How to identify Quartz with Feldspar

To correctly identify Quartz with Feldspar (SiO2 (Quartz) and (K,Na)AlSi3O8 (Feldspar)), 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

   Quartz: Typically colorless or white, but can be pink, purple (amethyst), yellow (citrine), brown (smoky quartz), green, or black. Feldspar: Typically white, pink, gray, or green. K-feldspar is often salmon-pink to reddish-brown; plagioclase feldspar is typically white to gray.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: Vitreous (glassy). Feldspar: Vitreous to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   In rocks, quartz often appears anhedral (irregular grains) filling spaces, or as euhedral crystals in vugs. Feldspar often forms subhedral to euhedral crystals, sometimes exhibiting twinning. The overall texture of a rock containing both will depend on its origin (e.g., phaneritic in granite, foliated in gneiss, clastic in sandstone).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms terminating in hexagonal pyramids (when euhedral). Often anhedral in massive rocks. Feldspar: Prismatic, blocky crystals, often twinned (e.g., Carlsbad twinning in K-feldspar, polysynthetic twinning in plagioclase).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: No cleavage, conchoidal fracture. Feldspar: Two distinct cleavage planes intersecting at or near 90 degrees (orthoclase) or slightly less than 90 degrees (plagioclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found together in felsic igneous rocks (granite, granodiorite, rhyolite), many metamorphic rocks (gneiss, schist, granulite), and clastic sedimentary rocks (sandstone, arkose). They are indicative of continental crustal compositions.

Key Facts

Hardness

Quartz: 7 (Mohs). Feldspar: 6-6.5 (Mohs).

Specific Gravity

Quartz: 2.65 g/cm3. Feldspar: 2.55-2.76 g/cm3 (varies with composition).

Crystal System

Quartz: Trigonal. Feldspar: Monoclinic (K-feldspar) or Triclinic (Plagioclase).

Color

Quartz: Colorless, white, gray, purple, pink, yellow, brown, black. Feldspar: White, pink, gray, green, reddish-brown.

Luster

Quartz: Vitreous. Feldspar: Vitreous to pearly.

Transparency

Quartz: Transparent to translucent. Feldspar: Translucent to opaque.

Fracture

Quartz: Conchoidal. Feldspar: Uneven to conchoidal.

Cleavage

Quartz: None. Feldspar: Two distinct cleavages at or near 90 degrees.

Composition

Quartz: Silicon dioxide (SiO2). Feldspar: Aluminosilicates of potassium, sodium, and calcium ((K,Na)AlSi3O8 for alkali feldspar).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular. Feldspar: Prismatic, tabular, blocky, massive.
• Cleavage Type: Quartz: Absent. Feldspar: Perfect to good in two directions.
• Fracture Type: Quartz: Conchoidal. Feldspar: Uneven to conchoidal.
• Tenacity: Quartz: Brittle. Feldspar: Brittle.
• Luster Type: Quartz: Vitreous. Feldspar: Vitreous to pearly.

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