Perthite: The Pale Streaks Hidden Inside Some Feldspar
A pink or pale feldspar crystal may contain thin, lighter streaks. Some of these patterns are perthite: an intergrowth of two feldspars within what appears to be one grain.
A mixture that separates while cooling
At high temperatures, alkali feldspar can contain both sodium and potassium in its structure. As conditions change during cooling, an intermediate composition can separate into sodium-rich and potassium-rich parts.
Geologists call this process exsolution. The separation happens within the solid material. It does not require the crystal to melt again.
What the streaks contain
In perthite, the main host is potassium-rich feldspar. The lighter intergrown material is commonly albite, a sodium-rich feldspar. The intergrowth may appear as thin lines, patches, or irregular bands.
Some examples are visible by eye. Finer examples need a microscope, and very fine intergrowths require other methods to study.
Why the texture matters
Perthite gives geologists information about changes within feldspar after its early growth. It is a texture, rather than a new mineral species.
Not every pale line in feldspar is perthite. Cracks, alteration, and other internal patterns can look similar. A careful description should separate an observation—“pale streaks”—from a confirmed identification.
Next time you look at a feldspar specimen, try looking inside the grain as well as at its outer shape. Small patterns can reveal a long cooling history.
Sources: Open University: Feldspars; Mindat: Perthite.
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