- A student identifies a mineral as sulfur based solely on its distinct yellow color. However, a subsequent test shows the mineral can easily scratch a copper penny (Mohs hardness ≈ 3.5). Why is the student's initial identification likely incorrect?
Answer: The mineral's hardness is greater than that of sulfur.
This question requires evaluating conflicting evidence. While color can be a helpful clue, it can also be misleading. Hardness is a more reliable diagnostic property. Sulfur has a very low hardness of 1.5-2.5. The test showing the mineral can scratch a copper penny proves its hardness is greater than 3.5. This directly contradicts the known hardness of sulfur, making the identification incorrect. While sulfur is an element, it is also classified as a mineral when found in its natural crystalline form.
- A mineral has a metallic luster and is black in color. A geology student performs a streak test on an unglazed porcelain plate. Which of the following observations would be the least likely result of this test?
Answer: The streak is white.
Minerals with a metallic luster almost always have a dark-colored (black, gray, brown, green) streak. A white or colorless streak is characteristic of non-metallic, light-colored minerals. Therefore, a white streak would be the least likely observation for a black, metallic mineral. Reddish-brown is typical for hematite, greenish-black for pyrite, and dark gray for galena.
- A student describes a mineral's luster as "shiny like a metal, but dark and somewhat dull, like tarnished silver." Which of the following technical terms for luster best fits this specific description?
Answer: Submetallic
This question tests nuanced terminology. While the mineral is metallic-looking, the qualifiers 'dull' and 'tarnished' point specifically to the term 'submetallic'. This luster is characteristic of opaque minerals that are not as reflective as true metals (e.g., galena, pyrite). Adamantine is exceptionally brilliant (like diamond), vitreous is glassy, and metallic is a broader category for highly reflective, opaque minerals.
- An explorer uses a mineral sample to scratch a line on a copper plate (Mohs hardness ≈ 3.5). Later, the explorer finds that a steel knife (Mohs hardness ≈ 5.5) easily scratches the mineral sample. Based on these two tests, what can be concluded about the mineral's hardness?
Answer: The mineral's hardness is between 3.5 and 5.5.
This is a two-step logic problem. The first test shows the mineral is harder than the copper plate, so its hardness must be greater than 3.5. The second test shows the mineral is softer than the steel knife, so its hardness must be less than 5.5. Combining these two facts establishes a hardness range between 3.5 and 5.5.
- A geologist finds a dark-colored, metallic mineral that forms cubic crystals. A streak test produces a dark gray powder. Which of the following tests would be most useful to definitively distinguish between galena and pyrite?
Answer: Testing its hardness against a copper penny (hardness ≈ 3.5).
Both galena and pyrite are metallic, form cubic crystals, and have a dark streak. The most effective distinguishing property is hardness. Galena has a hardness of 2.5, while pyrite has a hardness of 6-6.5. A copper penny (hardness ≈ 3.5) will scratch galena but will not scratch pyrite, making this a definitive test. Reaction to acid is used for carbonates, crystal face angles for both are 90°, and neither is strongly magnetic.
- A student analyzes a transparent mineral. It cannot be scratched by a fingernail (H=2.5) but can be scratched by a copper penny (H=3.5). When struck, it splits along three planes that are not at 90-degree angles. Which mineral is most likely being described?
Answer: Calcite
The properties described point to calcite. The hardness is between 2.5 and 3.5 (calcite's is 3). Splitting along three planes not at 90 degrees describes rhombohedral cleavage, which is characteristic of calcite. Halite has cubic cleavage (3 planes at 90°). Quartz has no cleavage (fracture) and is much harder (H=7). Fluorite has octahedral cleavage (4 planes) and is harder (H=4).
- A student has two visually similar, transparent, crystalline mineral samples. Sample 1 can scratch a glass plate (H=5.5). Sample 2 cannot scratch the glass plate, but a drop of dilute HCl placed on it fizzes. What are the likely identities of Sample 1 and Sample 2?
Answer: Sample 1 = Quartz, Sample 2 = Calcite
Sample 1 scratches glass (H=5.5), so its hardness is > 5.5. Quartz (H=7) is a common transparent mineral that fits this description. Sample 2 has a hardness < 5.5 and fizzes in HCl. The reaction with acid is the definitive test for calcite (a carbonate mineral). Calcite's hardness is 3, which is consistent with H < 5.5. The other pairs have incorrect properties.
- A student is using a Mohs hardness kit. The unknown mineral sample is scratched by the topaz point (H=8) but is not scratched by the quartz point (H=7). The sample successfully scratches the quartz point. What is the hardness of the sample?
Answer: Between 7 and 8
The tests establish a range for the mineral's hardness. 'Scratched by topaz (H=8)' means the mineral's hardness is less than 8. 'Not scratched by quartz (H=7)' and 'scratches quartz' both mean the mineral's hardness is greater than 7. Therefore, the hardness must be between 7 and 8. Stating an exact value like 7.5 is not possible with these tools; only a range can be determined.
- An 80.0 cm³ sample of a mineral with perfect cubic cleavage is found to have a mass of 176.0 g. If this sample were broken into several smaller pieces along its cleavage planes, which of the following statements would be true for each piece?
Answer: The density would be 2.2 g/cm³.
This is a multi-step problem. First, calculate the density of the original sample: D = mass/volume = 176.0 g / 80.0 cm³ = 2.2 g/cm³. Density is an intrinsic physical property, meaning it does not change with the size or shape of the sample. When the mineral is broken along its cleavage planes, each smaller piece will still be composed of the same substance and will therefore have the same density of 2.2 g/cm³. The cleavage property also remains.
