Most marble forms at convergent plate boundaries where large areas of Earth's crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.
Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes. In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small. In a freshly-broken hand specimen, they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light.
As metamorphism progresses, the crystals grow larger and become easily recognizable as interlocking crystals of calcite. Recrystallization obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation, which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary.
Recrystallization is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals become larger as the level of metamorphism progresses. Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases.
While there are a few types of calcium based marble that are also green – Cippolino and Verde Antigua – the vast majority of green stones that are called marbles, are actually serpentine. Serpentine is a group of minerals made of mostly magnesium silicate. Serpentines are usually green, but may vary in tone from nearly black to yellow, and may have some amount of white veining throughout their surface. Some studies carried out on green marbles used in architecture discovered that most of stones labeled as marble, but green in color contained 75 to 90% serpentine, with little or no calcite at all. The benefit of using a serpentine is the fact that’s actually more durable than true marble.
Now how did those veins get there?
Marble is a calcite (calcium carbonate), a type of metamorphic rock formed from limestone, that is heated and subject to fairly high pressure. It is not igneous. I suspect the marbling comes from the different deposition layers as they're laid down, and deformation from the subsequent pressure and heat.
Swirls and veins in marble are formed by a both primary (sedimentary) and secondary (metasomatic and metamorphic) processes. Can I have different type of 'veins' and swirls in the metamorphosed limestone in comparison with the initial limestone? YES. Why? Because during the metamorphic phase, pressure and heat is applied to the rock. So, we might have a partial melt and re-crystallization, a pressure slide forming swirls and new 'veins'.
To log this earthcache, send the answers to my email or geocaching account.
1. What color are the calcite veins in the marble?
2. What is the average size of the calcite veins?
3. Do the calcite veins seem to be uniform, or vary in appearance?
4. In your own words, explain how the calcite veins formed.
5. Post a picture of yourself (face not required) or a personal item without spoiling the answers.