Minerals, Rocks and Their Identification; Petrogenetic Processes
Domain 4 tests mineral properties, rock classification and the petrogenetic paths that connect them. Expect to identify a sample from description or to place a rock in the cycle using texture and mineralogy - a common Canarian exam item, since the islands supply basalt, trachyte, phonolite, ignimbrite, tuff and calcrete in one small area.
Minerals: properties that decide an identification
Minerals are naturally occurring, inorganic, crystalline solids with a defined composition. Key properties: hardness (Mohs 1 talc to 10 diamond), cleavage versus fracture, lustre, colour and streak (streak is more reliable than colour), density, magnetism, effervescence in HCl (calcite), and taste/solubility (halite). Silicate structures control properties: isolated tetrahedra (olivine), chains (pyroxene, amphibole), sheets (mica, clay), frameworks (quartz, feldspar).
Rock classification
Igneous: plutonic (coarse, phaneritic) versus volcanic (fine, aphanitic, or glassy), classified by silica and mineralogy - QAPF for the alkaline suite. Sedimentary: clastic by grain size, chemical/biochemical by composition. Metamorphic: foliated (slate, schist, gneiss) versus non-foliated (marble, quartzite, hornfels).
Petrogenetic processes
Partial melting → magma → differentiation (fractional crystallization, assimilation, magma mixing) → eruption or intrusion. Bowen's series: discontinuous (olivine → pyroxene → amphibole → biotite) and continuous (calcic → sodic plagioclase). Weathering → transport → diagenesis → sedimentary rock; heat and pressure → metamorphism; further heat → melting, closing the cycle.
Worked example
Two dark, fine-grained lavas are given. Sample A is dense, dark, with abundant olivine phenocrysts and no quartz; sample B is grey-green, with abundant plagioclase laths and visible pyroxene, and no olivine. Reasoning: A is an alkali basalt/basanite - low silica, olivine-bearing, mantle-derived, typical of Canarian shields; B is also basaltic but more evolved and plagioclase-rich, consistent with shallower fractionation at lower pressure. Textural difference (porphyritic versus intergranular) records the crystallization history, not the age.
Traps that cost marks
Identifying by colour alone - trachyte and phonolite are both pale. Calling obsidian a mineral. Using 'basalt' for any dark rock including gabbro (same chemistry, different texture). Forgetting that ignimbrite is a pyroclastic rock, not a lava.
- Texture records cooling history; mineralogy records composition.
- Calcite: hardness 3, perfect rhombohedral cleavage, fizzes in dilute HCl.
- Quartz: hardness 7, conchoidal fracture, no cleavage.
- Gabbro = basalt's plutonic equivalent; rhyolite = granite's volcanic equivalent.
- Trachyte (alkali feldspar) and phonolite (feldspathoid, rings when struck) are the evolved Canarian terms.