What Type Of Rock Is Marble?

When you think of structures made of stone, there tend to be a small handful of styles and types we think of. First is whatever things like castles, the pyramids and other ancient monuments were hewn from (usually sandstone or limestone), marble and granite. What’s more, the latter two are often mistaken for one another, as some marbles look like granite and vice-versa.

What exactly is marble, though? We can picture it in our mind, and we imagine sweeping echoing courts occupied by Roman emperors, ornate monuments, or lavish mansions and estates belonging to nobility or the fabulously wealthy. It has a speckled, grained or striated appearance, the most common cliché being black and white in appearance.

It’s regarded by most people as beautiful, and a hallmark of wealth and success. In spite of this, while it’s not cheap, it’s become more affordable, allowing it to be accessible to people who aren’t fabulously wealthy. Marble has risen in popularity recently, due in part to the ability to manufacture synthetic marble, new sources of natural marble, and improved quarrying and processing/cutting techniques making the sky the limit with what you can do with this stone.

This doesn’t answer our question, though, does it? What is marble? What’s it made of? It’s not on the periodic table of elements, so it must be made of complex minerals, right? That’s correct, marble is a conglomerate of some base elements, with various trace and contaminant elements giving it the grain and hue that distinguishes various types of marble.
Let’s take a moment to explore the composition of marble, how it forms, and take a brief look at the slightly soft definition of marble in the stone industry.

Marble’s Composition

Marble’s made primarily of a mineral known as calcite, which forms the bulk of its composition. Calcite is, by itself, a very bright white compound made of calcium and carbon mainly, and its chemical nature actually means it gets a lot of use outside just construction material.

In fact, bulk of marble – a purer, softer stone by nature, is crushed to produce things like calcium bicarbonate and other acid-neutralizing bases. Antacids and other common compounds are produced from this naturally-occurring resource.

However, when calcite combines with other elements such as mica, clay, pyrite, iron oxide (rust) and graphite, it becomes much harder. These elements reinforce the base structure, as well as adding unique visual flairs. Red, brown and pink marbles, for example, contain a lot of iron and clay minerals. Sparkly, gray marbles with a lot of shine tend to have a lot of quartz deposits, while dark marbles tend to contain more mica and graphite. Yellow marbles will have more pyrite, and blue marbles will even have some copper deposits in them.

There are a host of other trace minerals that can occur in marble, producing a very wide range of different styles, patterns and hardnesses of marble. Nonetheless, while construction marble is considerably harder than pure calcite, it’s much softer than something like granite, which has a higher amount of crystalline minerals, and was formed under intense pressure and/or heat. While heat and pressure do often play a role in marble’s formation, it’s much less intense than the likes of granite.

How Marble Forms

If you ask a stone expert or a geologist how marble forms, they’ll be happy to tell you, but their explanations will be intricate, involved and full of jargon. You’ll hear the term “metamorphism” and “dolomitic” thrown around a lot in these explanations. In general terms, metamorphism is just pressure, chemical processes or heat causing molecular structures to come apart and reform in a new state with distinctly different properties than the previous state.

Dolomitic stone is any of a variety of stones (such as its namesake, dolomite) which contains calcium and magnesium compounds, which are very good at aiding in crystallization and metamorphism, as well as forming solid bonds. Limestone is dolomitic, most marble is dolomitic.

So, now that we’ve had a brief introduction to geological chemistry, let’s run through how marble is formed. Generally, marble forms when pressure, heat and chemical processes occur across an exposed strata of the earth’s crust. Calcite, which usually originates from organic deposits, dolomitic compounds and other trace minerals will be subjected to the metamorphism described a minute ago. This can be brought on by heat, by pressure, or by some acidic reactions going on in the localized section of the crust.

The result will recrystallization of the calcite, quarts, trace minerals and dolomites, producing the harder, striated marble we’re all familiar with. Other stones often mistaken for marble (and sometimes considered types of marble by stone dealers for simplicity’s sake) are formed in very similar ways.

Marble, in the long run, is the result of complex chemical interactions between fossil organics and various reactive elements, long story short. This means that a lot of locations where marble is easily found tend to be sites of large extinctions, long-lost ancient forests, jungles or swamps, and places once having a lot of high-energy geological activity.

Loose Definition

It’s important to take a moment to discuss the fact that marble by geological definition is not always the same as the definition used by stone dealers and contractors. This is not done in an attempt to deceive anyone, but for the sake of simplicity and less voluminous classification. In other words, it’s to simplify things in a non-condescending way for customers.

Some other stones with similar appearances and properties (often of similar formation origins) are sometimes classified as types of marble, when by geological definitions, they are not. Among these are travertine (which is a love it or hate it stone, but quite exquisite), verd antique (which is not seen very often these days), serpentine, and some polished limestones. It’s also worthy of note that limestone is actually a very close cousin to actual marble, the recrystallization we mentioned a moment ago being when the stone becomes one or the other – the size and distribution of the calcite crystals being the determining factor.

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