For reasons to be explained, this series of minerals has spawned a vast
literature. But happily for most of us, the essentials can be sifted rather
The way the books often present the subject is with a triangle. So let’s
try that. At the top we may visualize potassium (K, for kalium, the original
name); at the lower left-hand corner we’ll put Na (for sodium) and at the
right hand corner put Ca, for calcium.
The sides of the triangle represent percentages so for example as one moves
from the Na corner to the Ca corner we assume zero Na to 100% Ca. This side
represents the most important ones for general use, though strictly, this happy
simplicity is confounded by the fact that the mixture of Na (AlSi3O8)
dubbed Ab (for albite) to Ca (Al2Si2O8)
dubbed An (for anorthite) is continuous.
As for the pointy end of the diagram it’s usually labeled Or for
orthoclase, formula K (ALSi3O8).
Further confusion is injected by the effects of pressure and temperature of
formation which changes the crystal structure, though a lump of say the
potassium feldspar orthoclase may look like a lump of the potassium feldspar
But have heart, dear readers, all is not obfuscation (?).
The series Ab to An has been arbitrarily sectioned off into six handy blocks
depending on the Na (Ab) to Ca (An) content. Ready? – albite, oligoclase,
andesine, labradorite, bytownite and anorthite. Ab used here stands for albite,
a white feldspar rich in sodium (Na). An is used to represent the calcium-rich
feldspar called anorthite, dark gray to black.
How to tell them? Tough in the field but generally a clue is color; the more
Na (Ab) the lighter the color; the more Ca (An), the darker. And one,
labradorite is famous as building stone for its silky blue sheen in a gray mass.
Potassium feldspars – orthoclase and microcline – are generally
flesh-to-salmon pink, have very good cleavage and, if green, the microcline kind
makes a nice gemstone as amazonite.
As for those "big six" above, they are lumped together as plagioclases.
So you see there is a way out of this mess!
Uses? Not many considering their abundances. Mostly microcline and orthoclase
are used in ceramic glazes and mild abrasives in cleaners (H6 on Mohs scale).
Kaolin, a valuable white clay may form when feldspars decay and most soil clays
owe their origin to feldspars.
Now the good news.
The test announced for tomorrow on feldspars has been cancelled indefinitely.
Life is not all gloom!