Rocks and Minerals

Overview

Rocks and Minerals emphasizes rapid, justified identification and linking properties to formation processes. Learn to deploy a minimal test set efficiently and to explain the reasoning behind each ID.

Field and bench tests (conceptual)

• Hardness (Mohs): fingernail (~2.5), copper penny (~3.5 newer: ~3), steel nail (~5.5), glass (~5.5–6), porcelain streak plate (~6.5–7). Test on fresh surfaces; beware embedded grit.
• Streak: powder color on unglazed porcelain; more reliable than bulk color (hematite streak red‑brown; pyrite streak greenish‑black).
• Luster: metallic vs non‑metallic (vitreous, pearly, silky, resinous, earthy). Describe precisely.
• Cleavage/fracture: number and angles of planes; conchoidal fracture (quartz, obsidian). Distinguish cleavage faces from crystal faces.
• Specific gravity (heft): galena notably dense; pumice unusually light.
• Reaction: dilute HCl effervescence (calcite strong; dolomite slow or powdered). Magnetism (magnetite); fluorescence (concept only if permitted).

Mineral ID quick keys (selected)

• Quartz: H≈7, no cleavage, vitreous, conchoidal fracture, any color, streak white.
• Feldspars: two cleavages ~90°; K‑spar often salmon/pink (microcline/orthoclase), plagioclase shows striations.
• Micas: perfect basal cleavage; biotite (dark), muscovite (light/transparent), flexible sheets.
• Carbonates: calcite (strong acid effervescence, rhombohedral cleavage, H≈3), dolomite (weak acid unless powdered, rhombohedral).
• Halite: cubic cleavage, salty (do not taste unless explicitly permitted); sylvite bitter (avoid tasting; treat as conceptual distinction).
• Pyrite vs gold: pyrite brassy with striated cubes, H≈6–6.5, streak greenish‑black; gold malleable, streak yellow; density much higher.

Igneous rocks

• Textures: phaneritic (coarse, intrusive), aphanitic (fine, extrusive), porphyritic (two‑stage), glassy, vesicular (gas bubbles), pyroclastic (tuff, ash). Phenocrysts indicate slower early cooling.
• Composition series: felsic (quartz, K‑feldspar; light) → intermediate (andesite/diorite) → mafic (pyroxene, Ca‑plagioclase; dark) → ultramafic (olivine). Use color index cautiously; confirm with minerals.
• Keys: granite (phaneritic felsic), rhyolite (aphanitic felsic), diorite (phaneritic intermediate), andesite (aphanitic intermediate), gabbro (phaneritic mafic), basalt (aphanitic mafic), obsidian (glassy), pumice/scoria (vesicular; pumice light).

Sedimentary rocks

• Clastic: classify by grain size (conglomerate/breccia—rounded vs angular clasts; sandstone—quartz/arkose/lithic; siltstone; shale/mudstone). Sorting/rounding indicate transport.
• Chemical/biochemical: limestone (calcite reactions), dolostone, chert (microcrystalline quartz, conchoidal), evaporites (halite, gypsum), coal (peat→lignite→bituminous→anthracite).
• Structures: bedding/laminations, cross‑beds, ripple marks, graded beds (turbidites), mud cracks; fossils and trace fossils.

Metamorphic rocks

• Texture: foliated (slate→phyllite→schist→gneiss sequence) vs non‑foliated (marble, quartzite, hornfels). Foliation from directed pressure; index minerals (chlorite, biotite, garnet, staurolite, kyanite, sillimanite) track grade (qualitative).
• Parentage: marble from limestone (effervesces), quartzite from sandstone (hard, sugary surface; scratches glass), slate from shale (slaty cleavage).

Worked micro‑examples

1. Quartz vs calcite

• Both can be clear. Quartz: H=7 scratches glass; no effervescence; conchoidal fracture. Calcite: H=3 (scratched by copper), strong HCl fizz, rhombohedral cleavage.

