Metamorphic Rocks: Marble, Quartzite, Schist, Gneiss and Related Stones
Metamorphic rocks are rocks transformed by heat and pressure without complete melting: limestone becomes marble, sandstone becomes quartzite. Garnet, lapis lazuli and jade all come from this process.
Key points
| Origin | Solid-state transformation of a pre-existing rock (protolith) |
|---|---|
| Causes | Heat, pressure, hot fluids |
| Types | Contact (thermal), regional, high pressure, impact |
| Examples | Marble, quartzite, slate, schist, gneiss, eclogite |
| Common sign | Minerals aligned in sheets (foliation) |
| Related stones | Garnet, kyanite, staurolite, lapis lazuli, marble ruby, jade, serpentine, soapstone |
Transforming without melting
Metamorphism (from the Greek “change of form”) refers to all the transformations undergone by a rock that is buried or heated, without it fully melting. Under the effect of temperature (from about 200 °C), pressure and hot fluids, minerals recrystallise, new minerals appear, and grains grow larger. The starting rock is called the protolith (parent rock). If the temperature rises further, the rock begins to melt and we move into the realm of igneous rocks, with intermediate states called migmatites.
The types of metamorphism
| Type | Cause | Examples |
|---|---|---|
| Contact (thermal) | Heat from a magma that has intruded nearby | Marble and hornfels around a granite, chiastolite, lapis lazuli |
| Regional | Pressure and heat during the formation of mountain ranges | Schist, mica schist, gneiss |
| High pressure | Rapid burial, for example in a subduction zone | Eclogite, blueschist, jadeitite |
| Shock | Impact of a meteorite | Coesite, impact glasses, impact diamonds |
These categories overlap. A rock is described as low, medium or high grade depending on the intensity of the transformation.
From protolith to metamorphic rock
| Starting rock | Metamorphic rock | Change |
|---|---|---|
| Limestone | Marble | Recrystallisation of calcite into welded grains that sparkle |
| Sandstone | Quartzite | Quartz grains and their cement fuse: the rock breaks through the grains |
| Clay or claystone | Slate, then phyllite, mica schist, gneiss (increasing grade) | Appearance of aligned micas, then of large grains |
| Basalt | Greenschist, amphibolite, eclogite | New minerals (chlorite, amphibole, garnet) |
| Granite | Orthogneiss | Minerals stretched into bands |
| Ultramafic rock | Serpentinite, then soapstone (talc) | Hydration, creation of serpentine then talc |
In clay-rich rocks, the Scottish geologist George Barrow described in 1893 a sequence of minerals that appear as temperature rises: chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Each one signals a degree of transformation: these are “index minerals”. They explain why garnets are found in mica schists and kyanite in some gneisses. The foliation comes from the alignment of flat minerals, such as mica, perpendicular to the pressure.
Stones and gems born of metamorphism
- Garnet (almandine): abundant in mica schists and gneisses. See garnet.
- Kyanite, staurolite, andalusite: typical minerals of these rocks; cross-shaped staurolite twins are called “fairy crosses”. Chiastolite (andalusite) shows a dark cross formed by carbon inclusions, born of contact metamorphism. See staurolite and chiastolite.
- Lapis lazuli: a rock formed from limestone metamorphosed in contact with magmas, mined for more than 6,000 years in Badakhshan, Afghanistan. See lapis lazuli.
- Marble ruby: the famous rubies of Mogok, in Myanmar (Burma), are found in marbles. See ruby.
- Jade: jadeite forms under high pressure in subduction zones; nephrite forms in other settings, in contact with serpentinites. See jade.
- Green aventurine: a quartzite speckled with green mica. See green aventurine.
- Serpentine and soapstone (steatite): soft, used for carving and for old cooking vessels. See serpentine and soapstone.
- Emeralds: some deposits (Zambia, Brazil, the Urals) lie in schists; others, such as Colombia, have a different, sedimentary and hydrothermal origin.
Marble and quartzite: trade terms and geological terms
The marble of the stonemason is a limestone that takes a polish. The marble of the geologist is a recrystallised limestone. Carrara marble, for example, fits both definitions. Another frequent confusion is between sandstone (sedimentary, visible grains, breaks between the grains) and quartzite (metamorphic, denser, breaks through the grains).
Slate splits into thin parallel sheets: this is what makes it usable as roofing. It is quarried in France, for example in Anjou. This ease of splitting comes from the foliation of the micas.
Precautions and care
- Marble and other limestone rocks are sensitive to acids (vinegar, lemon): they lose their shine.
- Quartzite and gneiss are very hard, but their surface can be rough or cracked.
- Talc and soapstone are very soft (hardness 1 to 2.5): they can be scratched with a fingernail.
- Avoid breathing in dust from sanding or cutting: dust from quartzite and other silica-rich rocks can cause silicosis over the long term in exposed professionals. Do not sand dry at home.
See also mineral or rock to distinguish minerals (garnet, kyanite) from the rocks that contain them (mica schist).
Frequently asked questions
What is the difference between a metamorphic rock and an igneous rock?
An igneous rock comes from the cooling of a liquid (magma). A metamorphic rock is already solid and has been transformed by heat and pressure without fully melting.
Is marble a mineral?
No, it is a rock made mostly of interlocked grains of calcite (or dolomite).
Is lapis lazuli metamorphic?
Yes: it forms in limestones transformed by contact metamorphism, which gives it its mixed composition of several minerals.
Where does the layering of schist come from?
From the alignment of flat minerals, such as micas, under the pressure of crustal deformation.
What are migmatites?
Transitional rocks between metamorphism and melting, in which part of the rock has begun to melt. They show light and dark bands.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.