Altered Feldspar

Description

Feldspar with secondary mineralization refers to feldspar grains (most commonly K-feldspar or plagioclase) that have undergone chemical alteration, resulting in the formation of new, secondary minerals within or replacing the original feldspar crystal. The type of secondary mineral depends on the original feldspar composition, the chemistry and temperature of the altering fluids, and the pressure conditions. Common secondary minerals include chlorite, epidote, sericite (fine-grained muscovite), clay minerals (e.g., kaolinite, illite), and sometimes carbonates or zeolites. This alteration often changes the color, luster, and overall appearance of the feldspar, making it appear dull, cloudy, or stained with the color of the secondary mineral.

How to Identify

Color

Highly variable, depending on the secondary mineral. Chlorite imparts green hues; epidote, yellowish-green to pistachio green; sericite, silvery-white to pale green; clay minerals, dull white, gray, or yellowish. The original feldspar color may be obscured or mottled.

Luster

Can range from dull to earthy (due to clay minerals or fine-grained alteration) to silky (due to sericite) or waxy/greasy (due to epidote). Original vitreous luster of feldspar is often diminished.

Texture

Often appears cloudy, turbid, or opaque. Fine-grained aggregates of secondary minerals may replace the original feldspar crystal, sometimes preserving the original crystal shape (pseudomorphism). Can feel softer or more friable than fresh feldspar.

Crystal Form

Original feldspar crystal forms (e.g., tabular, prismatic) may be preserved as pseudomorphs, but the internal structure is replaced by fine-grained aggregates of secondary minerals. Individual secondary mineral crystals are often microscopic.

Cleavage

Original feldspar cleavage planes (two at nearly 90 degrees for K-feldspar, two at 90 degrees for plagioclase) may be partially preserved, but the altered material can be more brittle or granular along these planes. Secondary minerals may have their own distinct cleavages (e.g., perfect basal cleavage in sericite).

Geological Environment

Common in hydrothermally altered zones around ore deposits, geothermal fields, fault zones, contact metamorphic aureoles, and regionally metamorphosed terrains (especially greenschist facies). Also found in weathered igneous and metamorphic rocks.

Key Facts

• Hardness: Variable, typically 2-6 on Mohs scale, depending on the degree and type of alteration. Softer than fresh feldspar (6-6.5).
• Specific Gravity: Variable, typically 2.5-2.8 g/cm³, similar to or slightly lower than fresh feldspar (2.5-2.7 for K-feldspar, 2.6-2.8 for plagioclase).
• Crystal System: Original feldspar crystal system (monoclinic for K-feldspar, triclinic for plagioclase) may be preserved in pseudomorphs, but the secondary minerals have their own systems (e.g., monoclinic for chlorite and epidote, monoclinic for sericite).
• Color: Highly variable: green (chlorite, epidote), yellowish-green (epidote), white (sericite, clay minerals), gray, pinkish-white.
• Luster: Dull, earthy, silky, waxy, greasy, or sub-vitreous.
• Transparency: Opaque to translucent, rarely transparent.
• Fracture: Uneven to conchoidal, often more brittle than fresh feldspar.
• Cleavage: Original feldspar cleavage (two directions at or near 90 degrees) may be partially preserved, but often obscured or replaced by the cleavage of secondary minerals (e.g., perfect basal cleavage of sericite).
• Composition: Original feldspar (KAlSi3O8 or NaAlSi3O8-CaAl2Si2O8) partially or wholly replaced by secondary minerals such as chlorite ((Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6), epidote (Ca2(Al,Fe)3(SiO4)3(OH)), sericite (KAl2(AlSi3O10)(OH)2), and various clay minerals (e.g., Al2Si2O5(OH)4 for kaolinite).

Quick Check

• Color: Variable (green, yellowish-green, white, gray, silvery) often mottled or cloudy.
• Luster: Dull, earthy, silky, waxy, or greasy, often lacking the vitreous luster of fresh feldspar.
• Streak: White to pale green/yellow (depending on secondary mineral), often lighter than the apparent body color.

Physical Characteristics

• Crystal Habit: Pseudomorphic replacement of original feldspar crystals, or fine-grained aggregates within the feldspar. Individual secondary minerals are often microscopic.
• Cleavage Type: Variable, depending on the secondary mineral. Feldspar's two good cleavages may be partially preserved. Sericite has perfect basal cleavage. Chlorite has perfect basal cleavage. Epidote has one perfect cleavage.
• Fracture Type: Uneven, splintery, or conchoidal.
• Tenacity: Brittle to friable.
• Luster Type: Dull, earthy, silky, waxy, greasy, or sub-vitreous.

Formation

Secondary mineralization in feldspar occurs when primary feldspar minerals react with hydrothermal fluids, meteoric water, or during diagenesis and metamorphism. These fluids introduce new chemical components or leach existing ones, leading to the breakdown of the original feldspar structure and the formation of new, more stable secondary minerals. For example, K-feldspar can alter to sericite (fine-grained muscovite) in the presence of potassium-rich fluids, while plagioclase can alter to epidote, chlorite, and albite (saussuritization) in greenschist facies metamorphic conditions or hydrothermal alteration.

Usage

While the altered feldspar itself is not typically used directly, its presence is a crucial indicator in geological exploration for hydrothermal ore deposits (e.g., porphyry copper deposits where sericitic alteration is common), geothermal systems, and understanding metamorphic processes. It can also affect the engineering properties of rocks, making them weaker or more permeable.

Age Distribution

Can occur in rocks of all geological ages, as alteration is a post-formational process.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates (Tectosilicates for feldspar, Phyllosilicates for chlorite/sericite/clays, Sorosilicates for epidote)

Group

Feldspar Group (original mineral) with various secondary mineral groups (e.g., Chlorite Group, Epidote Group, Mica Group, Clay Mineral Group)

Crystal System

Variable (original feldspar: monoclinic or triclinic; secondary minerals: monoclinic, triclinic, or hexagonal)

Chemical Formula

Highly variable, representing a mixture of original feldspar and secondary minerals. E.g., KAlSi3O8 + (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6 (K-feldspar + chlorite)

Composition

A composite of the original feldspar's chemical components and those of the newly formed secondary minerals, often involving hydration and metasomatic exchange of elements (e.g., K, Na, Ca, Mg, Fe, Al, Si, H2O).

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