Explanation:
Oxygen is overwhelmingly the most abundant chalcogen in the crust, sulfur is much more abundant than selenium and tellurium, and polonium occurs only in minute amounts as a radioactive decay product.
2. The predominance of oxygen in the Earth’s crust is best explained by its occurrence mainly as:
A.
Native oxygen trapped in metallic ores
B.
Oxides, silicates, carbonates, sulfates, and water
C.
Sulfides of iron and copper only
D.
Elemental oxygen dissolved in sulfide minerals
Correct Answer:
Oxides, silicates, carbonates, sulfates, and water
Explanation:
Oxygen is extensively distributed in the crust in oxide and oxygen-containing minerals, especially silicates, as well as in water and several oxoanionic minerals.
3. Which mineral association is most characteristic of the geochemical occurrence of sulfur?
A.
Sulfur occurs almost exclusively as oxide minerals
B.
Sulfur occurs as native sulfur and in sulfide and sulfate minerals
C.
Sulfur occurs only as dissolved sulfate in seawater
D.
Sulfur is found mainly as a constituent of aluminosilicate lattices
Correct Answer:
Sulfur occurs as native sulfur and in sulfide and sulfate minerals
Explanation:
Sulfur has a diverse geochemical distribution: it may occur in the native state near volcanic regions and also as sulfide minerals and sulfate minerals such as gypsum and barite.
4. Why are selenium and tellurium commonly recovered as by-products rather than mined from large independent deposits?
A.
They are geochemically associated with sulfide ores of metals such as copper, lead, and silver
B.
They are converted completely into atmospheric gases before mineral formation
C.
They occur only in sedimentary carbonate rocks
D.
They are present mainly as insoluble silicates in granite
Correct Answer:
They are geochemically associated with sulfide ores of metals such as copper, lead, and silver
Explanation:
Selenium and tellurium are relatively scarce chalcogens and commonly substitute for sulfur or occur in related sulfide mineral environments, so they are obtained during the processing of metallic sulfide ores.
5. A geological sample contains abundant copper sulfide minerals and trace selenium and tellurium. Which interpretation is most consistent with chalcogen geochemistry?
A.
Selenium and tellurium are likely associated with the sulfide ore-forming environment
B.
The sample must have formed exclusively from atmospheric oxygen
C.
The trace elements prove that the deposit is composed mainly of sulfate minerals
D.
Selenium and tellurium cannot occur in the same geological setting as sulfur
Correct Answer:
Selenium and tellurium are likely associated with the sulfide ore-forming environment
Explanation:
Selenium and tellurium commonly occur in trace concentrations in metallic sulfide deposits because of their chemical and structural relationship with sulfur.
6. Which statement best distinguishes the geochemical occurrence of sulfur from that of selenium and tellurium?
A.
Sulfur has important native and sulfate occurrences, whereas selenium and tellurium are generally much rarer and associated with sulfide ores
B.
Sulfur occurs only in sulfides, whereas selenium and tellurium occur abundantly as native elements
C.
Sulfur is less abundant than both selenium and tellurium in the crust
D.
Selenium and tellurium are major constituents of silicate rocks, unlike sulfur
Correct Answer:
Sulfur has important native and sulfate occurrences, whereas selenium and tellurium are generally much rarer and associated with sulfide ores
Explanation:
Sulfur is comparatively abundant and has significant native, sulfide, and sulfate forms, while selenium and tellurium are scarce and are most characteristically linked with sulfide mineral deposits.
7. Polonium is absent as a major constituent of ordinary mineral deposits primarily because it:
A.
Is highly soluble in groundwater
B.
Has no naturally occurring isotopes
C.
Is radioactive and is formed only in minute amounts in decay series
D.
Forms stable silicate minerals that are too deep to sample
Correct Answer:
Is radioactive and is formed only in minute amounts in decay series
Explanation:
Naturally occurring polonium is generated in the radioactive decay series of uranium and thorium, and its short-lived isotopes prevent accumulation as an abundant crustal mineral.
8. Which pair correctly matches a chalcogen with its principal geochemical mode of occurrence?
A.
Oxygen — chiefly as sulfide minerals
B.
Sulfur — native sulfur, sulfides, and sulfates
C.
Tellurium — major constituent of silicate minerals
D.
Polonium — abundant stable oxide deposits
Correct Answer:
Sulfur — native sulfur, sulfides, and sulfates
Explanation:
Sulfur displays all three important forms of occurrence: native sulfur, sulfide minerals, and sulfate minerals, unlike the other listed mismatched associations.
9. The occurrence of native sulfur near volcanic regions is most directly related to:
A.
Deposition or crystallization of sulfur from sulfur-bearing volcanic gases
B.
Reduction of all atmospheric oxygen by silicate minerals
C.
Weathering of tellurium-rich carbonate rocks
D.
Radioactive decay of oxygen isotopes
Correct Answer:
Deposition or crystallization of sulfur from sulfur-bearing volcanic gases
Explanation:
Volcanic gases can contain sulfur compounds such as hydrogen sulfide and sulfur dioxide; reactions, cooling, and oxidation near volcanic vents can lead to deposition of native sulfur.
10. Which inference about chalcogen distribution is most scientifically justified by the occurrence of selenium and tellurium in copper-refining residues?
A.
Their concentration during processing reflects their low but significant association with copper sulfide ores
B.
They are more abundant than oxygen but are removed during silicate formation
C.
They originate exclusively from decomposition of atmospheric water
D.
Their presence proves that copper ores formed as pure sulfate deposits
Correct Answer:
Their concentration during processing reflects their low but significant association with copper sulfide ores
Explanation:
Selenium and tellurium commonly accompany copper and other metals in sulfide ores; metallurgical processing can therefore concentrate them in residues even though their crustal abundance is low.
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Geochemical Distribution of Chalcogens
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