Feldspar Diagram Quiz and Flashcards for Learning

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Questions and Answers

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

Plagioclase feldspars have a continuous solid solution at high temperatures.

Calcium is more abundant in the crust than potassium. (correct)

Plagioclase feldspars can substitute Al3+ for Si4+ to maintain charge balance.

Alkali feldspars have a more limited potassium content due to ionic radius differences.

How are plagioclase feldspars able to maintain charge balance when Na+ and Ca2+ ions are substituted?

By substituting Al3+ for Si4+. (correct)

By substituting Si4+ for Al3+.

Plagioclase feldspars cannot maintain charge balance during Na+ and Ca2+ substitution.

By increasing the lattice size to accommodate the charge difference.

Why is the amount of potassium that can enter the alkali feldspar lattice limited?

Alkali feldspars have a discontinuous solid solution series.

Potassium has a higher abundance in the crust compared to calcium.

Potassium has a larger ionic radius compared to sodium and calcium. (correct)

Alkali feldspars do not have the ability to substitute Al3+ for Si4+.

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. Signup and view all the answers

What is the general trend in plagioclase composition from granitic to mafic igneous rocks?

Plagioclase becomes more calcium-rich (more anorthite) in mafic rocks. Signup and view all the answers

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. Signup and view all the answers

What is the meaning of the word 'Feldspar'?

A combination of 'field' and 'flake' Signup and view all the answers

Why was the term 'Feldspar' introduced in the 18th century?

To describe rocks found in the fields Signup and view all the answers

Which compositional endmember of feldspar contains calcium?

Anorthite Signup and view all the answers

Why are there no feldspars with compositions intermediate between K-feldspar and Ca endmembers?

They have different ionic radii and charges Signup and view all the answers

Which feldspar contains barium in its crystal structure?

Celsian Signup and view all the answers

Why are celsian and hyalophane considered to be volumetrically restricted?

Because they are less common than other feldspars Signup and view all the answers

Which of the following best describes microcline?

A triclinic K-feldspar with a crosshatched twinning pattern Signup and view all the answers

Which feldspar is found in volcanic and subvolcanic rocks?

Sanidine Signup and view all the answers

What is the structural state of alkali feldspars dependent on?

The temperature of crystallization and subsequent cooling history Signup and view all the answers

Which of the following is NOT a K-feldspar?

Anorthoclase Signup and view all the answers

What is the characteristic feature of adularia?

It is structurally similar to microcline but without the cross-hatched twinning Signup and view all the answers

Which feldspar occurs in plutonic rocks?

Orthoclase Signup and view all the answers

Study Notes

Plagioclase Feldspars

Plagioclase feldspars are the most common feldspar minerals due to their widespread occurrence in igneous, metamorphic, and sedimentary rocks.
They maintain charge balance during Na+ and Ca2+ ion substitution through the incorporation of additional Al3+ ions in the structure, allowing for negative charge balance.
The amount of potassium that can enter the alkali feldspar lattice is limited by the lattice dimensions, which restrict larger ions like K+ due to their size compared to Na+ and Ca2+.

Classification of Feldspars

Plagioclase feldspars are classified based on their composition, primarily with respect to the ratios of Na and Ca, while alkali feldspars are classified primarily based on the presence of K+.
The general trend in plagioclase composition shifts from Na-rich minerals in granitic rocks to Ca-rich minerals in mafic rocks.
Boundaries between alkali feldspar minerals on the ternary diagram lack structural significance as they do not correspond to distinct changes in crystal structure.

Etymology and Composition

The term 'Feldspar' derives from German, meaning 'field stone,' as they were originally found in many field settings.
The term 'Feldspar' was introduced in the 18th century to categorize the group of minerals characterized by their shared structure and composition.
The compositional endmember of feldspar that contains calcium is anorthite.

Intermediate Compositions

There are no feldspars with compositions intermediate between K-feldspar and Ca endmembers due to the dominance of the specific ionic sizes and crystal preference in the structure.
Celsian is the feldspar that contains barium (Ba) in its crystal structure.
Celsian and hyalophane are considered volumetrically restricted because their occurrences are relatively rare compared to other feldspar types.

Specific Feldspar Characteristics

Microcline is best described as a low-temperature polymorph of potassium feldspar with a distinct microcline structure.
Alkali feldspar found in volcanic and subvolcanic rocks typically includes sanidine due to its formation from rapid crystallization.
The structural state of alkali feldspars is dependent on temperature conditions during crystallization.
Adularia, a type of K-feldspar, is characterized by its twinning and occurs in veins and cavities.
Plagioclase occurs commonly in plutonic rocks, reflecting its formation under slow cooling conditions within the earth's crust.

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Description

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.