K-feldspar and plagioclase feldspar

Feldspars have two cleavage planes that intersect at 90°. 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 feldspars are divided into two main groups: Potassium feldspar ("K-spar") and plagioclase ("plag"). Both display two cleavages and an overlapping range of colors, but only plagioclase displays tiny grooves on one cleavage known as striations. The photos below show several examples of feldspar. Click on any photo to enlarge!

This white feldspar shows two cleavages (top/bottom and sides) plus the fracture surface (front).

Both plagioclase and K-spar are commonly white; the lack of striations suggests that this is K-spar.

The salmon-pink color is typical of K-spar. Note the two cleavage surfaces and one fracture surface.

There are no striations.

Here are some of the more common colors of feldspar.

Broken feldspars do not always make perfect cleavage blocks for many reasons. Just look to see two planes intersecting at 90°!.

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.

These are two well-formed feldspar crystals taken from a weathered granite in the Mojave Desert of California.

The blocky shape of well-formed crystals plus the two cleavages at right angles to each other mean that feldspar crystals in rocks tend to have straight edges and planar surfaces.

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