How to identify Epidote-Feldspar Rock

To correctly identify Epidote-Feldspar Rock (Epidote-Feldspar rock), 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

   Typically green (epidote) intergrown with white, pink, or grey (feldspar). The green can range from yellowish-green to pistachio green to dark green. Feldspar colors depend on its type (e.g., orthoclase is often pink, plagioclase can be white to grey).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Epidote typically has a vitreous to resinous luster. Feldspar has a vitreous to pearly luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granular, often interlocking crystals. Can be massive, fine-grained, or coarse-grained. The epidote may appear as disseminated grains, veinlets, or larger crystals within the feldspar matrix.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Epidote often forms prismatic crystals, sometimes acicular, or granular aggregates. Feldspar forms blocky, tabular, or prismatic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Epidote has perfect cleavage on {001} and good cleavage on {100}. Feldspar has two distinct cleavages at or near 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in regionally metamorphosed mafic to intermediate igneous rocks (e.g., metabasalts, metagabbros, metadiorites) and calc-silicate rocks. Also found in hydrothermally altered igneous rocks and skarns.

Key Facts

Hardness

Epidote: 6-7; Feldspar: 6-6.5 (Mohs scale)

Specific Gravity

Epidote: 3.3-3.5; Feldspar: 2.5-2.8

Crystal System

Epidote: Monoclinic; Feldspar: Monoclinic (Orthoclase) or Triclinic (Plagioclase)

Color

Epidote: Pistachio green, yellowish-green, dark green; Feldspar: White, grey, pink, red, green (amazonite)

Luster

Vitreous to resinous (epidote), vitreous to pearly (feldspar)

Transparency

Epidote: Transparent to translucent; Feldspar: Transparent to translucent

Fracture

Epidote: Uneven to conchoidal; Feldspar: Uneven to conchoidal

Cleavage

Epidote: Perfect on {001}, good on {100}; Feldspar: Two distinct cleavages at or near 90 degrees

Composition

Epidote: Ca2(Al,Fe)3(SiO4)3(OH); Feldspar: KAlSi3O8 (Orthoclase) or NaAlSi3O8-CaAl2Si2O8 solid solution (Plagioclase)

Physical characteristics

• Crystal Habit: Epidote: Prismatic, acicular, granular, massive; Feldspar: Tabular, blocky, prismatic, massive
• Cleavage Type: Epidote: Perfect basal, good prismatic; Feldspar: Perfect basal and good pinacoidal
• Fracture Type: Uneven to conchoidal for both
• Tenacity: Brittle for both
• Luster Type: Vitreous to resinous (epidote), Vitreous to pearly (feldspar)

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