Feldspar
Aluminosilicate group (K,Na,Ca)(Al,Si)4O8
The most abundant mineral group in Earth's crust, feldspars are aluminosilicates that form much of granite and many igneous rocks.
- Mohs hardness
- 6-6.5
- Color
- white, pink, grey, cream, salmon, to greenish or bluish
- Type
- mineral
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Overview
Feldspar is not a single mineral but a group of closely related aluminosilicate minerals that together make up roughly 60% of Earth's crust, making them the most abundant minerals on the planet.
The group splits into the alkali (potassium) feldspars — orthoclase, microcline, sanidine — and the plagioclase series, which ranges continuously from sodium-rich albite to calcium-rich anorthite. Colors range from white and grey to salmon-pink, with gem varieties including moonstone, labradorite, sunstone, and amazonite.
Feldspars are defining components of granite, syenite, basalt, and countless other rocks, and weather into the clay minerals that form soils.
Formation & geology
Feldspars crystallize directly from cooling magma and are primary minerals in nearly all igneous rocks: potassium feldspar dominates granite and rhyolite, while calcium-rich plagioclase dominates basalt and gabbro.
They also form in metamorphic rocks such as gneiss and in some sedimentary rocks (arkose) where feldspar grains survived weathering. Large, gem-quality crystals grow slowly in pegmatites.
Because feldspars are unstable at Earth's surface, they gradually weather to clay, releasing potassium and sodium — a process central to soil formation. They occur in granites and pegmatites worldwide.
How to identify it
Key features are two cleavage directions meeting at roughly 90 degrees, a hardness of 6-6.5 (scratches glass), and a vitreous-to-pearly luster on cleavage faces. Streak is white.
Plagioclase often shows fine parallel striations (twinning lines) on cleavage surfaces; microcline may be green (amazonite); orthoclase is commonly salmon-pink. Many show a flash of color (schiller/labradorescence) in gem varieties.
Look-alikes: Quartz is harder (7), has no cleavage, and breaks with conchoidal fracture rather than flat faces. Calcite is softer and fizzes in acid. The 90-degree cleavage and glass-scratching hardness separate feldspar from most look-alikes.
Uses & significance
Industrially, feldspar is essential to the ceramics and glass industries, where it acts as a flux that lowers melting temperature, and as a filler in paint, plastics, and rubber. It is a major raw material for porcelain, tiles, and glazes.
Gem and ornamental feldspars are highly valued: moonstone, labradorite, sunstone, amazonite, and orthoclase are cut for jewelry and carvings.
Feldspar is also mined for its potassium content. In metaphysical traditions the gem varieties carry their own meanings (intuition, transformation), but the group's main importance is geological and industrial.
Frequently asked questions
What is the difference between feldspar and quartz?
Both are common in granite, but feldspar has flat cleavage faces meeting near 90 degrees and is slightly softer, while quartz has no cleavage and breaks with curved, glassy fractures.
Is moonstone a feldspar?
Yes. Moonstone, labradorite, sunstone, and amazonite are all gem varieties of the feldspar group.
Why is feldspar important?
Feldspar is the most abundant mineral group in the crust and is a critical flux in glass and ceramics manufacturing.
What does feldspar weather into?
At Earth's surface feldspar gradually breaks down into clay minerals such as kaolinite, which form much of the world's soil.
Feldspar guides
In-depth guides for identifying, valuing, and understanding Feldspar.
