Rock & Mineral ID Basics

Calcite — Group

Mineral

Calcite — Chemical group (subgroup)

Carbonate

Calcite — Luster

Vitreous (glassy) to pearly

Calcite — Color

White, colorless, gray, pink — highly variable

Calcite — Hardness (Mohs scale)

3

Calcite — Harder than…

Fingernail (~2.5) and penny (~3) — about the same as a penny

Calcite — Streak

White

Calcite — Cleavage / Breakage

3 directions of cleavage NOT at 90° (rhombohedral) — produces rhombus-shaped fragments

Calcite — Other diagnostic properties

Reacts VIGOROUSLY with dilute HCl (fizzes strongly). Double refraction visible through clear crystals.

Halite — Group

Mineral

Halite — Chemical group (subgroup)

Halide

Halite — Luster

Vitreous (glassy)

Halite — Color

Colorless, white, sometimes pink or orange

Halite — Hardness (Mohs scale)

2.5

Halite — Harder than…

Fingernail (barely) — softer than a penny

Halite — Streak

White

Halite — Cleavage / Breakage

3 perfect cleavage directions at 90° — produces perfect cubic fragments

Halite — Other diagnostic properties

TASTES SALTY — definitive test. Cubic crystals.

Quartz — Group

Mineral

Quartz — Chemical group (subgroup)

Silicate (tectosilicate)

Quartz — Luster

Vitreous (glassy)

Quartz — Color

Any color — colorless, white, purple (amethyst), smoky brown, yellow (citrine), pink (rose)

Quartz — Hardness (Mohs scale)

7

Quartz — Harder than…

Glass plate (~5.5) and steel knife/nail (~5.5–6) — cannot be scratched by either

Quartz — Streak

White

Quartz — Cleavage / Breakage

NO cleavage — conchoidal (curved, shell-like) fracture

Quartz — Other diagnostic properties

6-sided hexagonal crystals. Extremely hard for a non-metallic mineral. Wide color range but always H=7.

Gypsum — Group

Mineral

Gypsum — Chemical group (subgroup)

Sulfate

Gypsum — Luster

Vitreous, pearly, or silky depending on variety

Gypsum — Color

White, colorless, gray, occasionally pinkish

Gypsum — Hardness (Mohs scale)

2

Gypsum — Harder than…

Nothing — softer than a fingernail (H~2.5). Fingernail scratches it easily.

Gypsum — Streak

White

Gypsum — Cleavage / Breakage

1 perfect cleavage (splits into flat sheets); 2 imperfect directions also present

Gypsum — Other diagnostic properties

Only common mineral scratched by a fingernail. Two varieties: selenite (clear, peelable sheets) and alabaster (earthy/massive). No HCl reaction.

Hematite — Group

Mineral

Hematite — Chemical group (subgroup)

Oxide

Hematite — Luster

Metallic (specular/crystal form) or dull earthy (massive form)

Hematite — Color

Black or silver-gray (specular form) OR reddish-brown (earthy form) — your lab has both

Hematite — Hardness (Mohs scale)

5.5–6.5

Hematite — Harder than…

Glass plate — harder than glass, scratched by streak plate

Hematite — Streak

REDDISH-BROWN — key ID even for the black metallic variety

Hematite — Cleavage / Breakage

No distinct cleavage; irregular fracture

Hematite — Other diagnostic properties

Reddish-brown streak is the definitive ID regardless of which form. Lab sample labeled 'Hematite / Specular Hematite'.

Biotite — Group

Mineral

Biotite — Chemical group (subgroup)

Silicate (phyllosilicate)

Biotite — Luster

Vitreous to submetallic (shiny on cleavage faces)

Biotite — Color

Black, dark brown, or dark greenish-black

Biotite — Hardness (Mohs scale)

2.5–3

Biotite — Harder than…

Fingernail — softer than a penny

Biotite — Streak

White to grayish

Biotite — Cleavage / Breakage

1 PERFECT cleavage — peels into thin sheets like pages of a book

Biotite — Other diagnostic properties

Dark-colored MICA. Sheets are ELASTIC — they spring back when bent. Distinguishes from chlorite (flexible but not elastic). Lab label: 'Biotite Mica'.

Galena — Group

Mineral

Galena — Chemical group (subgroup)

Sulfide

Galena — Luster

Bright, shiny metallic

Galena — Color

Lead gray

Galena — Hardness (Mohs scale)

2.5

Galena — Harder than…

Fingernail — softer than a penny

Galena — Streak

Gray (lead gray)

Galena — Cleavage / Breakage

3 perfect cleavage directions at 90° — cubic cleavage, produces small perfect cubes

Galena — Other diagnostic properties

VERY high specific gravity (~7.5) — feels extremely heavy for its size. Lead ore mineral. Cubic crystal form common.

Hornblende — Group

Mineral

Hornblende — Chemical group (subgroup)

Silicate (double-chain inosilicate — amphibole family)

Hornblende — Luster

Vitreous

Hornblende — Color

Dark green to black

Hornblende — Hardness (Mohs scale)

5–6

Hornblende — Harder than…

Glass plate (barely) — about same hardness as glass

Hornblende — Streak

Gray-green

Hornblende — Cleavage / Breakage

2 cleavage directions at 56° and 124° — NOT at right angles (amphibole signature)

Hornblende — Other diagnostic properties

Amphibole family (lab label says so). Elongate crystals. Cleavage angle (56°/124°) distinguishes from augite/pyroxene (87°/93°).

Fluorite — Group

Mineral

Fluorite — Chemical group (subgroup)

Halide

Fluorite — Luster

Vitreous (very glassy)

Fluorite — Color

Highly variable — purple, green, blue, yellow, colorless; often multiple colors in one sample

Fluorite — Hardness (Mohs scale)

4

Fluorite — Harder than…

Fingernail and penny — softer than glass

Fluorite — Streak

White

Fluorite — Cleavage / Breakage

4 perfect cleavage directions (octahedral) — cuts corners of a cube

Fluorite — Other diagnostic properties

Hardness exactly 4 is diagnostic. Wide color range. Often cubic crystals. Very glassy appearance.

Orthoclase — Group

Mineral

Orthoclase — Chemical group (subgroup)

Silicate (tectosilicate — K-feldspar / potassium feldspar)

Orthoclase — Luster

Vitreous to pearly

Orthoclase — Color

Pink (salmon), white, cream, or gray

Orthoclase — Hardness (Mohs scale)

6

Orthoclase — Harder than…

Glass plate — about the same or slightly harder than glass

Orthoclase — Streak

White

Orthoclase — Cleavage / Breakage

2 cleavage directions at 90° (two perfect directions forming right angles)

Orthoclase — Other diagnostic properties

NO striations on cleavage faces — key distinction from plagioclase. AKA potassium feldspar. Lab label: 'Orthoclase AKA Potassium Feldspar'. Salmon-pink color is common.

Pyrite — Group

Mineral

Pyrite — Chemical group (subgroup)

Sulfide

Pyrite — Luster

Bright, shiny metallic

Pyrite — Color

Brassy / pale gold ('fool's gold')

Pyrite — Hardness (Mohs scale)

6–6.5

Pyrite — Harder than…

Glass plate AND steel knife — harder than both

Pyrite — Streak

BLACK (greenish-black) — NOT gold

Pyrite — Cleavage / Breakage

No distinct cleavage — conchoidal or irregular fracture

Pyrite — Other diagnostic properties

Black streak + brassy color + hardness > glass = definitive ID. Cubic crystals with striated faces, or 12-sided crystals. 'Fool's gold' because color looks like gold but streak and hardness are completely different.

Plagioclase — Group

Mineral