Mineral Identification

Identify minerals in the field using physical properties, systematic elimination, and field tests.

When to Use

• You find an unknown rock or mineral specimen and want to identify it
• You are prospecting and need to assess whether a site shows valuable mineral indicators
• You want to distinguish ore-bearing rock from barren rock in the field
• You are building geological literacy through systematic observation

Inputs

• Required: A mineral specimen or outcrop to examine
• Optional: Streak plate (unglazed porcelain tile or bathroom tile back)
• Optional: Steel nail or knife blade (hardness ~5.5)
• Optional: Glass plate (hardness ~5.5)
• Optional: Copper coin (hardness ~3.5)
• Optional: Hand lens (10x)
• Optional: Dilute hydrochloric acid (10% HCl) for carbonate test

Procedure

Step 1: Observe Without Touching

Before handling, observe the specimen in context.

Field Context:
+--------------------+------------------------------------------+
| Observation        | Record                                   |
+--------------------+------------------------------------------+
| Host rock          | What type of rock is it in/on?           |
|                    | (granite, basite, sandstone, schist...)   |
+--------------------+------------------------------------------+
| Geological setting | Vein, disseminated, massive, placer,     |
|                    | weathering surface, cave deposit          |
+--------------------+------------------------------------------+
| Associated         | What other minerals are nearby?           |
| minerals           | (quartz veins often host gold; iron       |
|                    | staining suggests oxidation zone)        |
+--------------------+------------------------------------------+
| Crystal form       | Visible crystals? Habit? Size?           |
| (if visible)       | (cubic, prismatic, tabular, massive)     |
+--------------------+------------------------------------------+

Got: Field context recorded before handling the specimen.

If fail: If geological context is unclear (loose specimen, urban find), proceed with physical properties only — context narrows candidates but is not required.

Step 2: Test Physical Properties

Apply the diagnostic tests systematically.

Diagnostic Property Tests:

LUSTER (how it reflects light):
- Metallic: reflects like metal (pyrite, galena, gold)
- Vitreous: glassy (quartz, feldspar)
- Pearly: like a pearl (muscovite, talc surfaces)
- Silky: like silk fibers (asbestos, satin spar gypsum)
- Earthy/dull: no reflection (kaolin, limonite)
- Adamantine: brilliant, diamond-like (diamond, zircon)

HARDNESS (Mohs scale — scratch test):
+------+-----------+----------------------------------+
| Mohs | Reference | Can Be Scratched By              |
+------+-----------+----------------------------------+
| 1    | Talc      | Fingernail                       |
| 2    | Gypsum    | Fingernail (barely)              |
| 3    | Calcite   | Copper coin                      |
| 4    | Fluorite  | Steel nail (easily)              |
| 5    | Apatite   | Steel nail (just)                |
| 6    | Feldspar  | Steel nail cannot scratch        |
| 7    | Quartz    | Scratches glass                  |
| 8    | Topaz     | Scratches quartz                 |
| 9    | Corundum  | Scratches topaz                  |
| 10   | Diamond   | Scratches everything             |
+------+-----------+----------------------------------+

Test: scratch the specimen with each reference tool, soft to hard.
Hardness sits between the tool that fails and the tool that succeeds.

STREAK (powder colour on porcelain):
- Drag the specimen firmly across an unglazed porcelain tile
- Record the colour of the powder line
- Streak colour is often different from specimen colour
- Critical: hematite is grey-black but streaks RED
- Critical: pyrite is gold but streaks BLACK
- Minerals harder than the streak plate (~7) leave no streak

CLEAVAGE AND FRACTURE:
- Cleavage: breaks along flat planes (mica: 1 direction, feldspar: 2)
- Fracture: breaks irregularly (conchoidal = curved like glass, uneven, fibrous)
- Note number of cleavage directions and angles between them

SPECIFIC GRAVITY (heft test):
- Hold the specimen and assess: heavier or lighter than expected for its size?
- Heavy: possible metallic ore (galena, gold, magnetite)
- Light: possible pumice, sulfur, or organic material

Got: A profile of the specimen: luster, hardness range, streak colour, cleavage/fracture type, and relative density.

