When Density Collides With Hardness and Acid Tests in a Mineral Lab
A lab group's results for three mineral samples are ambiguous, and their identification key sorts by density first:
| Sample | Mass (g) | Water before (mL) | Water after (mL) | Hardness observation | Breakage | Dilute HCl |
|---|---|---|---|---|---|---|
| A | 81.0 | 50.0 | 80.0 | scratched by a steel nail (H ≈ 5.5); scratches glass | conchoidal, no cleavage | no reaction |
| B | 54.0 | 50.0 | 70.0 | scratched by a copper coin (H ≈ 3) | three cleavages near 90° | vigorous fizzing |
| C | 43.2 | 50.0 | 70.0 | scratched by a fingernail (H ≈ 2.5) | cubic cleavage | no reaction |
The group concluded that A and B are the same mineral and that C is unidentifiable. One student wants to confirm C by tasting it; another insists B cannot be calcite because a calcite sample used by last year's class did not fizz.
1. Compute the densities, explain why the group's density-first key failed, and identify each sample with the evidence that supports the identification.
Read the worked answer
Densities from displacement: A = 81.0 g ÷ 30.0 mL = 2.70 g/cm³; B = 54.0 g ÷ 20.0 mL = 2.70 g/cm³; C = 43.2 g ÷ 20.0 mL = 2.16 g/cm³.
The key fails because quartz (about 2.65) and calcite (about 2.71) differ by only about 0.06 g/cm³, which is smaller than the error of this method: a ±1 mL meniscus reading on 20–30 mL is roughly ±0.1 g/cm³, and trapped air or a wet sample pushes the result further. Density alone cannot separate A from B, so the key collapses them.
The other properties do separate them. A is harder than steel (>5.5) and scratches glass, has no cleavage and conchoidal fracture, and does not react with acid - that is quartz. B is soft enough for a copper coin (H ≈ 3), breaks into rhombohedra with three cleavages near 90°, and fizzes vigorously - that is calcite. C is very soft (H ≈ 2.5), has cubic cleavage, and has a density of 2.16 g/cm³, which is diagnostic for halite. A revised key should branch on reaction to acid and cleavage before density, using density only as a confirmatory check at the end.
2. Explain why last year's calcite sample may not have fizzed, and describe how to run the acid test so the result is trustworthy.
Read the worked answer
There are several ordinary explanations, and all of them should be checked before concluding that B is not calcite. First, a weathered or dusty surface: a thin coating of clay, dust, or secondary minerals can shield the carbonate from the acid, so a drop placed on an old surface produces few or no bubbles. Second, the sample may have been dolomite rather than calcite; dolomite reacts only weakly with cold dilute HCl and often only when powdered, so a teacher's 'calcite' labeled from a supply catalog may be mislabeled or may be a dolomite specimen. Third, the acid may have been too dilute or applied to a wet surface, diluting it further. Fourth, the observer may have looked for bubbles in the wrong place - fizzing is best seen where the drop sits on a roughened surface.
The reliable procedure: place a fresh, dry specimen on a white streak plate, scratch a small amount of powder from an inconspicuous corner, add one drop of 10% HCl to the powder, and observe with a hand lens. Powdering increases surface area and exposes unweathered material, so calcite fizzes vigorously, dolomite fizzes weakly or only as powder, and quartz or halite do not react. Students should record 'vigorous / weak / none' rather than yes/no.
3. Respond to the tasting suggestion, and identify an additional test that would confirm C without acid or taste.
Read the worked answer
Tasting is never acceptable in a school lab. Unknown specimens may carry dust, labeling residues, or other contaminants, and halite's salty taste is a field observation, not a laboratory test; the correct response is to redirect the student to a property that is safe, observable, and diagnostic. Instruct the group that the 'taste' property is excluded from the key by policy, and have them write the reason in their notebooks.
C can be confirmed safely by combining three independent observations. Its cubic cleavage is distinctive and distinguishes it from calcite's rhombohedral cleavage. Its hardness of about 2.5 means a fingernail scratches it but a copper coin does too, which fits halite and excludes quartz. Its density of 2.16 g/cm³ is far below both quartz and calcite and matches halite. A confirming test is solubility: a small chip stirred into 20 mL of water dissolves completely and the solution conducts electricity, whereas quartz and calcite do not dissolve under those conditions.
If the acid supply is exhausted, A and B can still be separated by hardness - quartz scratches glass, calcite does not - and by calcite's double refraction, which produces a doubled image of a line drawn on paper beneath a clear cleavage fragment.