Separate igneous texture, mineral modes and mass composition

1. Define the geological question

Igneous rocks crystallize from melt, but composition and cooling history answer different questions. Coarse interlocking crystals commonly indicate prolonged growth below the surface; fine textures indicate less growth time, while glass records inhibited crystallization. A porphyritic texture combines larger early crystals with a finer later matrix and can record changing conditions.

2. Build the mechanism and evidence ledger

Felsic compositions commonly include quartz and feldspars; mafic compositions contain more iron/magnesium-rich minerals. Granite and rhyolite illustrate broadly related felsic compositions with different textures; gabbro and basalt illustrate a mafic comparison. Color and crystal size alone are insufficient: alteration, vesicles and overlapping mineral assemblages complicate simple labels.

A modal analysis measures component volume fractions. To infer mass, multiply each component volume by its density. The original aggregate has a dense component at 3.3 g/cm³ and a light component at 2.6 g/cm³, with no pores. If the dense volume fraction is f and total volume V, its mass is 3.3fV; the light mass is 2.6(1−f)V.

Total mass divided by total volume gives aggregate density. Dividing dense mass by total mass gives a different composition measure from f. This is a controlled material-accounting example, not a calibration that classifies all rocks. Vesicles, fluid, weathering and mineral substitutions would require a revised model.

3. Reconstruct a worked example

At f=0.25 and V=200 cm³, dense/light volumes are 50/150 cm³ and masses 165/390 g. Density is 555/200=2.775 g/cm³, while the dense mass fraction is 165/555≈0.2973, not 0.25.

4. Predict and investigate

Predict first: Will doubling sample size change its density or its component mass fraction?

Hold volume fraction fixed and double total volume, then hold volume fixed and change fraction. Identify extensive quantities that double versus intensive quantities that remain fixed. Explain which independent texture observation is needed before proposing a cooling history.

5. Test a changed case

Apply the mechanism to a changed case

At equal component volumes in a 100 cm³ sample, find total mass, density and dense mass fraction.

Retain the calculation, evidence and assumptions before opening public feedback.

Compare a worked response

Dense/light masses are 165/130 g, total 295 g, density 2.95 g/cm³ and dense mass fraction 165/295≈0.5593. Equal volume is not equal mass.

6. Retain evidence and its limits

Keep the first prediction, complete ledger, changed case and rejected alternative in the lesson notebook. Supplied observations and bounded CPU models are not actual specimen handling, fieldwork or independently assessed competence. Record source purpose and uncertainty rather than treating public feedback as independent reasoning.

Mechanism and classification reference, checked 30 October 2026. Original teaching prose, numerical packets and questions; no source illustration or exercise is copied.