Which of the following statements is FALSE?

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Question focus: identifying which statement about isomorphous substitution is false.
Option 1: 'Isomorphous substitution is the same as cation exchange' – This is incorrect as a general statement. Isomorphous substitution is a process during crystal lattice formation where one ion substitutes for another of similar size within the mineral’s structure, often yielding permanent lattice charge. Cation exchange, by contrast, is a surface process where ions are exchanged between the mineral surface and the surrounding solution without altering the crystal lattice itself. So equating the two conflates a lattice substitution mechanism with a surface exchange process.
Option 2: 'Isomorphous substitution leads to a permanent negative charge on the surface of a 2:1 cystalline silicate clay minerals' – This is essentially correct. When a tetrahedral or octahedral site in a 2:1 clay layer is substituted by a ion of lower valence (e.g., Al3+ for Si4+), a net negative charge is introduced that is structural and persistent, independent of pH. This qualifies as a permanent charge feature of many 2:1 clays.
Option 3: 'Isomorphous substitution occurs during the formation of secondary clay mineral crystalline structure.' – This statement is true. Isomorphous substitution happens during mineral crystallization when different ions occupy sites in the crystal lattice, contributing to the mineral’s identity and charge characteristics.
Option 4: 'In isomorphous substitution the substituting element has similar size but lower valence than the element being replaced in the crystalline structure' – This describes the common scenario: ion size compatibility allows substitution without major lattice distortion, and a lower valence introduces the negative charge. This is accurate as a general rule.
In summary, the statement that is false is Option 1, because isomorphous substitution differs from cation exchange and involves lattice-level replacement rather than surface ion exchange.

MERGE_2025_0322 Quiz 3 Week 5 revision

Which of the following statements is FALSE?

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