Are there any rocks that don't fit neatly into the igneous/metamorphic/sedimentary classification system?
Posted Mar 27, 2022, 2:04 PM UTC
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Some (additional) examples:
High temperatures experienced in medium- to high-grade metamorphism can cause the rock to partially melt and recrystallize, producing migmatite, a composite rock with metamorphic and igneous layers.
There isn't a precise boundary between diagenesis and metamorphism, which can lead to ambiguities between sedimentary and metamorphic rocks.
Unlike sedimentary bituminous coal and lignite, anthracite coal is generally, but not universally, classified as a low-grade metamorphic rock. There are three grades within the anthracite rank, in order of increasing grade/metamorphism: semi-anthracite coal, which is intermediate between bituminous coal and anthracite (and therefore the most ambiguous); ("normal") anthracite; and meta-anthracite.
Is it a quartz sandstone or a lightly metamorphised quartzite? Generally sandstone will break along grain boundaries, while quartzite will break through grains, but in practice this detemination may require microscopic examination. (To complicate things further, orthoquartzite--or in older literature even just quartzite--is an old term for some highly pure quartz sandstones, hence the seemingly redundant term meta-quartzite for what would otherwise today be called quartzite.)
Tuff is a lithified deposit of volcanic ash. It is usually considered with other volcanic, and therefore igneous, rocks. But its formation, texture, and layering are sometimes considered from a sedimentological point of view.
While a bit of a tangent, natural ice is technically a mineral, so glaciers are technically rocks--primarily metamorphic rocks. Compressed snow "sediment" undergoes diagenesis to firn. At greater depths with further compression, firn becomes glacial ice. Somewhere along the way the glacial ice may be considered metamorphosed. At the base and edges of glaciers, melting and refreezing occur.
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ice is technically a mineral
could you elaborate on that? I wouldn't have thought that the solid state of water would be enough to qualify as a mineral.
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stolen from https://rockhoundresource.com/is-ice-a-mineral-the-definitive-answer/ :
"Ice is definitely a mineral. It meets all four of the
requirements set forth in the definition of a mineral: It is naturally
occurring, solid, has an organized crystal structure, and has a
well-defined chemical composition. Ice is also technically a rock.
Water, however, is not a mineral."
Ice is also homogeneous, meaning it has a consistent internal molecular makeup, which qualifies it for the state of mineral.
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ok, so liquid water = not a mineral, but frozen water / ice = a mineral. Gotcha.
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To extend the idea further than is reasonable, you could say that glaciers are igneous rocks, seeing as they solidified from the molten form. So water is a kind of low temperature lava resulting from the melting of the ice. The oceans are huge lava lakes filled with lava monsters.
Like I said, extending the whole idea further than is reasonable. But it’s consistent with ice being a mineral, which it 100% is.
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The mineral definition I learned in mineralogy also includes "inorganic" (which ice is) in addition to those four other criteria. We also learned explicitly that ice is considered a mineral.
The organic criterion is potentially confusing because high school/intro science courses often taught that organic just means a compound that contains carbon, which is necessary but not sufficient. This matters in mineralogy because the minerals calcite and aragonite (CaCO3), dolomite (MgCO3), and diamond and graphite (C) are inorganic compounds that do contain carbon.
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The mineral definition I learned in mineralogy also includes "inorganic" (which ice is) in addition to those four other criteria. We also learned explicitly that ice is considered a mineral.
The organic criterion is potentially confusing because high school/intro science courses often taught that organic just means a compound that contains carbon, which is necessary but not sufficient. This matters in mineralogy because the minerals calcite and aragonite (CaCO3), dolomite (MgCO3), and diamond and graphite (C) are inorganic compounds that do contain carbon.
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Very true! Especially since carbonite minerals can be formed by organic processes, such as when plankton form carbonaceous shells which later undergo sedimentation after the plankton die.
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I would also add that there is a whole additional category of rock, hydrothermal, which includes rock that crystallizes directly from hot (generally water-rich) fluids. Many types of ores are found in this kind of rock. Because this category is not that common, it is often omitted from Geology 101 classification schemes.
https://uwaterloo.ca/earth-sciences-museum/resources/hydrothermal-minerals
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"hydro" part of hydrothermal is the sticking point on this classification. Stalactites, stalagmites, etc. are roughly sedimentary, as is hydrothermal. Water picking up something and depositing it somewhere else. Ion vs solid.
I don't think this is a hill I'd feel comfortable to die on, just saying that there are differing opinions on this rather shade of grey scale of definition. But that's the beauty of geology, mathematical definitions are for engineers.
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Speaking of classifying deposited minerals, I know biogenic stuff is sedimentary, and it makes sense when we are talking about, eg, chalk or seams of coal. But it feels a bit weirder when talking about, say, the interior of a coral head. Stuff built by an anima as a cohesive unit doesn't necessarily feel sedimentary.
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Fordite! Aka motor city agate. Tels are another interesting human deposition. Those are both pretty clearly sedimentary though
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There are primitive rocks, which form in space directly from condensing protoplanetary disk material. You can find these in some asteroids.
Why would those not qualify as sedimentary? Are they fusing together in ways to change the molecular structure too much? If so, why might they not be metamorphic?
I think it's more because they don't form by sedimentation
Sedimentation implies water I believe?
No, it does not. Aeolian (wind) transport and deposition can and does occur. The Navajo sandstone (SW US) is a spectacular example of lithified aeolian dunes.
You are correct, buy I think gravity induced layering is implied in sedimentation.
A conglomeration like a carboniferous contrite is not volcanic, sedimentary or metamorphic. It consists of a matrix of near spherical grains of differing sizes that condensed directly from the primordial dust cloud ~4.5 Billion years ago with very little intervening processing.