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Sanidine
Sanidine is tectosilicate (alkali feldspar, high-temperature), (K,Na)AlSi₃O₈. In the hand, it rates 6 on the Mohs scale, and it runs water clear and smoky gray. The volcanic feldspar, glassy water-clear tabular crystals frozen in young lavas, like windowpanes embedded in stone. Young volcanics: Eifel (Germany, gemmy giants), Itrongay (Madagascar, golden gem feldspar), Colorado (USA).
The geology.
Sanidine carries the formula (K,Na)AlSi₃O₈. It belongs to the monoclinic system. It is classified as tectosilicate (alkali feldspar, high-temperature).
It sits at 6 on the Mohs scale. The surface reads vitreous, glassy. Light behaviour is transparent to translucent. Specific gravity runs 2.52 to 2.62. The streak is white. It breaks with two cleavages at 90°.
The volcanic feldspar, glassy water-clear tabular crystals frozen in young lavas, like windowpanes embedded in stone.
You will most often meet it as glassy tabular crystals in lava, loose gemmy cleavages or phenocrysts in trachyte and rhyolite.
The origins.
Young volcanics: Eifel (Germany, gemmy giants), Itrongay (Madagascar, golden gem feldspar), Colorado (USA).
On value, the database notes: affordable; gem Eifel and Itrongay stones are collector items.
Traditional associations.
In energy work traditions, sanidine has little established lore; Madagascar's golden gem form borrows sunstone warmth themes.
People buy on this language, so it belongs on the page. It belongs under the facts, though, not over them.
Spotting the real thing.
The volcanic feldspar, glassy water-clear tabular crystals frozen in young lavas, like windowpanes embedded in stone.
| Looks like | How to tell them apart |
|---|---|
| Orthoclase | Same chemistry, slower biography, orthoclase is the cloudy pink plutonic form; sanidine is volcanic and glass-clear |
| Quartz phenocrysts | Quartz shows no cleavage flash and rounder outlines |
| Calcite cleavages | Calcite fizzes and is far softer |
At-home tests.
You can run every one of these at a kitchen table. Good light matters more than gear when you are looking at sanidine.
| Test | What you should see |
|---|---|
| By eye | Glass-clear with right-angle cleavage flash |
| By eye | Embedded in young volcanic rock |
| Scratch check | Scratches glass with effort |
Anything that scratches, burns or dissolves the piece is a lab job. Get it appraised rather than testing it apart.
Care & handling.
At 6 on the Mohs scale it handles careful wear but scratches against quartz and dust. Store it separately and take rings off before rough work.
It breaks with two cleavages at 90°, which is worth knowing before you set it or drop it.
Warm water and a soft cloth clean almost everything. Ultrasonic tanks, steamers and household cleaners are where pieces get ruined.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
The formula on record is (K,Na)AlSi₃O₈. Crystal system: Monoclinic. Specific gravity: 2.52 to 2.62. Streak: White. Cleavage and fracture: Two cleavages at 90°.
How it reaches the market
Young volcanics: Eifel (Germany, gemmy giants), Itrongay (Madagascar, golden gem feldspar), Colorado (USA).
What it gets confused with
Orthoclase: Same chemistry, slower biography, orthoclase is the cloudy pink plutonic form; sanidine is volcanic and glass-clear. Quartz phenocrysts: Quartz shows no cleavage flash and rounder outlines. Calcite cleavages: Calcite fizzes and is far softer.
What it is worth
Affordable; gem Eifel and Itrongay stones are collector items.
One more thing
Sanidine is feldspar quenched young, erupt it fast and it stays glassy; let it sit underground and it matures into ordinary orthoclase.
An education-first guide.
No product links here. We do not sell sanidine, and a guide that only covers our own shelves would not be much of a guide.
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