Still working away at making sense of Geology. Started the day with thos question
to Copilot:
Geology; the terms 'felsic', 'intermediate', 'mafic' and 'ultramafic' are only apllied to igneous rocks. Why is that?
They’re restricted to igneous rocks because the whole felsic–intermediate–mafic–ultramafic scheme is fundamentally a classification of magma chemistry, not of rocks in general. Once you anchor the system to magma, the boundaries make sense; outside of igneous processes, they break down.
🧱 What these terms actually measure
All four terms describe bulk silica content (SiO₂) and the associated ferromagnesian vs. silica‑rich mineralogy of a melt:
Felsic — high SiO₂, rich in quartz + feldspar
Intermediate — moderate SiO₂
Mafic — lower SiO₂, rich in pyroxene + olivine
Ultramafic — very low SiO₂, dominated by olivine + pyroxene
These categories map directly onto Bowen’s reaction series and the crystallization path of a cooling magma. They are therefore meaningful only when the rock’s mineral assemblage reflects equilibrium crystallization from a melt.
This is why the Wikipedia page you have open emphasizes mafic as a term for silicate minerals or igneous rocks and notes that it is “used primarily as a field term to describe dark-colored igneous rocks.”
🧭 Why the system doesn’t apply to sedimentary rocks
Sedimentary rocks do not inherit their composition from a single melt. They are:
mixtures of grains from many sources
altered by weathering, transport, sorting, and diagenesis
often dominated by quartz and clays regardless of the parent rock chemistry
A sandstone made from eroded basalt is still quartz‑rich because basalt weathers into clays and releases silica. A shale made from granite is still clay‑rich. The original magma chemistry is lost.
So calling a sandstone “felsic” or “mafic” would be misleading: its composition no longer reflects a melt, only weathering products.
🔥 Why the system doesn’t apply to metamorphic rocks
Metamorphic rocks form by solid‑state recrystallization, not by cooling of a melt. Their mineralogy is controlled by:
pressure–temperature conditions
fluid chemistry
protolith composition
deformation
A mafic protolith (e.g., basalt) may metamorphose into amphibolite, greenschist, blueschist, or eclogite depending on P–T conditions. The resulting mineral assemblage no longer corresponds to the igneous silica‑based categories.
Metamorphic petrology uses different classification systems (e.g., pelitic, mafic, ultramafic protoliths; facies like greenschist, amphibolite, granulite).
🧩 The deeper reason: the terms encode igneous processes
The felsic–mafic continuum is not just a chemical scale; it encodes:
melt viscosity
eruption style
crystallization sequence
density and buoyancy of magma
tectonic setting
These relationships do not exist in sedimentary or metamorphic contexts.
That’s why the IUGS classification explicitly restricts these terms to igneous rocks and minerals.
🪨 A useful nuance
Geologists do sometimes use “mafic protolith” or “ultramafic protolith” in metamorphic studies, but that refers to the original igneous rock, not the metamorphic rock itself.
If you want, we can map these categories onto specific Quebec rock types or look at how ultramafic rocks behave in metamorphic terrains like the Appalachians near you.
https://www.britannica.com/science/igneous-rock/Mineralogical-components; been working my way through the Britannica entries on igneous rocks. A summary would be useful...
A clear, structured summary of the Mineralogical components section of the Britannica article on igneous rocks, anchored to what the page actually contains and to the broader framework you’ve been building as you work through igneous petrology.
🧱 Core idea: igneous rocks are defined by the minerals that crystallize from a melt
Britannica’s treatment emphasizes that the identity of an igneous rock is fundamentally the identity of its crystallized mineral assemblage, which in turn reflects the chemistry of the magma and the conditions of cooling. The minerals are not random: they follow predictable stability sequences (Bowen’s reaction series) and cluster into compositional families.
🧩 The major mineral groups in igneous rocks
Britannica organizes igneous minerals into a few key categories, each tied to specific magma chemistries and tectonic settings.
1) Silicate framework minerals (felsic end)
These dominate silica‑rich magmas and define the felsic end of the spectrum.
Quartz — pure SiO₂; hallmark of highly evolved, high‑silica magmas.
Feldspars — the most abundant minerals in Earth’s crust.
Plagioclase (Na–Ca series)
Alkali feldspar (K‑rich)
These minerals crystallize late in Bowen’s series and produce light‑colored, low‑density rocks like granite and rhyolite.
2) Ferromagnesian minerals (mafic–ultramafic end)
These are rich in Fe + Mg, crystallize early, and dominate dark, dense rocks.
Olivine — first to crystallize; signature of ultramafic and primitive mafic magmas.
Pyroxenes — major mafic minerals in basalt and gabbro.
Amphiboles — hydrated ferromagnesian silicates; appear in more evolved mafic/intermediate magmas.
Biotite — Fe‑Mg mica; appears in intermediate to felsic rocks.
These minerals define the mafic/ultramafic categories and reflect hotter, lower‑silica melts.
3) Accessory minerals
Small in volume but geochemically powerful.
Magnetite, ilmenite — record oxidation state of the magma.
Zircon — preserves U‑Pb ages and trace‑element signatures.
Apatite — records phosphorus and volatile content.
Even a tiny amount of zircon can anchor the age and evolution of a pluton.
🔥 How mineralogy encodes magma evolution
Britannica stresses that mineral assemblages are not arbitrary; they reflect:
Silica saturation (undersaturated → oversaturated)
Alkali content (Na–K vs Ca)
Water content (controls amphibole/biotite stability)
Temperature of crystallization (olivine first, quartz last)
Fractional crystallization (removal of early mafic minerals drives magma toward felsic compositions)
This is why the same chemical continuum (ultramafic → mafic → intermediate → felsic) appears in both mineralogy and rock classification.
🧭 Mineralogy as the basis for igneous rock names
Britannica ties mineralogy directly to the formal IUGS classification:
Plutonic rocks (granite, diorite, gabbro) are classified by the relative proportions of quartz, alkali feldspar, and plagioclase (the QAPF diagram).
Volcanic rocks (rhyolite, andesite, basalt) are classified by bulk chemistry, but the mineralogy still reflects the same compositional families.
Ultramafic rocks (peridotite, dunite) are defined by dominance of olivine ± pyroxene.
Mineralogy is therefore both descriptive and genetic: it tells you what the rock is and how the magma evolved.
🧨 Volatiles and hydrous minerals
Britannica highlights the special role of water:
Hydrous minerals (amphibole, biotite) only crystallize when the melt contains enough H₂O.
Water lowers melting temperatures and changes the crystallization path.
This is why subduction‑zone magmas (wet) produce andesite/dacite with amphibole, while mid‑ocean ridge basalts (dry) do not.
This ties mineralogy directly to tectonic setting.
🧱 Textural context: why mineralogy must be read with cooling history
Minerals record chemistry, but textures record cooling rate:
Phaneritic (coarse‑grained) → slow cooling in plutons
Aphanitic (fine‑grained) → rapid cooling in lavas
Porphyritic → two‑stage cooling history
Glassy → quenching before crystals can form
Britannica emphasizes that mineralogy + texture together give the full igneous story.
🧭 How this fits your broader reading
You’ve been working through the felsic–intermediate–mafic–ultramafic continuum, and this Britannica section gives you the mineralogical backbone behind those terms. It reinforces the point from your earlier question: these categories are meaningful only because igneous rocks inherit their mineralogy directly from a melt.
If you want, I can also synthesize the entire Britannica igneous‑rock sequence (origins → magma types → textures → mineralogy → classification → volcanic forms) into a single integrated map you can use as a reference while you read.
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