Important marble architecture like the Parthenon in Greece has lost some of its detail from rain exposure.
Marble is a durable material with large crystals and low porosity that makes it easy to polish. It is used in building, art and landscaping to add beauty and strength. However, marble has properties that make it subject to chemical weathering to a degree other materials, such as granite, are not. Its interaction with any acid substance, particularly rainwater with a low pH, leads to a chemical reaction and consequences less than ideal for builders, artists, and landscapers. Marble is best used either in regions with less rainfall or in home locations where it can be protected by wind or overhangs from the corrosive effects of rain.
Erosion
Metamorphic rock begins as another type of rock and is changed by geological forces.
Marble is created when limestone is subject to heat and pressure. It is a metamorphic rock (limestone is a sedimentary rock, created through eons of layering and pressure). Marble and limestone both are composed of calcium carbonate, a substance that reacts with the sulfuric acid in rainwater that has been exposed to industrial pollutants (acid rain). Any acidic substance will cause this reaction, but rainwater is the most likely contact marble will have with acidity. Relief and inscription is most likely to be noticeably effected by this chemical weathering. Inscribed plaques or paving stones, decorated columns or gravestones should be protected from rainfall if you want them to keep their detail.
Blackening
Gypsum can lead to discoloration in exposed marble.
When calcium carbonate molecules in the marble dissolve in an acidic solution, aqueous ions are produced. These wash into the crevices. The ions and sulphur create, when dry, gypsum. Gypsum accumulating in sections of the marble can collect material that blackens those areas, marring the beauty of the marble stone.
Browning
The red-brown color of the Colorado River is due to oxidation of iron.
A number of different kinds of marble are mined from mountain areas in different regions of the world, and when the marble erodes, it can expose materials that present their own aesthetic problems with exposed to the air. Hematite and pyrite in Tuckahoe marble, for example, can carry iron that oxidizes when exposed to air, tinting the marble with a rust color. Again, a dry or protected environment can help mitigate this.
Marble Mulch
Calcium that decomposes into soil from mulch can raise the acidity level (lower the pH) of a soil bed. Landscape plants grow best in a pH range specific to each species, and too low a pH can cause nutrient absorption problems in plants that need less acid. Chlorosis, or yellowing, is a common problem seen following poor iron absorption from too-acid soil. If your shrubbery or flower bed is watered with rain, marble is probably a poor mulch choice. Irrigated beds should be marble mulched only after the pH of irrigation water is confirmed. A few plants, such as azaleas, thrive with lower pH soil and may not be harmed by leaching from the marble.
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