Feldspar Origins and Meaning

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Questions and Answers

Which of the following statements accurately describes the difference between orthoclase and sanidine?

  • Orthoclase and sanidine are both triclinic K-feldspars, but sanidine occurs in volcanic rocks, while orthoclase occurs in plutonic rocks. (correct)
  • Orthoclase is monoclinic and occurs in plutonic rocks, while sanidine is triclinic and occurs in volcanic rocks.
  • Orthoclase is triclinic and occurs in volcanic rocks, while sanidine is monoclinic and occurs in plutonic rocks.
  • Orthoclase and sanidine are both monoclinic K-feldspars, but orthoclase occurs in volcanic rocks, while sanidine occurs in plutonic rocks.

Which of the following statements accurately describes the structural state of alkali feldspars?

  • The structural state is dependent on the composition and the degree of ordering of the aluminum and silicon atoms.
  • The structural state is dependent on the presence of potassium or sodium in the feldspar composition.
  • The structural state is dependent on the temperature of crystallization and subsequent cooling history. (correct)
  • The structural state is dependent on the degree of twinning and the optical properties of the feldspar.

Which of the following statements accurately describes the characteristics of adularia?

  • Adularia is a K-feldspar produced by low-temperature hydrothermal or authigenic processes, and it is structurally similar to microcline but without the cross-hatched twinning. (correct)
  • Adularia is a Na-feldspar produced by low-temperature hydrothermal or authigenic processes, and it is structurally similar to sanidine but without twinning.
  • Adularia is a K-feldspar produced by high-temperature magmatic processes, and it is structurally similar to orthoclase but with cross-hatched twinning.
  • Adularia is a Na-feldspar produced by high-temperature hydrothermal or authigenic processes, and it is structurally similar to anorthoclase but with cross-hatched twinning.

Which of the following statements accurately describes the characteristics of anorthoclase?

<p>Anorthoclase is the only alkali feldspar that is not a K-feldspar; it is sodic in composition, optically triclinic, and characterized by a twinning similar to microcline but on a smaller scale. (A)</p> Signup and view all the answers

Which of the following statements accurately describes the cooling history of volcanic feldspars?

<p>Volcanic feldspars tend to retain their high-temperature ordering because the cooling is very rapid, resulting in the formation of sanidine. (B)</p> Signup and view all the answers

Which of the following statements accurately describes the characteristics of microcline?

<p>Microcline is a triclinic K-feldspar and has a characteristic crosshatched tartan pattern of twinning seen with a petrographic microscope. (B)</p> Signup and view all the answers

What is the origin of the word 'feldspar'?

<p>It is derived from the German words 'Feld' (field) and 'Spat' (flake) (B)</p> Signup and view all the answers

What is the approximate percentage of feldspars in the Earth's crust?

<p>60% (D)</p> Signup and view all the answers

What is the relationship between the ionic radii and charges of the K and Ca endmembers of feldspars?

<p>They have different ionic radii and charges, making the structure unstable (A)</p> Signup and view all the answers

What is the relationship between the compositional endmembers of feldspars?

<p>K-feldspar (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8) are the three endmembers (C)</p> Signup and view all the answers

What is the relationship between alkali feldspars and plagioclase feldspars?

<p>Alkali feldspars are compositionally between the K-feldspar and albite endmembers, while plagioclase feldspars are between the albite and anorthite endmembers (C)</p> Signup and view all the answers

What is the significance of the barium-containing feldspars celsian and hyalophane?

<p>They are volumetrically very restricted and not as common as other feldspars (D)</p> Signup and view all the answers

What is the primary reason that plagioclase feldspars are the most common feldspar minerals?

<p>Calcium is more abundant in the Earth's crust than potassium. (D)</p> Signup and view all the answers

Why is the substitution of ions in plagioclase feldspars a coupled process?

<p>Because the substitution of Na+ and Ca2+ ions requires a charge balance to be maintained. (B)</p> Signup and view all the answers

What is the significance of the boundaries in the ternary diagram for feldspar mineral classification?

<p>The boundaries have no structural significance, but are justified because plagioclase is a very common mineral. (D)</p> Signup and view all the answers

How are alkali feldspars classified, in contrast to plagioclase feldspars?

<p>Alkali feldspars are classified based on their chemical composition and optical properties. (C)</p> Signup and view all the answers

What is the relationship between the composition of plagioclase feldspars and the type of igneous rock they are found in?

<p>Plagioclase feldspars with more calcium are found in mafic igneous rocks, while those with more sodium are found in felsic igneous rocks. (D)</p> Signup and view all the answers

What is the primary difference between the classification of plagioclase feldspars and alkali feldspars?

<p>Plagioclase feldspars are classified into a continuous solid solution series, while alkali feldspars are classified into separate mineral species. (A)</p> Signup and view all the answers

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