Question 1 of 12: Feldspar and Olivine — Silicate Group Differences

Question 1: Feldspar and Olivine — Silicate Group Differences (Part 1 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Feldspar and olivine sit at opposite ends of the silicate structural classification — a fully-polymerized framework silicate versus an unpolymerized island silicate — and that single structural contrast controls every other diagnostic difference between them.

Feldspar — tectosilicate (framework silicate)

Feldspar (example: plagioclase, specifically labradorite, $(\text{Ca,Na})(\text{Al,Si})_4\text{O}_8$) is a tectosilicate: every $\text{SiO}_4^{4-}$ tetrahedron shares all four of its oxygens with neighbouring tetrahedra (with roughly half the Si substituted by Al to balance charge), building a fully three-dimensional framework with Si:O = 1:2. This dense, cross-linked framework gives feldspar two cleavage directions at (or near) 90°, hardness 6, and a light (white/gray/pink) colour, and it crystallizes over a wide temperature range as a continuous or near-continuous solid solution (Ca-Na plagioclase; K-Na alkali feldspar).

Olivine — nesosilicate (island silicate)

Olivine (example: forsterite, $\text{Mg}_2\text{SiO}_4$, or the Mg-Fe solid solution generally) is a nesosilicate: isolated $\text{SiO}_4^{4-}$ tetrahedra share no oxygens with each other at all (Si:O = 1:4), held together only by ionic bonds through interstitial $\text{Mg}^{2+}/\text{Fe}^{2+}$ cations. With no plane of weakly-bonded shared oxygens to cleave along, olivine shows no cleavage (conchoidal fracture instead), is denser and harder (H = 6.5–7) than feldspar, has an olive-green colour, and — being the first mineral to crystallize from a mafic magma per Bowen's reaction series — is stable only at high temperature, making it the least chemically weathering-resistant common silicate (Goldich stability series).

A rock containing both

Olivine basalt (or its plutonic equivalent, olivine gabbro) contains both minerals together: calcic plagioclase forms the bulk of the groundmass/phenocrysts while olivine occurs as early-formed phenocrysts, because both are early, high-temperature crystallizing phases from a mafic (basaltic) parent magma.