldspar group is very large and there are many different chemical formulas.
Before a feldspar can be used its chemical composition must be determined. Different feldspars will behave differently. Some uses for the feldspar family in general include:. Wood Cox Mine, Bancroft. Feldspar is an incredibly abundant mineral.
It has been found on the moon and in some meteorites. Feldspars are found in the every section of the rock cycle. Many feldspars are igneous as they commonly precipitate out in magma as it cools. They may also be formed as metamorphic minerals in veins of other rocks. This process involves both heat and pressure.
Fragments of pure feldspar crystals thus tend to form rectangular blocks with irregular ends. Feldspars have vitreous lusters and occur in opaque shades of white to gray to pink to very dark gray.
Other rarer colors are also possible! The dark and pink samples display the tiny veinlets commonly seen in K-spar. This photo shows the very thin surface striations that are seen on some cleavages of plagioclase, but never in K-spar. Unlike the tiny veinlets of the K-spar sample above, the striations form on the surface of a cleavage plane the veinlets are internal color variations and the striations are perfectly parallel.
Click here to see the right-angle cleavages of this sample.
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