Restoration & Maintenance Corner: “Green Marble” (Serpentine)
Bob Murrell
Stone Restoration
Photos provided by Bob Murrell
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Green marble, as it is often misnamed, is a very popular material here in the USA. You see it on floors and counters, both commercial and residential, in both tile and slab forms.
It is used for vertical applications both interior and exterior. It is everywhere. What it is not, is marble.
The marble group is composed of metamorphic limestone, calcium carbonate, and dolomite. The serpentinite or “green marble” group is primarily composed of serpentine and other minerals.
Serpentine is mostly magnesium and iron which has been intruded by hot, chemically rich fluids and metamorphosed at fairly deep levels in the earth’s crust. Of course there are also associated high temperature levels at these depths as well.
Serpentine is generally harder than most marble at up to 5 on the Mohs Scale and generally does not react to acid like the calcium-based materials do. However, other constituent minerals in the serpentinite can affect the overall hardness and some are also acid sensitive.
So to complicate things, there are many minerals associated with the “green marbles.” Some minerals that add variation without being detrimental to the stone are Siderite, which can show up as light to dark brown areas or veins, Albite, and the blackish and magnetic Magnetite which are both fairly hard on the Mohs Scale.
Other minerals which can contribute to the early wear or degradation of the stone are Brucite, Chrysotile, Asbestos, and Chlorite which are all very soft. Some of the white to almond veins associated with the serpentines can be either calcium or magnesium carbonates. The calcium carbonates can be distinguished using an acid test and they will react, whereas the magnesium carbonate will not. Of course one of the reasons for using “green marble” is that it can be acid resistant for use on tables and counters.
I tell you all of this not to act like I am a geologist, as I am certainly not, but to show the complexity and variations of these stones. Serpentines or Serpentinites are quarried all over the world and various amounts of the above mentioned constituent minerals and more are present depending on the location from which they come. These different minerals in the stone react differently in certain installations as well as in restoration and maintenance procedures.
Back in the old days when I was training contractors in the back of our warehouse, I remember we acquired a fairly large 2cm (3/4˝) slab of serpentine. We trained on Friday and Saturday doing most of the wet grinding and honing on Friday. We left the serpentine slab on the concrete floor in some remaining water.
When we came back to finish up on Saturday, the slab had warped. When I say warped, I mean I could stand on it and rock it back and forth! It did not break and I believe that it may have even reverted back to its original dimensions after completely drying out, if my memory serves me well. This is why areas that are subject to moisture, like a shower, may not be good applications for serpentine. They may also be required to be set in epoxy to help with the moisture and adhesion issues.
When I finally got a nice 1,000 sq. ft. training room in the late 1990s, we had some serpentine installed along with many other stones like marble, granite, and terrazzo. Later we discovered that some of the serpentine tiles had loosened from the thin set (the tile warped) and now had hollow areas underneath. Of course we used this as a training moment by tapping on the tile for students to show them how the hollow spots sounded.
I have been involved with many serpentine projects all over the Caribbean and North America. One thing I have learned is that there are so many varieties, you can expect almost anything. Poor quality stones containing Chrysotile, talc, asbestos, or Chlorite can wear at much faster rates and even erode. This is from traffic wear and water penetration from maintenance and weather issues, especially in exterior applications. The carbonates can also be water sensitive and wear faster too.
Restoration and maintenance procedures for Serpentine are generally assigned as to somewhere between marble and granite. The serpentines are harder than most marble and also normally acid resistant. Despite this fact, crystallizers generally work well when polishing, as do some granite compounds or powders. Like granite, a higher hone level may be required before polishing.
I have found that by crystallizing with a #0 steel wool pad after an 1,800 grit or 3,500 grit diamond, a really good polish can be achieved. In some cases, a better polish than that of the factory can be reached. I have also polished with granite polishing compound and similar type products as well with good results.
Maintenance is accomplished using either pH neutral cleaners designed for polished surfaces or a quality stone soap for honed surfaces. Periodic intensive cleaning of the grout lines is accomplished with deep cleaners and a brush. Re-polish traffic wear lanes as required.
As always, submit a test area for the desired results and to confirm the procedure. Consult a knowledgeable distributor of quality stone care products who can help guide you in the right direction. No need to reinvent the wheel.
Bob Murrell has worked as a supplier of products and technical support to the natural stone industry for over 35 years. He has written numerous articles for various trade publications and has also trained thousands of contractors over the last 25 years.