Question

Difficulty: HardProcedural Steps

To determine the age of archaeological sediments using Optically Stimulated Luminescence (OSL), researchers extract coarse-grain quartz under strictly controlled conditions to prevent resetting the geological signal. Under dim red light, the raw sediment is first sieved to isolate grains between 9090 and 250250 micrometers. Subsequently, the isolated fraction is treated with hydrochloric acid (textHCl\\text{HCl}) to dissolve carbonates, which could otherwise interfere with subsequent chemical treatments. Once the effervescence ceases, the sample is washed prior to being treated with hydrogen peroxide (textH2textO2\\text{H}_2\\text{O}_2) to remove organic matter. Crucially, the remaining minerals are then subjected to a dense liquid separation using sodium polytungstate at a specific gravity of 2.62textg/cm32.62\\text{ g/cm}^3 to float off potassium feldspars, which exhibit unwanted luminescence signals. Finally, the remaining heavy fraction is etched with concentrated hydrofluoric acid (textHF\\text{HF}) for 4040 minutes. This textHF\\text{HF} etching serves a dual purpose: it dissolves any remaining feldspars and simultaneously etches away the alpha-irradiated outer shell of the quartz grains, leaving pure quartz cores ready for measurement.

Based on the passage, if a researcher omits the dense liquid separation step using sodium polytungstate, which of the following is the most likely consequence for the final quartz sample?

  1. The final quartz sample will remain contaminated with potassium feldspars, as the 4040-minute hydrofluoric acid (textHF\\text{HF}) etch is only intended to clear residual traces rather than the bulk of these minerals.Answer
  2. B
    The initial sieving process will fail to isolate grains within the 9090 to 250250 micrometer range due to the presence of unseparated minerals.
  3. C
    The hydrochloric acid treatment will fail to dissolve carbonates because the potassium feldspars will shield them from chemical reaction.
  4. D
    The hydrogen peroxide treatment will react directly with potassium feldspars, preventing it from successfully removing organic matter.

Answer

The final quartz sample will remain contaminated with potassium feldspars, as the 4040-minute hydrofluoric acid (textHF\\text{HF}) etch is only intended to clear residual traces rather than the bulk of these minerals.
The correct answer is correct because the passage establishes that the dense liquid separation is used to float off potassium feldspars, and the subsequent hydrofluoric acid (textHF\\text{HF}) etching step is designed to dissolve 'any remaining feldspars.' If the dense liquid separation is omitted, the bulk of the feldspars will remain, and the 4040-minute hydrofluoric acid (textHF\\text{HF}) etching will not be sufficient to remove them all, resulting in a contaminated quartz sample.

Step-by-Step Solution

1
Identify the step that is omitted and locate its description in the passage.
The omitted step is the dense liquid separation using sodium polytungstate, which is used to float off potassium feldspars.
Understanding the function of the omitted step is necessary to determine its downstream consequences.
2
Analyze the subsequent steps in the procedure that follow the omitted step.
Following the dense liquid separation, the remaining heavy fraction undergoes a 4040-minute hydrofluoric acid (textHF\\text{HF}) etch, which dissolves 'any remaining feldspars' (residual traces) and etches the quartz cores.
Evaluating downstream steps reveals how the absence of the preceding step affects them.
3
Synthesize the relationship between the dense liquid separation and the hydrofluoric acid (textHF\\text{HF}) etching step to deduce the consequence of the omission.
Because the dense liquid separation is skipped, the bulk of the potassium feldspars is not floated off. Consequently, the 4040-minute hydrofluoric acid (textHF\\text{HF}) etch (which is only designed to handle remaining/trace feldspars) will be insufficient to dissolve all of them, leading to a contaminated quartz sample.
Connecting the roles of both steps allows for the correct logical deduction of the procedure's failure mode.

Key Concept

Identifying downstream consequences of omitting a step in a multi-stage scientific procedure by analyzing sequential relationships.
Estimated Time:2m 0s
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