How to identify Quartz with dark inclusions

To correctly identify Quartz with dark inclusions (Quartz (SiO2) with mafic mineral inclusions), 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 matrix is typically colorless, white, or smoky. Inclusions are dark, ranging from black, dark green, dark brown, to dark red, depending on the specific mineral. Examples: black for tourmaline (schorl), dark green for chlorite or actinolite, dark brown/red for goethite/hematite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Quartz: Vitreous (glassy). Inclusions: Can vary from vitreous to sub-metallic or dull, depending on the mineral.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Quartz: Smooth, conchoidal fracture surfaces. Inclusions: Can be acicular (needle-like), fibrous, platy, granular, or dendritic, creating a textured appearance within the quartz.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Hexagonal prisms terminated by rhombohedra. Inclusions: Often preserve their characteristic crystal habits (e.g., prismatic tourmaline, platy chlorite) within the quartz.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: None (or very poor basal cleavage, rarely observed). Inclusions: Varies by mineral (e.g., perfect basal cleavage in chlorite, prismatic cleavage in amphiboles).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermal veins, pegmatites, igneous rocks (e.g., granites, rhyolites), metamorphic rocks (e.g., schists, gneisses), and sedimentary environments where quartz recrystallizes or grows diagenetically.

Key Facts

Hardness

7 on the Mohs scale (for quartz). Inclusions will have varying hardnesses, but the overall specimen's scratch resistance is dominated by quartz.

Specific Gravity

2.65 g/cm³ (for pure quartz). Slightly higher if dense inclusions are abundant.

Crystal System

Trigonal (for quartz). Inclusions will have their own crystal systems (e.g., hexagonal for tourmaline, monoclinic for chlorite/amphiboles).

Color

Colorless, white, or smoky quartz with dark (black, dark green, dark brown) inclusions.

Luster

Vitreous.

Transparency

Transparent to translucent.

Fracture

Conchoidal (for quartz).

Cleavage

None (for quartz).

Composition

SiO2 (quartz) with various mafic mineral inclusions (e.g., (Na,Ca)(Mg,Fe,Al)3(Al,Fe)6(BO3)3(Si6O18)(OH)4 for tourmaline; (Mg,Fe)3(Si,Al)2O5(OH)4 for chlorite; Ca2(Mg,Fe)5Si8O22(OH)2 for actinolite/hornblende).

Physical characteristics

• Crystal Habit: Quartz: Prismatic, often terminated. Inclusions: Acicular, fibrous, platy, granular, or dendritic within the quartz.
• Cleavage Type: Quartz: Absent. Inclusions: Variable, depending on the mineral (e.g., perfect basal in chlorite, good prismatic in amphiboles).
• Fracture Type: Conchoidal (for quartz).
• Tenacity: Brittle.
• Luster Type: Vitreous.

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