Tachylite rock is an opaque black or brown basaltic glass with a greasy look and a resinous luster. It is a natural volcanic glass that forms when molten basaltic magma rapidly cools.

The rapid cooling results in little to no crystallization. Therefore, you will end up with a mostly amorphous solid.

Tachylite is the more common basalt glass. The other less common one is sideromelane. Sideromelane is a transparent, brownish to brownish yellow basaltic glass.

Others, like Pele’s hair, tears, or seaweed (Limu o Pele), usually have a basaltic composition but not always.

Some authors will call it tachylyte, an alternative spelling. We will use both the spellings in this post.

Lastly, this term may refer to rocks or interstitial glass with a tachylitic composition.

Appearance

Tachylite is black or brownish opaque glass. It looks somewhat like pitch resin and may be vesicular or non-vesicular. Some specimens form small, rounded bodies known as spherulites.

It is opaque because it has a magnetite or Fe-Ti oxide microscopic crystals or microlites scattered in an amorphous or glassy matrix.

Also, some hand specimens may have plagioclase and olivine minerals. You can see some of these tiny crystals with the naked eye.

On a thin section, it will appear brownish with magnetite granules or microlites. Also, you may see olivine or plagioclase crystals if present.

Usually, this rock is brittle and has a conchoidal fracture. It will readily break if you hammer it.

Lastly, it may resemble other volcanic glasses like obsidian or pitchstone. However, if you heat its splinters or powder, it will fuse more readily than obsidian or pitchstone.

Tachylite composition

Tachylite is a mafic or basic igneous rock with 45-52 wt.% silica (SiO₂). It also has considerable iron and magnesium and is lower in sodium and potassium oxides.

Basic rocks are igneous rocks with 45-52 wt.% silica. Mafic, on the other hand, are basic rocks with minerals rich in magnesium and iron.

Being glassy, tachylyte doesn’t have minerals except for the minute Fe-Ti oxide crystals or magnetite. Besides magnetite, it may have a small amount of other microlites like feldspar or olivine.

However, we call it mafic since it forms basaltic magmas. These magmas form basalt (fine-grained), diabase (medium-grained), or gabbro (coarse-grained) rock.

How does tachylite form?

Tachylite forms from the rapid cooling of basaltic magmas. The rapid cooling doesn’t allow much crystallization to occur.

Usually, tachylites cool slightly slower than sideromelane. Therefore, it has some microlites. The rapid cooling prevents the formation of microlites in sideromelane.

Lastly, tachylytes are less common than acidic or silicic volcanic glass like obsidian. They are less common since basaltic magmas are highly fluid and tend to crystallize those acidic lavas.

Where does tachylyte occur?

Tachylite may occur in basaltic lava flows or as scoria ejected during explosive basaltic eruptions.

Also, they may occur at the margins of basalt or diabase sill and dike margins or on pillow lava rinds together with sideromelane. Pillow lavas form during subaqueous or submarine basaltic lava eruptions. 

1. Basaltic lava flow

Basaltic tachylite lava flows with a few crystals of augite, olivine, feldspar, and iron ores are known in Hawaii. They form from highly fluid basaltic lava that rapidly cools. Most are spongy or have a vesicular texture.

2. Fine cinder or scoria ashes

Tachylite also occurs on fine cinders or scoria ashes. Such is vesicular and may have phenocrysts. Phenocrysts are larger crystals formed earlier and found in a glass or finer-grained matrix.

Examples of such cinders are in Italy at Stromboli Island and Mount Etna and Auvergne in Frace. Also, they occur in Iceland.

3. Sill and dike margins

Rapid quenching of basalt or diabase dike or sill margins will result in a thin (mm scale) layer of tachylite. Inward, these sills or dikes become more crystalline due to slower cooling.  

This tachylite layer is non-vesicular. However, it may show banding, made evident with spherulite (small, rounded rock bodies) rows. Those without spherulitic will be perlitic, i.e., have concentric cracks.

Lastly, these glassy surfaces may have crystals like augite, olivine, and feldspar and widespread, tiny microliths of magnetite.

Weathering and alterations

Tachylite hydration and oxidation convert them into palagonite, a red, brown, or yellow cryptocrystalline (made of minute crystals) material.

Frequently Asked Questions (FAQs)