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Lecture Notes for LEC172 Geology Lecture 5
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Lecture Notes for LEC172 Geology Lecture 5 - Minerals
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Thursday 25th November 2021 LEC 172Minerals
Solid Solutions:
Cations of similar sizes substitute for each other and make mixed compounds known as solid solutions.
E.g., Olivine is a solid solution of Fe and Mg
Pure Mg Olivine Is Mg2SiO4 = Forsterite
Pure Fe Olivine Is Fe2SiO4 = Fayalite
Natural olivine’s are intermediate between these, i.e. some cation sites are filled by Mg and some by Fe.
Olivine is written as (Mg, Fe)2SiO4 meaning that there are two Mg or Fe ions, in whatever combination, for
every silicate group.
All major silicate groups (except quartz) show some degree of solid solution, e.g.
Pyroxene Mg2+ and Fe2+
Plagioclase Na+Si4+ and Ca2+Al3+
Alkali feldspar Na+ and K+
Natural Glasses:
Natural glass, obsidian, forms when lava cools too quickly to form crystals. Atoms are arranged in silica
tetrahedra, with other cations within it but, instead of being linked in a regular manner, they are randomly
arranged as they would be in a liquid.
It is neither a rock nor a mineral.
Characteristics of obsidians:
Jet-black
Glassy (vitreous) lustre
Shiney
Conchoidal (smoothly curved) fracture
Lack, or have very small, crystals
May have vesicles (bubbles) and flow bands
Mineral Polymorphism:
When chemical substances form more than one kind of crystal structure; different structures are called
polymorphs.
E.g., CaCO3 - Calcium carbonate forms, predominately two different structures, each w/ their own name:
Calcite - = 2,710 kg m-3, low P
Aragonite - ρ = 2,930 kg m-3, high P
Shelly, invertebrate organisms (e.g., clams, foraminifera, corals) secrete calcite and/or aragonite.
E.g., C Graphite, = 2,250 kg m-3, low P
Diamond, = 3,500 kg m-3, high P
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