If fail: If a property is ambiguous (e.g., luster between metallic and vitreous — "sub-metallic"), record both options. Ambiguity lowers confidence but does not block identification.

Step 3: Apply Special Tests

Additional tests for specific mineral groups.

Special Field Tests:

MAGNETISM:
- Hold a magnet near the specimen
- Strong attraction: magnetite (or possibly pyrrhotite)
- Weak attraction: some iron-bearing minerals

ACID TEST (10% HCl):
- Drop acid on the specimen surface
- Vigorous fizzing: calcite (CaCO3)
- Fizzing on powder only: dolomite (scratch surface first, then apply acid)
- No fizzing: not a carbonate

TASTE (only for suspected halite):
- Salty taste: halite (NaCl)
- Do NOT taste unknown minerals — some are toxic

SMELL:
- Sulfur: rotten egg smell (sulfides when scratched)
- Clay: earthy "petrichor" smell when breathed on (clay minerals)

TENACITY:
- Brittle: shatters when struck (most silicates)
- Malleable: deforms without breaking (gold, copper, silver)
- Flexible: bends and stays (chlorite, some micas)
- Elastic: bends and springs back (muscovite mica)

Got: Additional diagnostic data narrows identification further.

If fail: If special tests are unavailable (no magnet, no acid), proceed with basic properties — sufficient for most common minerals.

Step 4: Identify by Elimination

Cross-reference the property profile against known minerals.

Common Mineral Identification Key (simplified):

METALLIC LUSTER:
- Black streak + hard (6+) + cubic crystals = PYRITE
- Black streak + soft (2.5) + heavy + cubic = GALENA
- Red-brown streak + hard (5-6) + heavy = HEMATITE
- Yellow streak + soft (1.5-2.5) + yellow = GOLD (if malleable)
  or CHALCOPYRITE (if brittle, harder, green-black streak)
- Black streak + magnetic = MAGNETITE

NON-METALLIC, LIGHT-COLORED:
- Vitreous + hard (7) + conchoidal fracture = QUARTZ
- Vitreous + hard (6) + 2 cleavage planes = FELDSPAR
- Vitreous + soft (3) + fizzes in acid = CALCITE
- Pearly + very soft (1) + greasy feel = TALC
- Vitreous + soft (2) + 1 perfect cleavage = GYPSUM

NON-METALLIC, DARK-COLORED:
- Vitreous + hard (5-6) + 2 cleavage at ~90 degrees = PYROXENE
- Vitreous + hard (5-6) + 2 cleavage at ~60/120 degrees = AMPHIBOLE
- Vitreous + soft (2.5-3) + 1 perfect cleavage + flexible = BIOTITE (mica)

Got: A mineral identification or a shortlist of 2-3 candidates with the distinguishing test to differentiate them.

If fail: If the specimen matches no common mineral, it may be a rock (aggregate of minerals) rather than a single mineral, or it may require laboratory analysis (thin section, XRD).

Validation

• Field context was recorded before handling
• Luster was assessed under natural light
• Hardness was tested against at least two reference materials
• Streak colour was recorded (if specimen is softer than streak plate)
• Cleavage or fracture pattern was noted
• Identification was reached by systematic elimination, not guessing
• Look-alike minerals were explicitly considered and differentiated

Pitfalls

• Confusing pyrite with gold: "Fool's gold" (pyrite) is harder (6 vs 2.5), brittle (gold is malleable), and streaks black (gold streaks gold). The tests are definitive — use them
• Ignoring streak: Specimen colour is unreliable (hematite can be grey, red, or black). Streak colour is consistent and diagnostic
• Scratching with contaminated tools: A rusty steel nail produces a false streak. Clean test tools before use
• Assuming crystal habit: Many minerals rarely show well-formed crystals in the field. Massive or granular forms are more common — do not require visible crystals for identification
• Confusing weathered surface with true colour: Break the specimen to expose a fresh surface before testing. Weathering rinds can disguise the mineral beneath

Related Skills

• gold-washing — alluvial gold recovery uses mineral identification to read stream deposits and assess gold-bearing gravels