14 October 2008

Marble and the metamorphics

That sounds like the name of a Do-Wop band. So since I touched on igneous rocks in general and granite in particular yesterday, today I want to talk about the metamorphics.

My diagram above shows sedimentary rocks on a sea floor being subducted under a tectonic plate. That sedimentary rock will turn into metamorphic rock given enough time, enough heat and enough pressure. So what was once sediment on the bottom of the sea turns into metamorphic rock as it gets pushed down into the slow cooker that sits directly below our feet. If it doesn't run into anything, it will turn into an igneous rock eventually. But then I wouldn't have anything to write about.

So in our case, the now metamorphic rock is going to run into another continental plate and instead of continuing its long journey down, it's going to be thrust upward like in this diagram.

The blue section in this diagram shows how metamorphic rocks end up back on the surface of the earth after their time spent below ground. That blue section is how we get three common metamorphic rocks used in homes: marble, quartzite, soapstone and a fourth, mysterious stone called serpentinite. I'm going to write a post on soapstone later, so for now I'm going to concentrate on marble, quartzite and serpentinite.

Everybody knows what marble is but what a lot of people don't know is that it used to be limestone, a sedimentary rock. So even though the chemistry of the original limestone and the resulting marble are identical, their molecular structures are different thanks to time spent in the depths of the earth. The same thing happens to turn sandstone into quartzite. Serpentinite is another metamorphic rock that's usually mislabeled as marble (Rainforest Green and Rainforest Brown are examples of serpentinite). Serpentinite starts out as ultramafic rock in the mantle of the earth that gets thrust upward. Serpentinite never spent any time at the surface of the earth and it isn't a transformed sedimentary rock. It's instead the transformed, formerly molten core of the earth.

OK, real life example time.

Here's a slab of Carrera marble. It's a pretty common stone so far as marbles go and most of it starts at around $60 a square foot when it's being made into a kitchen counter. It's composed of calcium carbonate and other minerals and that's what makes it softer and less stain resistant than granite. Despite what everyone claims are its negatives, I'd be safe in saying that it's my favorite counter top material. It's a classic, it holds a temperature perfectly, it looks great and 11 million Italian families can't be wrong. Embrace the stains people.

So here it is in a traditional kitchen. The subway tile on the back wall is also made from Carrera in this kitchen. In life, this is a gorgeous kitchen in a nearly timeless style.

Carrera also looks great in a modern setting. The William Ohs kitchen shown above proves that pretty well.

Marble can also be black with white veins of silica in it and that's stunning in its own right. Although at the end of the day, I think that white marble's easier to work with aesthetically.

This is a piece of white quartzite my client fell in love with last week. White quartzite often gets mislabeled as marble, but they are very different stones. Marble is composed of calcium carbonate and quartzite is composed of silica. The two stones behave very differently. As silica, quartzite will be harder and less prone to staining than marble will. If you look at quartzite up closely, you can see that it has a grainier appearance than marble does. That's from its once having been sandstone.

Quartzite usually has some bright colors in it on a white background. When it looks like this it's often mislabeled as granite and that's unfortunate because it's not granite. Quartzite is more porous than granite is despite their being made from the same elements essentially.

This is a quartzite from Brazil that's always mislabeled and sold as a granite. It's usually called Blue Louise and it is an eyeful. Again, it's beautiful as a piece of stone but as Tim Gunn would say, it's a lot of look. Proceed with caution.

The other big difference between quartzite, marble, serpentinite and granite is the price. Quartzite and serpentinite are always wickedly expensive whether they're labeled properly or not. Marble and granite tend to be more affordable.

Finally, the slab of stone above is always labeled as a marble and it's usually called Rainforest Brown or Rainforest Green. It's not a marble though, it's a serpentinite that's been pushed up from the very bowels of the earth. It's made from a veritable soup of elements ranging from manganese, cobalt, nickel, iron and silica. Even though it's sold as a marble, it's a lot more resilient than marble is and it's usually harder. These stones are great to use in bathrooms, but I wouldn't want to prepare food on them in a kitchen owing to their toxic mineral content.

Granted, the amounts of cobalt, chromium and the rest of them are trace amounts; but I'd like to keep my cobalt intake down to zero thank you.

So there's my run through of some common metamorphic rocks. Tune in tomorrow when I tackle the sedimentary rocks people welcome into their homes.


  1. I just started learning and researching kitchen countertops. I want maintenance-free white countertops for a 3ft x 13ft peninsula. From what I understand that leaves out granite and marble.

    I've been told quartz is my best option. At a showroom yesterday I spotted a white sample with grey streaks sample that I really liked and it was told it is dolomite, which I've never heard of before. I only saw a small section but it seems to look like your picture above that you said "This is a piece of white quartzite my client fell in love with last week."

    Is dolomite the same as quartzite? Is it a good choice for kitchens? Any maintenance? Will it last a long time? What do I need to know about dolomite to choose it as my countertops?


  2. Dolomite is another carbonate mineral, sort of like limestone. As such it will behave in similar ways that marble or limestone will once it's made into a kitchen counter. Based on what you're looking for in a counter material, your best bet is still a quartz composite. Look for the brands Caesarstone and Silestone, they have done the best job of imitating the look of natural marble out of all the brands of quartz composite out there.


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