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Commercial Limestone And Marble
What Are Commercial Limestone and
Marble?
Andrew Alden
We all encounter limestone buildings and marble statues
during our lives. But the scientific and commercial definitions of these two
rocks don't match.
When geologists enter the stone dealer's showroom, and when
lay people go out in the field, each has to learn a new set of concepts for
these two different names.
Limerock Basics
Limestone and marble are both limerocks, an old-fashioned
industrial term for stone that is roasted to produce lime, or calcium oxide.
Lime is a basic ingredient in cement and much else.
Cement makers look at limerock as chemical feedstock of
greater or lesser purity and expense. Beyond that, they are indifferent to what
geologists or stone dealers call it.
The key mineral in limerock is calcite, or calcium carbonate (CaCO3). Any other mineral is undesirable, but a
particularly bad one is dolomite (CaMg(CO3)2), which
interferes with lime manufacture.
In the past, quarriers, builders, craftsmen and manufacturers
called limerock used for industrial purposes limestone. That's how limestone
got its name in the first place.
Limerock suitable for structural and decorative purposes,
like buildings and statuary, was called marble.
The word comes from ancient Greek with the root meaning of
strong stone. Those historic categories are relevant to today's commercial
categories.
Commercial Limestone and Marble
Dealers in stone use "limestone" and
"marble" to denote a category of stone that is softer than commercial granite (or basalt or sandstone)
but does not split like slate.
Commercial marble is more compact than commercial
limestone, and it takes a good polish.
In commercial use, these definitions aren't limited to
rocks made of calcite; dolomite rock is just as good.
In fact, serpentinite too has minerals softer than
granite and is considered a commercial marble under the names serpentine marble, green marble or verd antique.
Commercial limestone has more pore space than commercial
marble and does not wear as well. This makes it suitable for less demanding
applications like walls and columns and patios.
It may have some flat layering, but generally it has a
plain appearance. It may be honed or polished smooth, but it is limited to a
matte or satiny finish.
Commercial marble is denser than commercial limestone, and
it's preferred for floors, doorways and steps.
Light penetrates farther into it, giving marble a glowing
translucency. It also commonly has attractive swirling patterns of light and
dark, although pure white marble is also prized for statues, gravestones and
decorative features.
To add a bit of confusion, marble used to be called
"crystalline limestone" in previous centuries. Its key feature is the
ability to take a high finish.
None of these categories mean what they mean to geologists.
Geologic Limestone and Marble
Geologists are careful to distinguish limestone from dolomite rock, classifying both of these
carbonate rocks as sedimentary rocks.
But with metamorphism both become marble, a metamorphic rock in which all the
original mineral grains have been recrystallized.
Limestone is not made of sediment derived from rocks, but
instead generally consists of the calcite skeletons of microscopic organisms
that lived in shallow seas.
In some places it's formed of tiny round grains called
ooids, formed as calcite precipitates directly from seawater onto a seed
particle. The warm seas around the islands of the Bahamas are an example of an
area where limestone is forming today.
Under gentle conditions underground that are not well
understood, magnesium-bearing fluids may alter the calcite in limestone to
dolomite.
With deeper burial and higher pressure, dolomite rock and
limestone both recrystallize into marble, wiping out any fossils or other
traces of the original sedimentary environment.
Which of these are the real limestone
and marble?
I'm prejudiced in favor of geologists, but builders and
carvers and lime makers have many centuries of history on their side. Just be
careful about how you use these rock names.
Andrew Alden
Professional geologist, writer, photographer,
and geological tour guide
Thirty-seven years of experience writing
about geological subjects
Six years as a research guide with U.S. Geological Survey (USGS)
Experience
Andrew Alden is a former writer for
ThoughtCo who contributed hundreds of articles for more than 17 years. Andrew
works as a geologist, writer, editor, and photographer. He has written on
geological subjects since 1981 and participates actively in his field. For
example, Andrew spent six years as a research guide with the U.S. Geological Survey,
leading excursions on both land land and at sea. And since 1992, he has hosted
the earthquakes conference for the online discussion platform, The Well,
which began as a dialogue between the writers and readers of the Whole Earth
Review.
In addition, Andrew is a longtime member of
the member of the Geological Society of America — an international
society that serves members in academia, government, and industry; and
the American Geophysical
Union — a community of earth and space scientists that
advances the power of science to ensure a sustainable future.
Andrew lives in Oakland, California; and
though he writes about the whole planet and beyond, Andrew finds his own
city full of interest too and blogs about its
geology.
Education
Andrew Alden holds a bachelor's
(B.A.) degree in Earth Science from the University of New Hampshire,
College of Engineering and Physical Sciences, in Durham, N.H.
Awards and Publications
Andrew Alden on Earthquakes (The Well Group, Inc.,
2011)
Assessment of River — Floodplain Aquifer Interactions (Environmental
and Engineering Geoscience, 1997)
Andrew
Alden on Hosting (The Well Group, Inc., 1995)
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