Mineral Identification

Field-ID minerals via physical properties + systematic elimination + simple field tests.

Use When

• Unknown rock/mineral specimen → identify
• Prospecting → assess site for valuable mineral indicators
• Distinguish ore-bearing from barren rock in field
• Build geological literacy via systematic observation

In

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

Do

Step 1: Observe Without Touching

Before handling, ctx.

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)     |
+--------------------+------------------------------------------+

→ Field ctx recorded before handling.

If err: ctx unclear (loose specimen, urban find) → physical properties only. Ctx would narrow candidates, not strictly required.

Step 2: Test Physical Properties

Apply 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

→ Profile: luster, hardness range, streak colour, cleavage/fracture, relative density.

If err: ambiguous (luster between metallic + vitreous → "sub-metallic") → record both. Reduces confidence, doesn't prevent ID.

Step 3: Special Tests

Additional for specific 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)

→ More diagnostic data, narrows ID further.

If err: special tests unavailable (no magnet/acid) → basic properties sufficient for most common minerals.

Step 4: ID by Elimination

Cross-ref profile vs. known.

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)

→ Mineral ID or 2-3 candidates + distinguishing test.

If err: no match common minerals → may be rock (aggregate) not single mineral, or needs lab (thin section, XRD).

Check

• Field ctx recorded before handling
• Luster assessed under natural light
• Hardness tested vs. ≥2 references
• Streak recorded (if softer than plate)
• Cleavage/fracture noted
• ID via systematic elimination, not guess
• Look-alikes considered + differentiated

Traps

• Pyrite vs. gold: "Fool's gold" (pyrite) harder (6 vs. 2.5), brittle (gold malleable), streaks black (gold streaks gold). Tests definitive — use them
• Ignore streak: Specimen colour unreliable (hematite grey/red/black). Streak consistent + diagnostic
• Contaminated tools: Rusty nail → false streak. Clean tools first
• Assume crystal habit: Many rarely show crystals in field. Massive/granular more common — don't require visible crystals
• Weathered vs. true colour: Break specimen → fresh surface before testing. Weathering rinds disguise mineral

→

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