Mineral Identification with Conflicting Student Data
During a 45-minute mineral identification lab, 28 ninth graders rotate through seven stations. At station 4, students record for Sample 7: hardness 6.5 (scratches glass, not quartz), pinkish color, white streak, two cleavage planes meeting at about 90°, vitreous luster, and no reaction to dilute HCl. Student A labels it quartz because 'it scratches glass and has a white streak.' Student B labels it potassium feldspar because 'the flat surfaces are cleavage, not fracture.' The teacher's reference collection contains quartz, orthoclase feldspar, calcite, halite, mica, and pyrite, but the key was misplaced. A second sample, Sample 8, has hardness 3, rhombohedral cleavage, and vigorous effervescence in HCl; one group calls it halite. The class has five minutes left, and a student at the next station has just spilled a small amount of dilute HCl on the bench top. The teacher must decide what to emphasize in the remaining time and how to close the lab safely.
1. Evaluate the two identifications of Sample 7 using the recorded properties. Which claim is better supported, and what evidence would resolve the conflict?
Read the worked answer
Sample 7 is better identified as potassium feldspar. Quartz has a hardness of 7, a white streak, and no cleavage; it breaks with conchoidal fracture. The recorded two cleavage planes at about 90° are diagnostic against quartz, and the pinkish color and hardness of 6.5 fit orthoclase feldspar. 'Scratches glass' is not diagnostic because feldspar also scratches glass, and white streak occurs in both minerals. To resolve the conflict, students should compare the broken surfaces: quartz should show curved, glassy fracture surfaces, whereas feldspar should show flat, repeating cleavage planes. A confirmatory test could be a hardness comparison with a known quartz point or a check for cleavage in multiple orientations. The claim is not proven by one property; it rests on the combination of no acid reaction, two cleavages, and hardness. The teacher should ask which single observation most strongly excludes quartz.
2. Diagnose the error in the Sample 8 identification and describe a safe, quick closure for the lab given the spill.
Read the worked answer
Sample 8 is calcite, not halite. Halite has cubic cleavage, a hardness of about 2.5, and no effervescence in dilute HCl; calcite has rhombohedral cleavage, hardness 3, and reacts vigorously with acid. The acid reaction is diagnostic for a carbonate and rules out halite. For the spill, stop acid testing at that station, put on goggles and gloves, and keep students away. Absorb the small spill with paper towels, neutralize the area with sodium bicarbonate if the kit provides it, and dispose of materials according to the school's chemical-waste protocol. Ventilate the area and wash hands. The teacher should assign one trained student pair to cleanup only after the rest of the class is seated, then finish with a two-minute verbal exit ticket rather than more acid tests.
3. How would you use the conflicting labels as an assessment opportunity rather than simply giving the answer?
Read the worked answer
Use the conflict as a short argumentation cycle. Ask each pair to list which observations are diagnostic and which are consistent but not decisive. Student A must explain why white streak and glass scratching do not rule out feldspar; Student B must explain why cleavage does rule out quartz. Then have pairs write a one-sentence revision: 'Our identification changed because...' Collect these as an exit ticket. For Sample 8, ask students to contrast the acid reaction with halite's expected behavior. This assesses whether students can use properties as evidence rather than match a label. The teacher should not simply announce the key; the instructional goal is the reasoning, and the misplaced key is an opportunity to model how scientists handle uncertainty.