Feldspar, a group of rock-forming minerals, constitutes approximately 60% of Earth’s crust. It is a widespread component found in various igneous, metamorphic, and sedimentary rocks. There is no single answer to the question “what color is feldspar,” as this mineral group exhibits a remarkable spectrum of colors, making it one of the most visually diverse mineral families.
Feldspar’s Diverse Color Palette
Feldspar minerals display an extensive range of colors. Common hues include white, gray, and various shades of pink, from pale flesh tones to brick-red. Feldspar can also appear in orange, red, green, blue, colorless, or nearly black. The specific color can vary considerably even within the same type of feldspar.
Factors Influencing Feldspar’s Color
Several factors influence feldspar’s color. The presence of trace elements and impurities within the mineral’s chemical structure often dictates its hue. For example, iron can impart yellow, red, or blue colors, depending on its oxidation state and location within the crystal lattice. Lead, even in minute quantities, can contribute to blue or green coloration in some feldspar varieties.
Imperfections or specific arrangements within the crystal structure, known as lattice defects, also affect how light interacts with the mineral. These structural variations alter light absorption and reflection patterns, leading to distinct visual effects. Radiation damage, often from the decay of naturally occurring elements like potassium-40, can also induce color changes.
Tiny inclusions of other minerals are another significant factor. Microscopic platelets of hematite or goethite can create a spangly, glittery appearance. Copper inclusions can produce vibrant red or green colors, particularly in some plagioclase feldspars. These inclusions scatter light, contributing to the overall visual effect.
Some feldspars exhibit unique optical phenomena, such as iridescence or adularescence, caused by light scattering from microscopic internal structures. Labradorescence, seen in labradorite, creates a shimmering play of colors like blue, green, yellow, orange, and red due to light interference within fine exsolution lamellae. Adularescence, characteristic of moonstone, produces a milky, bluish sheen as light scatters through thin, alternating layers of different feldspar minerals.
Common Feldspar Varieties and Their Hues
Orthoclase, a potassium-rich feldspar, often presents in pink, white, or gray hues. Its presence is responsible for the pink coloration seen in many granites. Transparent orthoclase can also be colorless or pale yellow.
The plagioclase series encompasses a continuous range of feldspar minerals, varying in sodium and calcium content. Albite and anorthite, at opposite ends of this series, are typically white or gray. Intermediate members, such as labradorite, are often dark with a background color of gray, black, or brown, but are prized for their dramatic iridescent flashes of blue, green, yellow, orange, and even purple, an effect known as labradorescence. Oligoclase, another plagioclase, can be colorless, white, or tinted gray, green, or red, and may display iridescence.
Amazonite, a distinct blue-green variety of microcline (a potassium feldspar), owes its vibrant color to lead and water within its structure. This striking blue-green can also appear mottled with lighter green specks.
Moonstone, primarily a variety of orthoclase or albite, is renowned for its adularescence, a milky, bluish sheen that appears to glide across its surface. Its body color can be white, cream, peach, gray, green, or blue. Sunstone, often an oligoclase or orthoclase, exhibits aventurescence, a spangly appearance caused by light reflecting off tiny inclusions of minerals like hematite or copper. This creates warm colors such as orange, red, and yellow, with some rare specimens displaying green or blue flashes.