- A student is testing an unknown mineral. The mineral scratches a glass plate (hardness 5.5) but is scratched by a quartz crystal (hardness 7). It exhibits a greasy luster and breaks along curved, irregular surfaces. Using the Mineral Properties table, which mineral is the student most likely testing?
Answer: Olivine
The student must synthesize four clues. First, the hardness is between 5.5 and 7. Second, the luster is greasy. Third, breaking along curved, irregular surfaces indicates conchoidal or irregular fracture, not cleavage. The student must consult the table. Olivine fits all criteria: hardness of 6.5-7, nonmetallic (vitreous/greasy) luster, and conchoidal fracture. Quartz has a hardness of 7 and might seem plausible, but the stem says it is scratched by quartz, implying its hardness is less than 7. Plagioclase has cleavage, and Halite is much too soft.
- A student describes a mineral as feeling "soapy" or "greasy" to the touch. The mineral is extremely soft, easily scratched by a fingernail, and leaves a white streak. Which underlying physical property is primarily responsible for the soapy feel?
Answer: Its very low hardness and perfect basal cleavage.
The description points to the mineral talc. The characteristic 'soapy' or 'greasy' feel is a direct result of its physical structure. Talc has a hardness of 1 on the Mohs scale and perfect basal cleavage, meaning its atoms are bonded into sheets that are held together by extremely weak forces. When touched, these sheets easily slide past one another, creating the soapy sensation. Its luster is pearly or greasy, not vitreous, and its specific gravity is average.
- A student rubs a mineral vigorously against a standard porcelain streak plate (hardness ≈ 7), but no powdered streak is produced. Instead, a scratch appears on the surface of the streak plate. What is the most reliable conclusion that can be drawn from this result?
Answer: The mineral's hardness is greater than 7.
The streak test relies on powdering the mineral against the plate. If the mineral is harder than the streak plate, it will scratch the plate instead of being powdered. Since the mineral scratched the plate (hardness ≈ 7), the mineral's hardness must be greater than 7. This is a valid and informative result, not an error. The conclusion that the streak is white is a common misconception; the white powder seen is from the scratched plate itself, not the mineral.
- A mineral fragment is an elongated, six-sided prism. When the geologist tries to break it, it fractures irregularly across the prism, rather than splitting parallel to any of its long crystal faces. This observation suggests the mineral primarily exhibits:
Answer: a well-developed crystal form but lacks cleavage.
The six-sided prism shape describes the mineral's crystal form or habit, which is its external growth shape. The fact that it breaks irregularly (fractures) rather than splitting along flat planes parallel to its faces indicates a lack of cleavage. Many minerals, like quartz, have a distinct crystal form but do not have cleavage. Basal cleavage would be a single flat break perpendicular to the prism's length, which was not observed.
- A perfectly cubic crystal of a mineral with a metallic luster has a side length of 2.0 cm and a mass of 60.0 g. Based on the provided data, what is the identity of this mineral?
Answer: Galena (D ≈ 7.5 g/cm³)
The student must first calculate the volume of the cube and then its density. The volume of a cube is side³, so V = (2.0 cm)³ = 8.0 cm³. Density is mass/volume, so D = 60.0 g / 8.0 cm³ = 7.5 g/cm³. The mineral has a metallic luster and a density of 7.5 g/cm³, which matches the properties of galena. Pyrite is a plausible distractor because it is also metallic and cubic, but its density is incorrect.
- A field geologist needs to distinguish between earthy hematite and cuprite, which can appear visually similar. Only two quick field tests can be performed without specialized equipment. Which pair of tests would be most effective?
Answer: Hardness and Streak
Both luster (earthy/dull) and color (reddish-brown) can be very similar between these two minerals, making them poor diagnostic tests. Density is a good differentiator but is difficult to measure accurately in the field. The most effective quick tests are hardness and streak. Hematite (H=5-6) is significantly harder than cuprite (H=3.5-4), a difference easily tested with a pocket knife. Their streaks, while both reddish, are subtly different and a key diagnostic tool.
- A mineralogist strikes a mineral, and it breaks into several smooth, flat surfaces oriented in three directions, all meeting at 90-degree angles. A second mineral, when struck, shatters into irregular, curved pieces. What properties are demonstrated by the first and second minerals, respectively?
Answer: Cubic cleavage; Conchoidal fracture
The first mineral demonstrates cleavage, which is the tendency to break along planes of weak atomic bonding. Breaking into flat surfaces in three directions at 90° is specifically called cubic cleavage. The second mineral shatters irregularly with curved surfaces, which is known as conchoidal fracture. Fracture is breakage that is not along a cleavage plane.
- A mineralogist is examining two mineral specimens. Specimen A consistently breaks into thin, flat sheets. Specimen B consistently breaks into blocky, rhomb-shaped pieces with no right angles. Which statement accurately describes the cleavage of these two specimens?
Answer: Specimen A has one direction of cleavage; Specimen B has three directions of cleavage.
Cleavage describes how a mineral breaks along flat planes. Specimen A breaking into sheets (like mica) demonstrates perfect cleavage in one direction. Specimen B breaking into rhombs (like calcite) demonstrates cleavage in three directions, but not at 90-degree angles. 'Fracture' is the term for irregular breakage, so distractors C and D use incorrect terminology.
