How to identify Altered Feldspar

To correctly identify Altered Feldspar (Feldspar (likely K-feldspar) with secondary alteration/mineralization (e.g., chlorite or epidote)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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