Serpentine vs Marble
Sometimes in the stone industry it can be a challenge to have a clear understanding of a specific material. This is because there are so many materials used for hard surfaces. Additionally, the materials are usually named or labeled based on the practical nature of their properties. As a result, a material that would be geologically classified as one material may be communicated to a consumer market as another material because it behaves as, looks like, or generally reminds people of another material altogether. In this article we are going to look at two such materials. As we consider pitting serpentine vs marble we will consider the geological classification of these and see how the naming conventions in the commercial stone realm differ. So let's get to comparing serpentine and marble.
Brief Summary of Serpentine
Serpentine may not be a term that you hear on a regular basis. Perhaps it is because the term is not actually a material in and of itself. Rather, the term serpentine, used in a geological sense refers to a group of minerals. This Wikipedia.org page describes it this way:
Serpentine subgroup (part of the kaolinite-serpentine group in the category of phyllosilicates) are greenish, brownish, or spotted minerals commonly found in serpentinite. They are used as a source of magnesium and asbestos, and as decorative stone. The name comes from the greenish colour and smooth or scaly appearance from the Latin serpentinus, meaning "serpent rock".
Thus, the rock is actually called serpentinite and it contains a variety of minerals. Geology.com says this regarding the mineral content of seprentine:
Serpentine is not the name of a single mineral. Instead it is a name used for a large group of minerals that fit this generalized formula: (X)2-3(Y)2O5(OH)4.
This means that the surface material that is referred to as serpentine can vary quite a bit in composition. The minerals' collective hardness affects the overall hardness of the stone. Serpentine is at times called by other names. One of these is "green marble". Perhaps this is because some varieties of serpentine can be nearly as soft as marble. At any rate, this creates a situation where a material that is geologically classified as serpentine (serpentinite) is referred to in a practical context as "marble". Looking objectively at this naming convention, it makes sense. After all, professionals are generally aware of some of the traits of marble; one being its low hardness rating.
In contrast with the previous material, marble is a commonly promoted material. Geology.com says this about what marble is:
Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite.
This is a very different composition from serpentine's. Notice in the explanation above that natural marble is the result of limestone undergoing a metamorphosis. This fact regarding marble also leads to naming conventions in the commercial stone industry that are inaccurate from a geological standpoint but at times make practical sense. Sometimes limestone, banded calcite (also known as onyx), and travertine (which is a specific kind of limestone) get labeled as marble even though these have not met the geologic definition for marble. However, it is easy to see why this would make sense from a practical standpoint. All of those materials are calcareous (or composed mainly of calcium carbonate), so they will have many of the same characteristics.
Now that we have established what each of our materials in this serpentine vs marble match up is, and how the naming can be ambiguous, let's move into our comparison of these materials.
Comparing Serpentine and Marble
As you may have concluded from the information thus far, similarities between marble and serpentine seem to rest on the fact that serpentine can be a rather soft stone because of its varying mineral content. Likewise marble is a soft stone.
Marble and Serpentine Similarities
Since marble has a hardness of 3-4 on the Mohs scale and serpentinite (serpentine) ranges from 3-6, depending on the mineral content, it is easy to see that there are slabs of each out there with a hardness of 3 or 4. In these cases, marble and serpentine would have similar hardness and could reasonably be cut, shaped, and worked in similar ways. However a shared label and similar hardness does not make the two materials the same; as we will next see.
Differences Between Serpentine and Marble
In addition to concluding similarities between these two natural stone materials, you may have also deduced that these materials will be fundamentally different based on the different mineral content. What do we mean by that?
One of the references shown above lists numerous minerals which can be present in serpentine. Yet, there is one specific mineral that is not listed. That mineral is calcite (or calcium carbonate). Furthermore, the other reference mentioned clearly states that marble (as well as the other calcareous materials that receive the label "marble") "is composed primarily of the mineral calcite". Since geologically classified serpentine contains virtually no calcite and geologically classified marble must be primarily composed of calcite, these two materials cannot be the same material (in the geological sense). Why is this information of practical value for fabricators and even consumers?
Working with Soft and Hard Stone Materials
Knowing the hardness of the material being worked is important because hard materials and softer stone materials have different tooling. For example, there are various diamond blades made for fabricating natural stone, sintered stone, and engineered stone.
So think about a scenario where you are trying to cut a slab that is called "green marble". Maybe the slab is "connemara marble" and you reach for a marble blade. But the connemara slab has minerals in it that make this stone have a hardness that is more toward the upper end of the range for serpentine stone. In this scenario, you would be, in effect, cutting a stone with a hardness that is close to that of natural granite. So, your results might be better if a granite blade was used instead of a marble blade.
What's in a Name?
Natural stone that is classified as "calcareous" is stone that is composed of calcium carbonate. As we have already seen, scientifically classed "marble" fits this definition. Now, there are other natural stones that are also "relatively soft" materials as we discussed above. But soft does not necessarily equate to the stone being "marble". The mineral content is part of the scientific definition. So, just because a stone has a hardness of 3-4, that does not mean that it is must be named "marble". The opposite is also true. A stone slab having a label that says it is named "marble" does not mean that the stone is a geologically classified marble; as we mentioned earlier. But why does this matter?
The name "calcareous" stone conveys an important detail about the stone being observed. It says that the rock is composed of calcite. This property brings with it a whole host of traits. We already mentioned one; the hardness. Another quality is that calcareous stone reacts with acid. Thus, true marble will etch during an acid test. On the other hand, serpentine by definition (scientific definition) has virtually no calcite in it (although it is possible that there might be minor amounts). So the main part of serpentine will not etch during an acid test. This could even be true if the serpentine is of the soft variety.
In conclusion, serpentine vs marble comparisons could produce a range of results. Comparing a softer serpentine with a genuine marble slab would reveal similarities in how the materials respond to diamond tools. In another marble vs serpentine match up, you may have a hard variant of serpentine and the results would be different. However, as we have mentioned in this article, of the two, only marble (geologically) is calcareous. Because of this, performing an acid test along with a scratch test would prove to be valuable indeed.