How to identify Fuchsite

To correctly identify Fuchsite (K(Al,Cr)2(AlSi3O10)(OH)2), 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

   Vibrant emerald-green to pale green, directly proportional to the chromium content.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically occurs as fine-grained to medium-grained flakes, often forming schistose or foliated textures in metamorphic rocks. Can also be found as larger, individual flakes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually found as anhedral to subhedral flakes or scales. Macroscopic crystals are rare but can occur as pseudohexagonal plates. It is a monoclinic mineral, but its platy habit often masks this.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect basal cleavage {001}, allowing it to be split into thin, flexible, and elastic sheets.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks, particularly greenschists, quartzites, and marbles, that have undergone metamorphism in the presence of chromium. Often associated with ultramafic rocks (e.g., serpentinites) or chromite deposits.

Key Facts

Hardness

2.5-3 on the Mohs scale

Specific Gravity

2.77-2.88 g/cm³

Crystal System

Monoclinic

Color

Emerald-green to pale green

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to splintery (not typically observed due to perfect cleavage)

Cleavage

Perfect basal cleavage {001}

Composition

Potassium aluminum chromium silicate hydroxide

Physical characteristics

• Crystal Habit: Micaceous, flaky, scaly, foliated, massive. Rarely in distinct pseudohexagonal crystals.
• Cleavage Type: Perfect basal cleavage {001}, yielding thin, flexible, and elastic lamellae.
• Fracture Type: Not typically observed due to perfect cleavage; if present, it would be uneven to splintery.
• Tenacity: Flexible and elastic (lamellae)
• Luster Type: Vitreous to pearly on cleavage surfaces

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