2. Granite vs diorite

• Granite: K‑spar + quartz visible; often pink/white + black mica. Diorite: intermediate (plagioclase + amphibole), salt‑and‑pepper appearance; little/no quartz; no K‑spar pink.

3. Marble vs quartzite

• Both non‑foliated and light colored. Marble reacts with HCl (calcite); quartzite does not and scratches glass easily; quartz grains fused.

4. Basalt vs andesite

• Basalt: mafic (dark, aphanitic), may have vesicles; density higher. Andesite: intermediate gray, common small plagioclase phenocrysts; magnetism weaker than in iron‑rich basalts.

Pitfalls

• Relying on color alone; misreading weathered surfaces—always test fresh surfaces.
• Confusing cleavage with crystal habit; mistaking striations for cleavage.
• Testing on streak plate and then judging hardness from streak (streak plate is ~7; minerals softer will not scratch it but leave streak; harder scratch plate, often no streak).

Practice prompts

• Provide two lines of evidence for each ID in a mixed set of five hand samples (2 minerals, 3 rocks) and justify parent–product relationships in metamorphics.
• Interpret a sandstone thin section description (quartz percent, feldspar, lithics) into a sandstone name (quartz arenite vs arkose vs lithic arenite) and infer provenance.
• From a glacial outwash gravel description (subrounded, poorly sorted), infer transport history and likely depositional environment.

Quick tables

• Hardness anchors: Talc 1, Gypsum 2, Calcite 3, Fluorite 4, Apatite 5, Feldspar 6, Quartz 7, Topaz 8, Corundum 9.
• Cleavage counts: 1 perfect (mica), 2 at ~90° (feldspar), 3 at 90° (halite), 3 not at 90° (calcite), none (quartz).

References

• SciOly Wiki – Rocks and Minerals: https://scioly.org/wiki/index.php/Rocks_and_Minerals
• USGS rock/mineral ID guides (summaries)

Linking IDs to processes

Identification is stronger when tied to how the sample formed. Quartz’s lack of cleavage and conchoidal fracture reflect a continuous 3D SiO₂ network; feldspar’s two cleavages at ~90° come from weaker planes in its framework. Vesicular basalt points to volatile‑rich mafic lava degassing at the surface; porphyritic andesite records two‑stage cooling with early phenocryst growth followed by rapid quench. In sedimentary contexts, well‑sorted quartz arenite implies long transport and reworking, while arkose with abundant K‑feldspar signals short transport from granitic source and/or arid climates limiting chemical weathering. Metamorphic index minerals encode pressure–temperature histories—garnet growth marks rising grade, and staurolite/kyanite/sillimanite sequences track increasing temperature and, depending on assemblage, pressure.

Bowen’s reaction series as a reasoning tool

Bowen’s series predicts mineral stability with cooling: olivine and Ca‑rich plagioclase crystallize first at high temperature, followed by pyroxene and more intermediate plagioclase, then amphibole, biotite, K‑feldspar, muscovite, and quartz. Use it qualitatively to justify associations and weathering resistance. Mafic rocks rich in early‑forming minerals weather faster and appear darker; felsic rocks dominated by late‑forming quartz and K‑spar resist weathering and show lighter colors. In thin section descriptions or hand samples, pairing mafic phenocrysts with lighter groundmass supports an intermediate composition where crystallization bridged the series.

Metamorphic facies context

Facies group mineral assemblages by P–T conditions independent of bulk composition shorthand: zeolite and prehnite–pumpellyite at low T, greenschist with chlorite/actinolite/epidote at modest T, amphibolite with hornblende/plagioclase at higher T, and granulite at the dry, high‑T end. Blueschist and eclogite reflect high‑pressure subduction settings. For hand samples, you often cannot assign a precise facies, but recognizing chlorite‑rich schist versus hornblende‑bearing amphibolite lets you narrate a plausible tectonic path (e.g., burial and heating during orogeny, or high‑P low‑T subduction).