18 Questions
What is the main reason plagioclase feldspars are the most common feldspar minerals?
Calcium is more abundant in the crust than potassium.
How are plagioclase feldspars able to maintain charge balance when Na+ and Ca2+ ions are substituted?
By substituting Al3+ for Si4+.
Why is the amount of potassium that can enter the alkali feldspar lattice limited?
Potassium has a larger ionic radius compared to sodium and calcium.
What is the key difference in how plagioclase and alkali feldspars are classified?
Plagioclase is classified based on chemistry, while alkali feldspars are classified based on optical properties.
What is the general trend in plagioclase composition from granitic to mafic igneous rocks?
Plagioclase becomes more calcium-rich (more anorthite) in mafic rocks.
Why are the boundaries between the alkali feldspar minerals on the ternary diagram said to have no structural significance?
The use of the boundaries is justified because plagioclase is a very common and predictable mineral.
What is the meaning of the word 'Feldspar'?
A combination of 'field' and 'flake'
Why was the term 'Feldspar' introduced in the 18th century?
To describe rocks found in the fields
Which compositional endmember of feldspar contains calcium?
Anorthite
Why are there no feldspars with compositions intermediate between K-feldspar and Ca endmembers?
They have different ionic radii and charges
Which feldspar contains barium in its crystal structure?
Celsian
Why are celsian and hyalophane considered to be volumetrically restricted?
Because they are less common than other feldspars
Which of the following best describes microcline?
A triclinic K-feldspar with a crosshatched twinning pattern
Which feldspar is found in volcanic and subvolcanic rocks?
Sanidine
What is the structural state of alkali feldspars dependent on?
The temperature of crystallization and subsequent cooling history
Which of the following is NOT a K-feldspar?
Anorthoclase
What is the characteristic feature of adularia?
It is structurally similar to microcline but without the cross-hatched twinning
Which feldspar occurs in plutonic rocks?
Orthoclase
Learn about the origin and meaning of the term 'feldspar', its introduction as a specific term in the 18th century, and its abundance as a mineral group. Explore the influence of the English word 'spar' and the properties of feldspar minerals.
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