Sideromelane is a transparent or clear brownish or yellow-brown colored basaltic glass. It forms from rapid quenching of basaltic magma mostly (but not exclusively) in water or under ice caps.
Basaltic glass is a natural volcanic glass. It forms when magma quenches so rapidly that it freezes in place without crystallization. This creates an amorphous (structureless) solid. Since it forms from magma, it is essentially an igneous rock.
Sideromelane is one of the basaltic glasses. The other more common one is tachylite. Also, Pele’s hair, tears, and Limu o Pele are usually but not always basaltic.
Wolfgang Sartorius von Waltershausen, a German geologist, named this transparent, brownish, or yellowing glass sideromelane in 1853.
However, this term may also refer to a rock or interstitial glass.
Brownish, sideromelane volcanic glass: Photo credit: James St. John, CC BY 2.0, via Wikimedia Commons.
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Composition
Chemically, sideromelane is a mafic rock or volcanic glass with 45-52 wt.% silica. Rocks of such silica composition are also known as basic.
It is relatively high in mafic elements like iron and magnesium and lower in alkali oxides (sodium and potassium).
Lastly, this igneous rock is essentially a natural volcanic glass. Therefore, it doesn’t have minerals.
Formation and occurrence
Sideromelane usually (but not exclusively) occurs from rapid quenching of basaltic magma in water.
The water and magma interaction forms this basaltic glass at high temperatures. Examples of such interaction are known in Italy (underwater) and Iceland (under ice)
However, ash produced during explosive basaltic eruptions may have sideromelane shards.
Usually, sideromelane occurs with tachylite, microcrystalline, and broken phenocrysts. It is found in tuff, pillow lavas, or embedded hyaloclastite, or shards of this glass may form a sideromelane tuff.
Hyaloclastites are detrital basaltic glass formed from rapid quenching of lava. They are common on lava pillows and sheet flows.
Sideromelane vs. tachylite
Sideromelane is transparent pale-brown, brown, or yellowish-brown glass, while tachylite is opaque, resinous-looking black or dark brown.
It is transparent or clear because it cools fast and at higher temperatures. This doesn’t allow any crystals to grow.
On the other hand, tachylite is opaque and looks black or dark brown because it has iron-titanium oxide minute crystals scattered in the glass matrix. These minute crystals form as tachylite cools relatively slower.
Alteration
Low-temperature hydration of sideromelane forms palagonite. Pelagonite is a yellow, yellow-orange, or brownish material made of kaolinite, clay minerals, zeolites, or illite.
Significance
The occurrence and abundance of sideromelane indicate that the cooling of basaltic magma happened rapidly, usually under water or ice (wet environment).
Cooling rates are higher in magma-water contact than in magma-air contact. This happens because water has a higher conductivity and higher heat of vaporization than air. Thus, it cools magma faster.
Also, its occurrence shows solidification at higher temperatures and often closer to the lava or magma vent or fissure.