Feldspar is a common mineral found in igneous, metamorphic and sedimentary rocks. It makes up around 60% of the Earth’s crust. Feldspar minerals have very distinct properties that make them easy to identify.
Color
The most distinguishing characteristic of feldspar is its color. The name “feldspar” itself comes from the German “feld” meaning “field” and “spath” meaning “rock,” referring to the common grayish color of many feldspars. However, feldspars can exhibit a range of colors:
- Pink to red – potassium-rich feldspars such as microcline
- White to gray – sodium-calcium rich plagioclase feldspars such as albite
- Greenish – rare barium feldspars
The color is dependent on the chemical composition. Feldspars are primarily composed of silicon and aluminum, along with combinations of sodium, potassium, calcium and barium. Different elemental substitutions into the crystal lattice structure cause variations in color. The presence of iron, titanium and manganese as impurities also influences the color.
Luster
In addition to color, feldspar can be identified by its lustrous, glassy appearance. The minerals have a vitreous or pearly luster when fresh surfaces are fractured. This is caused by the way light reflects off the flat crystal faces of feldspar minerals. Microcline exhibits a flashier schiller effect with rainbow iridescence due to twinning planes in its structure.
Cleavage and Fracture
Feldspar crystals have good cleavage in two directions at 90 degree angles. This means they tend to break along smooth, flat faces that intersect at right angles. The cleavage planes are parallel to the length and width of the crystal faces, reflecting the underlying molecular structure of feldspar. Granite contains clumps of intergrown feldspar crystals that weather out in blocky fragments. In contrast, feldspar has a splintery fracture when broken in other directions besides cleavage.
Hardness
On the Mohs hardness scale, feldspars have a hardness of 6 to 6.5. They are harder than quartz (Mohs 7) but softer than most other common minerals like topaz, corundum and diamond. A steel knife blade can scratch feldspar. The hardness allows feldspars to persist through erosion and weathering processes, concentrating in sediments.
Specific Gravity
Feldspars tend to be relatively light for their volume. They have a specific gravity of 2.55 to 2.76. In other words, they are about 2.5 to 2.75 times as dense as water. Their lower density compared to other minerals allows feldspars to be concentrated in the upper parts of granitic magma chambers.
Crystal Form
Feldspar mineral species belong to the orthorhombic and triclinic crystal systems. The most common feldspar crystal forms are:
- Tabular – short and flat
- Lath-like – long and thin
- Square blocky – equal dimensions along length and width
Twinning, the intergrowth of two separate crystals, is common in feldspars and produces characteristic cross-hatch patterns. Granitic rocks often contain intergrown clusters of feldspar crystals with quartz and other minerals in a granite texture.
Common Feldspar Species
The feldspar mineral group includes several common rock-forming species:
Plagioclase Feldspars
- Albite (NaAlSi3O8) – sodium endmember, white
- Oligoclase (Na,Ca)(Al,Si)AlSi2O8) – sodium-calcium, colorless to creamy
- Andesine (Na,Ca)(Al,Si)AlSi2O8) – sodium-calcium, white to gray
- Labradorite ((Ca,Na)(Al,Si)AlSi2O8) – calcium-sodium, iridescent blue-green-gold
- Bytownite (CaAl2Si2O8) – calcium-rich, grayish white
- Anorthite (CaAl2Si2O8) – calcium endmember, white
Plagioclase feldspars form a solid solution series ranging between the sodium endmember albite and the calcium endmember anorthite. Intermediate compositions show finely intergrown albite and anorthite lamellae visible under a microscope.
Alkali Feldspars
- Orthoclase (KAlSi3O8) – potassium endmember, pink
- Microcline (KAlSi3O8) – potassium, gray with schiller
- Sanidine (KAlSi3O8) – high temperature potassium feldspar, glassy
- Anorthoclase – sodium-potassium feldspar
Alkali feldspars are potassium-rich and more abundant in felsic igneous rocks like granite. Orthoclase shows carlsbad twinning and microcline has distinctive tartan twinning.
