18 Questions
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.
Which of the following statements accurately describes the structural state of alkali feldspars?
The structural state is dependent on the temperature of crystallization and subsequent cooling history.
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.
Which of the following statements accurately describes the characteristics of anorthoclase?
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.
Which of the following statements accurately describes the cooling history of volcanic feldspars?
Volcanic feldspars tend to retain their high-temperature ordering because the cooling is very rapid, resulting in the formation of sanidine.
Which of the following statements accurately describes the characteristics of microcline?
Microcline is a triclinic K-feldspar and has a characteristic crosshatched tartan pattern of twinning seen with a petrographic microscope.
What is the origin of the word 'feldspar'?
It is derived from the German words 'Feld' (field) and 'Spat' (flake)
What is the approximate percentage of feldspars in the Earth's crust?
60%
What is the relationship between the ionic radii and charges of the K and Ca endmembers of feldspars?
They have different ionic radii and charges, making the structure unstable
What is the relationship between the compositional endmembers of feldspars?
K-feldspar (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8) are the three endmembers
What is the relationship between alkali feldspars and plagioclase feldspars?
Alkali feldspars are compositionally between the K-feldspar and albite endmembers, while plagioclase feldspars are between the albite and anorthite endmembers
What is the significance of the barium-containing feldspars celsian and hyalophane?
They are volumetrically very restricted and not as common as other feldspars
What is the primary reason that plagioclase feldspars are the most common feldspar minerals?
Calcium is more abundant in the Earth's crust than potassium.
Why is the substitution of ions in plagioclase feldspars a coupled process?
Because the substitution of Na+ and Ca2+ ions requires a charge balance to be maintained.
What is the significance of the boundaries in the ternary diagram for feldspar mineral classification?
The boundaries have no structural significance, but are justified because plagioclase is a very common mineral.
How are alkali feldspars classified, in contrast to plagioclase feldspars?
Alkali feldspars are classified based on their chemical composition and optical properties.
What is the relationship between the composition of plagioclase feldspars and the type of igneous rock they are found in?
Plagioclase feldspars with more calcium are found in mafic igneous rocks, while those with more sodium are found in felsic igneous rocks.
What is the primary difference between the classification of plagioclase feldspars and alkali feldspars?
Plagioclase feldspars are classified into a continuous solid solution series, while alkali feldspars are classified into separate mineral species.
Learn about the origins of the word 'feldspar' and its meaning. Discover how the term evolved over time and its association with rocks and minerals.
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