How to identify Quartz in matrix

To correctly identify Quartz in matrix (SiO2 in host 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

   Quartz itself is typically colorless or white, but can be various colors (e.g., purple amethyst, yellow citrine, pink rose quartz, smoky gray smoky quartz) due to impurities or irradiation. The matrix color will vary widely depending on the host rock (e.g., gray granite, red sandstone, black basalt).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for quartz. The matrix luster will vary (e.g., dull, earthy, sub-vitreous).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Quartz can be crystalline (euhedral to anhedral), granular, or massive. The matrix texture will be characteristic of the host rock (e.g., phaneritic for granite, aphanitic for basalt, clastic for sandstone, foliated for schist).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz often forms hexagonal prisms terminated by rhombohedra. In matrix, it can be well-formed crystals, irregular grains, or massive aggregates. The matrix will exhibit the characteristic crystal forms or textures of its constituent minerals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz lacks true cleavage, exhibiting conchoidal fracture. The matrix minerals may or may not exhibit cleavage (e.g., feldspar has good cleavage, mica has perfect basal cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Quartz in matrix is found in virtually all geological environments: igneous (e.g., pegmatites, granites, volcanic rocks), metamorphic (e.g., gneisses, schists, quartzites), and sedimentary (e.g., sandstones, chert nodules, hydrothermal veins cutting through various rock types).

Key Facts

Hardness

7 (Mohs scale) for quartz; matrix hardness variable.

Specific Gravity

2.65 g/cm³ for quartz; matrix specific gravity variable.

Crystal System

Trigonal for quartz; matrix crystal system variable depending on its constituent minerals.

Color

Colorless, white, or various hues for quartz; matrix color depends on host rock.

Luster

Vitreous (glassy) for quartz; matrix luster varies.

Transparency

Transparent to translucent for quartz; matrix transparency varies.

Fracture

Conchoidal for quartz; matrix fracture varies.

Cleavage

None for quartz; matrix cleavage varies.

Composition

Silicon dioxide (SiO2) for quartz; matrix composition depends on host rock.

Physical characteristics

• Crystal Habit: Prismatic, often with hexagonal cross-section, terminated by rhombohedra; massive, granular, cryptocrystalline. In matrix, it can be euhedral, subhedral, or anhedral.
• Cleavage Type: None (quartz); cleavage of matrix minerals varies.
• Fracture Type: Conchoidal (quartz); fracture of matrix minerals varies.
• Tenacity: Brittle (quartz); tenacity of matrix minerals varies.
• Luster Type: Vitreous (glassy) (quartz); luster of matrix minerals varies.

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