Less Common Feldspars
- Celsian (BaAl2Si2O8) – barium
- Hyalophane (K,Ba)[Al(Si,Al)Si2O8] – barium-potassium
Rarer barium-bearing feldspar varieties have a greener coloration. There are also some other uncommon sodium, calcium and potassium endmember feldspars.
Determining Feldspar Species
The specific feldspar species present in a rock sample can be determined through a few different methods:
Optical Microscopy
Examining a thin section of the rock under a petrographic microscope allows identification based on optical properties like relief, birefringence colors, twinning and cleavage angles. Chemical staining techniques can help differentiate alkali feldspars from plagioclase.
X-ray Diffraction
X-ray diffraction analysis of the crystal structure and atomic spacing provides a definitive identification of the feldspar species and composition.
Chemical Analysis
The proportions of elements like Na, K, Ca, Ba and Al can be quantified with x-ray fluorescence, electron microprobe or inductively coupled plasma mass spectrometry. Comparing the chemistry to feldspar endmembers indicates the specific species.
Fluorescence
Some feldspars like anorthite will fluoresce under shortwave ultraviolet light, providing a quick screening method to identify calcium-rich plagioclase.
Occurrences of Feldspar
Feldspars are the most abundant minerals in Earth’s crust, forming around 60% of continental rocks and even higher percentages of the Moon’s crust. Feldspars occur in many different igneous, metamorphic and sedimentary rock types and geologic settings:
Igneous Rocks
- Granite – Orthoclase and plagioclase are major minerals in granite along with quartz.
- Syenite – Alkali feldspar in syenite.
- Diorite – Andesine and labradorite in diorite.
- Andesite – Andesine phenocrysts in andesite porphyry.
- Rhyolite – Sanidine in rhyolite.
- Pegmatite – Large crystals of microcline, orthoclase and albite in granitic pegmatites.
Metamorphic Rocks
- Gneiss – Banded gneiss containing interleaved layers of quartz, feldspar and other minerals.
- Schist – Porphyroblasts of feldspar in schist.
- Amphibolite – Plagioclase crystals in amphibolite.
Sedimentary Rocks
- Arkose – Arkosic sandstone with feldspar grains.
- Greywacke – Dirty sandstone with feldspar, quartz and rock fragments.
- Conglomerate – Rounded pebbles of feldspar.
Economic Deposits
- China clay – Byproduct of feldspar weathering is kaolinite clay.
- Ceramic & glass raw material – Albite and microcline are used in ceramics.
- Gemstones – Orthoclase (moonstone), amazonite, labradorite.
Uses of Feldspar
The most common use of feldspar is as filler and flux material in glassmaking and ceramics. Some specific uses include:
- Glass – Feldspar provides alumina for glass production.
- Ceramics – Fluxing agent in ceramic glazes.
- Porcelain – Alkali feldspar used in porcelain and bathroom fixtures.
- Fiberglass – Microcline added to fiberglass.
- Abrasives – Microcline makes scouring powder.
- Gemstones – Faceted orthoclase and moonstone jewelry.
- Artificial teeth – Anorthite mimics natural tooth mineral composition.
Key Facts about Feldspar
- Most abundant mineral group in Earth’s crust.
- Found in igneous, metamorphic and sedimentary rocks.
- Distinctive glassy, pearly luster.
- Good cleavage at 90 degree angles.
- Hardness of 6 on Mohs scale.
- Colors include pink, white, gray, green.
- Microcline shows iridescent schiller effect.
- Chemical group is aluminum silicates with Na, K, Ca, Ba.
- Endmember compositions like albite, anorthite, orthoclase.
- Used as flux in glassmaking and ceramics.
Conclusion
In summary, feldspar is characterized by its glassy luster, visible cleavage planes, moderate hardness and distinctive mineral colors. Examining the color, luster, crystal form, cleavage and hardness allows easy identification of feldspar minerals. Chemical analysis can determine the specific feldspar endmember composition. Feldspars are essential components of abundant rock types like granite. Their chemical durability leads to concentration in sediments and metamorphic rocks. Feldspar’s importance in industrial uses like glass and ceramics make it a fundamentally important rock-forming mineral group.