Perthite Identification Guide
Recognizing perthite, a feldspar showing intergrown stringers of albite in K-feldspar, and separating it from plain feldspar and graphic granite.
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What Perthite Looks Like
Perthite is not a single mineral but an intergrowth: thin lamellae, veins, or patches of sodium feldspar (albite) exsolved within a host of potassium feldspar (orthoclase or microcline). The result is a feldspar showing a streaky, flame-like, or wavy two-tone pattern — typically a pinkish, salmon, or cream K-feldspar threaded with whiter albite stringers. Luster is vitreous to pearly, and it is translucent to opaque. The intergrowth often produces a subtle silvery sheen on cleavage faces, and amazonite perthite can be blue-green.
Step-by-Step Field ID Checklist
- Look for two intergrown feldspars. Pale stringers or patches within a pink/cream host are the signature.
- Catch the sheen. Tilt the stone; the lamellae can throw a soft silvery to bluish schiller.
- Find cleavage. Two cleavage directions near 90° confirm feldspar.
- Test hardness. Scratches glass at Mohs 6–6.5.
- Check the pattern scale. Perthite intergrowths are fine, irregular streaks (use a loupe); coarse, regular intergrowth with quartz is graphic granite instead.
Key Diagnostic Tests
- Hardness: 6–6.5; scratches glass.
- Streak: White.
- Cleavage: Two good cleavages meeting at ~90°, blocky stepped breaks.
- Luster: Vitreous to pearly, sometimes with schiller from the lamellae.
- Density: ~2.55–2.63, light.
- Acid/magnetism: No reaction; non-magnetic.
Common Look-Alikes and How to Tell Them Apart
- Plain orthoclase/microcline: Uniform single-color feldspar without the streaky two-feldspar intergrowth; perthite shows visible pale albite veining.
- Graphic granite: Regular, angular, runic intergrowth of quartz wedges in feldspar; quartz is harder (7) and glassier, whereas perthite's intergrown phase is another feldspar with the same hardness and cleavage.
- Moonstone: Shows a floating adularescent glow but lacks the visible separate pale stringers of perthite.
- Gneiss/migmatite banding: A rock-scale layering, far coarser than the mineral-scale intergrowth of perthite.
- Sunstone (aventurescent feldspar): Has metallic platelet glints rather than feldspar stringers.
Seeing two distinct but similarly soft, cleaving feldspar phases intergrown is the key to perthite.
Where Perthite Is Found
Perthite is extremely common in slowly cooled, potassium-rich igneous rocks — granites, syenites, and especially pegmatites — and in high-grade gneisses, because the intergrowth forms when an originally homogeneous alkali feldspar unmixes during slow cooling. The name comes from Perth, Ontario, Canada. Look in granite and pegmatite quarries worldwide.
Frequently asked questions
How do you identify perthite?
Look for a feldspar with pale albite stringers or patches intergrown in a pink or cream potassium-feldspar host, two cleavages near 90°, and a hardness of 6–6.5 that scratches glass.
What is the difference between perthite and graphic granite?
Perthite is an intergrowth of two feldspars (albite in K-feldspar), while graphic granite is a regular intergrowth of quartz in feldspar; the quartz in graphic granite is harder and glassier.
Is perthite a mineral or a texture?
Perthite is a texture or intergrowth, not a distinct mineral species. It forms when a single alkali feldspar slowly unmixes into separate potassium-rich and sodium-rich feldspar phases.
Why does perthite have stripes?
The stripes are exsolution lamellae: as alkali feldspar cools slowly, sodium and potassium feldspar become unstable as one phase and separate into alternating bands or veins.
Where does perthite form?
It forms in slowly cooled granites, syenites, pegmatites, and high-grade gneisses, allowing time for the feldspar to unmix; it is one of the most common feldspar textures in coarse igneous rocks.
Perthite identified by the community
Recent Perthite specimens identified with Rock Identifier.
