pjt222 Mineral Identification
Identify minerals in the field using physical properties, systematic elimination, and simple 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 indicators of valuable minerals
- 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) |
+--------------------+------------------------------------------+
Expected: Field context recorded before handling the specimen.
On failure: If geological context is unclear (loose specimen, urban find), proceed with physical properties only — context would have helped narrow candidates but is not strictly 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: try to scratch the specimen with each reference tool,
starting from soft to hard. The hardness is 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) will not leave a 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: does it feel heavier or lighter
than expected for its size?
- Heavy: possible metallic ore (galena, gold, magnetite)
- Light: possible pumice, sulfur, or organic material
Expected: A profile of the specimen: luster, hardness range, streak colour, cleavage/fracture type, and relative density.
On failure: If a property is ambiguous (e.g., luster between metallic and vitreous — "sub-metallic"), record both options. Ambiguity reduces confidence but does not prevent 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 generally — 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)
Expected: Additional diagnostic data that narrows the identification further.
On failure: If special tests are unavailable (no magnet, no acid), proceed with the basic properties — they are 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)
Expected: A mineral identification or a shortlist of 2-3 candidates with the distinguishing test needed to differentiate them.
On failure: If the specimen does not match any 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
Common 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 steel nail with rust 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 completely disguise the mineral beneath
Related Skills
gold-washing— alluvial gold recovery uses mineral identification skills to read stream deposits and assess gold-bearing gravels