Marble - Colorado Geological Survey

esult of brecciation, or fracturing, which has
allowed an influx of foreign material into a
body of marble. In general, the white marbles
are derived from pure calcitic or dolomitic limestones
and tend to be the most free of impurities.
Red, pink, or red-brown coloration is generally
caused by manganese oxides or hematite.
Yellow, yellow-brown, or cream color variations
are usually due to limonite. Gray to black tints
are caused by carbonaceous material contained
in the rock matrix (Bowles, 1958). Green shades
are due to the presence of micas, chlorite,
and/or silicates (MIA, 1985). A list of secondary
minerals frequently found in marbles would
include: brucite, diopside, epidote, feldspars,
forsterite, graphite, grossularite, humite, periclase,
phlogopite, pyrite, quartz, scapolite, serpentine,
sphene, spinel, talc, tremolite, vesuvianite,
and wollastonite (Dietrich and Skinner,
1979). The marble which occurs on the subject
property can be subdivided into many different
colors. The most prevalent is white marble
followed by a medium gray marble which was
previously mined. The amount of white marble
is misleading to even the casual field observer
because much of the white marble outcrops
have a medium gray stained or weathered rind
.5 inch to 1 inch thick. CGS also encountered
other marble of various shades of gray, green,
and a fine grained dark gray which was the
closest to black. None of the marbles were a
true black and no such true black is expected.
Texture
The texture of marble is the size, degree of uniformity,
and arrangement of constituent minerals.
Grain sizes may range from fine (greater
than one milimeter), to medium (one to five
milimeters), to coarse (greater than five milimeters)
for both calcitic and dolomitic marbles.
The marbles of the project area all appear to
fall into the medium grain category. Where calcite
and dolomite marbles occur together, the
calcite grains tend to be the more coarse. Also,
calcitic marbles tend to display anhedral, interlocking
grains with mosaic or sutured boundaries
whereas the dolomitic marbles tend to be
more granular or saccharoidal (Dietrich and
28
Skinner, 1979). Degree of uniformity can be variable,
ranging from equigranular, to non-equigranular,
to porphyritic textures in which large
euhedral grains are enclosed in a smaller
grained ground mass. None of the marbles
mapped fell into this category. All were equigranular
throughout the thickness of the bed
but might grade into finer or coarser grain size
over a long strike distance. The type, distribution,
and geometry of grains, along with the
nature of grain boundaries, is important in that
they will determine the relative physical properties
of a marble.
Translucence
Certain varieties of marble will allow light to
penetrate into the rock structure, creating a
very aesthetic "glow". This is a most desirable
quality in statuary marbles, some of which
have light penetration ranges on the order of .5
to 1.5 inch (Bowles, 1958). Some marbles have
a "waxy" luster which may also be attributable
to translucence. None of the polished samples
of the Colorado-Highland Marble quarry appear
to have significant translucence.
Rift-graln
The terms "rift" and "grain", when applied to
marble, are essentially synonymous and refer
to the direction(s) of easiest breakage or cleavage.
Frequently rift parallels bedding and is
probably caused by elongation of grains due to
pressure. The presence of platy or fibrous secondary
minerals, such as micas or actinolite,
will enhance rift because of preferred orientations
along axes of grain elongation (Bowles,
1958). Rift direction has been an historically
important consideration in quarry planning but
is recently becoming less important because of
the advent of modern diamond saws in the separation
of quarry blocks. As previously mentioned,
rift in Colorado-Highland marbles is
parallel to bedding, where present.
Soundness
The dimension of blocks to be quarried from a
particular deposit is greatly controlled by joint
spacing. Joints which are encountered in most