FELDSPAR
Feldspar: Properties, Varieties, Formation & Identification
Feldspar is the most abundant mineral group in Earth’s crust. Feldspars are aluminosilicates containing potassium, sodium, or calcium and occur in many igneous, metamorphic, and sedimentary rocks. They range from clear and white to pink, gray, green, and other colors, depending on composition and alteration.
Table of Contents
Feldspar Mineral Groups
Feldspars are commonly divided into potassium feldspars, such as orthoclase and microcline, and plagioclase feldspars, a series ranging from sodium-rich albite to calcium-rich anorthite. Their chemistry and crystal structure make them important clues to how rocks formed and cooled.
How Feldspar Forms
Feldspar crystallizes from many magmas and also forms or recrystallizes during metamorphism. Weathering can break feldspar down into clay minerals, while grains may survive transport and become part of sedimentary rocks. The mineral assemblage and texture help geologists infer a rock’s history.
Physical Properties
Feldspars generally have Mohs hardness around 6 and two cleavage directions that intersect close to right angles. Luster is typically vitreous, and fresh surfaces may show a pearly sheen. Some varieties display twinning or fine internal structures visible as lines or patterns.
Varieties and Optical Effects
Orthoclase and microcline are potassium feldspars; plagioclase includes albite and anorthite end members. Labradorite may show iridescent flashes known as labradorescence, while moonstone can display a soft floating glow called adularescence. These effects depend on internal structure and light scattering, not surface coatings alone.
Identification
Cleavage, hardness, crystal habit, twinning, and optical effects can help distinguish feldspar from quartz and other minerals. Quartz lacks feldspar’s prominent cleavage. Reliable identification of fine-grained rocks may require microscopy or laboratory analysis, since individual grains can be difficult to judge by eye.
Care and Collecting
Clean sturdy specimens with a soft brush and mild water-based cleaning only when the matrix is stable. Avoid strong chemicals and protect polished feldspar gems from hard impacts. With Mohs hardness near 6, feldspar can be scratched by harder minerals, so store specimens separately.
Frequently Asked Questions
Is feldspar one mineral?
No. Feldspar is a group of related minerals with different chemical compositions.
Is labradorite feldspar?
Yes. Labradorite is a plagioclase feldspar known for its characteristic flashes of color.
Further Reading & Sources
- Mindat — mineral data and localities.
- Handbook of Mineralogy — technical descriptions.
- Encyclopaedia Britannica: Feldspar — general overview.
- Wikipedia: Feldspar — background and references.