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Feldspar is not a single mineral but a group of tectosilicate minerals that constitute over 50% of the Earth's crust by weight. The term 'tumbled feldspar' refers to pieces of feldspar minerals that have been mechanically polished in a rock tumbler to achieve a smooth, rounded, and lustrous surface. This process enhances their aesthetic appeal, making them suitable for jewelry, decorative items, and collections. The specific properties of tumbled feldspar will vary depending on the particular feldspar mineral it is derived from (e.g., orthoclase, microcline, plagioclase).
How to Identify
- Color
- Highly variable, ranging from white, cream, pink, red, green, gray, to black. Often translucent to opaque.
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Smooth and polished due to tumbling. Untumbled, it exhibits distinct cleavage planes.
- Crystal Form
- Tumbled feldspar will not show original crystal forms. In its natural state, feldspar typically forms blocky, tabular, or prismatic crystals. Twinning (e.g., Carlsbad, Albite, Pericline) is common.
- Cleavage
- Two distinct cleavage directions intersecting at or near 90 degrees (orthoclase, microcline) or slightly less than 90 degrees (plagioclase). This is a key diagnostic feature, even in tumbled specimens where cleavage planes might be visible as internal reflections or subtle linear features.
- Geological Environment
- Found in virtually all types of igneous rocks (granite, basalt, gabbro, diorite), metamorphic rocks (gneiss, schist), and as a major component of many sedimentary rocks (arkose sandstone).
Key Facts
- Hardness: 6 to 6.5 (Mohs scale)
- Specific Gravity: 2.55 to 2.76 g/cm³ (varies with composition)
- Crystal System: Monoclinic (Orthoclase, Sanidine) or Triclinic (Microcline, Plagioclase)
- Color: White, pink, gray, green, red, brown, black; often translucent to opaque.
- Luster: Vitreous to pearly
- Transparency: Transparent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Two distinct cleavages, typically at or near 90 degrees.
- Composition: Aluminosilicates of potassium, sodium, and calcium (KAlSi3O8 - Orthoclase/Microcline; NaAlSi3O8 - Albite; CaAl2Si2O8 - Anorthite)
Quick Check
- Color: Variable (white, pink, green, gray, etc.)
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Blocky, tabular, prismatic; often twinned. Tumbled specimens are rounded and anhedral.
- Cleavage Type: Perfect in two directions, typically at 90° (orthoclase, microcline) or 86° (plagioclase).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Feldspars form under a wide range of geological conditions, including crystallization from magmas (igneous rocks), metamorphism (metamorphic rocks), and precipitation from hydrothermal solutions. They are primary constituents of most igneous and metamorphic rocks and are also found in sedimentary rocks as detrital grains.
Usage
Feldspar is a crucial industrial mineral. It is used extensively in ceramics (porcelain, tiles, sanitaryware), glass manufacturing (as a fluxing agent), and as a filler in paints, plastics, and rubber. Specific varieties are used as gemstones (e.g., moonstone, labradorite, amazonite, sunstone). Tumbled feldspar is primarily used for decorative purposes, lapidary arts, and in metaphysical practices.
Age Distribution
Ubiquitous throughout Earth's geological history, from Precambrian to Cenozoic.
Where to Find
Worldwide
Feldspar is the most abundant mineral group in the Earth's crust, found on every continent. Major commercial deposits are found in the USA (North Carolina, California), Canada, Mexico, Norway, Sweden, France, Italy, Germany, India, China, and Brazil.
Pegmatites
Large, well-formed feldspar crystals are often found in pegmatite dikes, which are coarse-grained igneous rocks.
Granitic Intrusions
Feldspar is a primary component of granites and related felsic igneous rocks.
Finding Tips
Look for Cleavage
Even in tumbled specimens, the characteristic two-directional cleavage of feldspar can often be observed as internal reflections or subtle linear features on the polished surface. This helps distinguish it from quartz, which lacks cleavage.
Check for Striations
Plagioclase feldspars often exhibit fine parallel striations (albite twinning lamellae) on one of their cleavage surfaces. This is a definitive characteristic for plagioclase.
Assess Hardness
Feldspar has a Mohs hardness of 6 to 6.5. It will scratch glass but can be scratched by quartz. This can help differentiate it from softer minerals.
Similar Rocks
Quartz
SiO2
Also known as: Silica
Calcite
CaCO3
Also known as: Calcium Carbonate
Nepheline
(Na,K)AlSiO4
Also known as: Nephelite
Scientific Classification
- Mineral Class
- Tectosilicates
- Group
- Feldspar Group
- Crystal System
- Monoclinic (K-feldspars like Orthoclase) or Triclinic (K-feldspars like Microcline, and Plagioclase series)
- Chemical Formula
- General formula: (K,Na,Ca)(Al,Si)4O8. Specific end-members: KAlSi3O8 (Orthoclase/Microcline), NaAlSi3O8 (Albite), CaAl2Si2O8 (Anorthite).
- Composition
- Solid solution series between potassium feldspar (K-feldspar) and plagioclase feldspar (Na-Ca feldspar). K-feldspars include Orthoclase, Microcline, and Sanidine. Plagioclase series includes Albite, Oligoclase, Andesine, Labradorite, Bytownite, and Anorthite.
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