MCQ Detailed View

The appearance and color of sedimentary rocks are important features that help geologists interpret the conditions under which the rocks were formed. Among the various factors influencing their coloration, iron plays the most dominant role.Sedimentary rocks are formed from the accumulation and cementation of sediments such as sand, clay, or organic remains. The natural minerals present in these sediments contribute to the final color of the rock. However, the most significant color variations are caused by iron compounds in different oxidation states.

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  Ferric Oxide (Fe₂O₃): Produces red, brown, or yellow shades. Rocks rich in hematite (a ferric oxide mineral) often appear red. This is commonly seen in red sandstones.

  
  


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  Ferrous Compounds (Fe²⁺): These produce darker colors such as gray, green, or even black, depending on the mineral and reducing environment. For example, iron in reduced form can cause a greenish tint in some shales.

  
  


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  Limonite (FeO(OH)·nH₂O): Produces yellowish-brown tones.

  
  


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  Magnetite (Fe₃O₄): Can contribute to darker black coloration.

  
  

Calcium, on the other hand, though abundant in rocks like limestone, does not significantly influence the rock’s color unless combined with other pigments. Similarly, zinc rarely plays any role in determining the coloration of sedimentary rocks.

Geologists use rock color as a field indicator of depositional environments:

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  Red and brown rocks → Formed in oxidizing environments (like deserts).

  
  


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  Green and gray rocks → Formed in reducing environments (like swamps or deep seas).

  
  


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  Black shales → Usually rich in organic matter, deposited in oxygen-poor environments.

  
  

Thus, the presence of iron in different forms and oxidation states is the primary factor controlling sedimentary rock colors.

✅ Key Points to Remember

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  Sedimentary rock color is mainly influenced by iron minerals.

  
  


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  Oxidized iron (Fe³⁺) → red, yellow, brown colors.

  
  


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  Reduced iron (Fe²⁺) → gray, green, or dark shades.

  
  


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  Calcium or zinc do not significantly affect rock coloration.

  
  

👉 Therefore, the correct answer is Iron (Option 2).