Pt.3) Grain Size, Texture, Mineral Phases and Compositions of the Marble: Building Stone of the Lincoln Memorial, by Elaine S. McGee

An investigation of differences in the durability of the Colorado Yule marble, a widely used building stone. By Elaine S. McGee.

U.S. GEOLOGICAL SURVEY BULLETIN 2162

Abridged and Presented for your enjoyment by Martin Cooney

Building Stone of the Lincoln Memorial

PART 3) GRAIN SIZE, TEXTURE, MINERAL PHASES AND COMPOSITIONS of the MARBLE

U.S. Department of the Interior / U.S. Geological Survey

Colorado Yule Marble — Building Stone of the Lincoln Memorial. Prepared in cooperation with the National Park Service Colorado Yule Marble — Building Stone of the Lincoln Memorial By Elaine S. McGee. U.S. DEPARTMENT OF THE INTERIOR: BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY: CHARLES G. GROAT, Director UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1999. Published in the Eastern Region, Reston, Va. For sale by U.S. Geological Survey Information Services Box 25286, Federal Center Denver, CO 80225 Library of Congress Cataloging in Publication Data. Manuscript approved for publication August 13, 1998.

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GRAIN SIZE AND TEXTURE

Tightly joined calcite grains produce a luminous surface that polishes well

The grain size and texture of marble influence both its appearance and its durability. Qualities that are often sought in marble are an even texture, a homogeneous appearance, and a luminous surface that polishes well.

Fine-grained marbles have a homogeneous appearance and may take a polish better than some coarser grained marbles.  Similarly, marbles with tightly joined calcite grains may appear more luminous when polished and may prove to be more durable than marbles that have large or loosely bonded grains.

Fine texture and fine crystallization

The fabric and texture of the Colorado Yule marble have been widely studied and examined. Early reports of the marble described its fine texture and fine crystallization (Lakes, 1910). The grain size of the Yule marble is reported as 2,000–3,000 grains to the square centimeter. Knopf (1949) reported grain sizes in the marble as 0.5–1.5 mm, and Thill and others (1969) reported an average grain size of 0.4 mm for the sample they studied.

Metamorphism means to change form leads to changes in the mineralogy, texture, and sometimes the chemical composition of rocks.

The edges of the calcite grains in the Colorado Yule marble are deeply crenelated (irregularly and minutely notched and scalloped); these crenelations are believed to account for the resistance to weathering of the marble.

Calcite Formations

Calcite grains determine texture and fabric characteristics of the stone

Bain also reported that the calcite crystals in the Yule marble are aligned so that the long axes of the grains are essentially perpendicular to the principal veining in the deposit.

As the major constituent of the marble, calcite grains determine texture and fabric characteristics of the stone.

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MINERAL PHASES AND COMPOSITIONS

Yule marble is predominantly composed of white calcite

Calcite Crystals

Samples taken from recently quarried and Lincoln Memorial stone

Acid dissolution and staining tests described by Knopf (1949) showed that the Colorado Yule marble is practically a pure calcite marble with very few inclusions of other minerals.

The accessory minerals reported by Thill and others (1969) differ slightly from the inclusion phases identified in the samples for this study.  However, Thill and others did not provide any more details about the sample or about the methods used for mineral identification.  Samples examined for this study—in hand specimen and in thin sections made from recently quarried samples of the marble and from pieces in the Lincoln Memorial—show that the Yule marble is predominantly composed of white calcite.

Quartz crystals

Calcite Crystals

Inclusions commonly occur in clusters

The most common inclusion in the marble is quartz.  Minor inclusions include muscovite, phlogopite, feldspar, pyrite, sphene, apatite, zircon, and rutile. In inclusion-rich samples of the marble, such as those from the Colorado Golden Vein Select and Colorado Golden Vein grades, the inclusions commonly occur in clusters and, with the exception of some quartz grains, are finer grained than the calcite.

Calcite

Quartz

Mica

Muscovite

Phlogopite

Feldspar

Pyrite

Sphene

Apatite

Zircon

Rutile

Sphalerite

Yule marble is nearly pure calcium carbonate

The calcite (CaCO3) in the Yule marble is nearly pure calcium carbonate. It does not vary significantly among the four grades of marble.  The compositions of the calcite grains in samples from the antefix, stylobate, and quarry dump are also very similar to the compositions in the graded marble samples.  Minor constituents determined in the calcite include MgO, MnO, FeO, and SrO; all are present in amounts less than 0.5 weight percent.

There is no correlation between the grade of the marble and the presence or amount of the minor constituents in the calcite. Quartz (SiO2) is the most abundant inclusion in these samples of the Yule marble.  The quartz occurs as rounded grains in clusters with other phases or in clusters of quartz grains.

Mica inclusions occur as thin gold to brown lines and streaks

Weathered quartz inclusions occur as rounded, translucent gray grains that may be slightly raised compared to the surrounding calcite. Some quartz inclusions form lines or veins, where they appear as clusters of small rounded grains.

Vanderwilt (1937) reported that the crystal grade of the Yule marble was not marketed after 1936 because it contained a quantity of chert that occurred in thin streaks; by that time, studies had shown that the chert weathered to an unacceptable, dark-gray color. It is probable that some of the quartz that occurs as distinct, dull-gray, rounded grains in the stone at the Lincoln Memorial corresponds to the chert described by Vanderwilt.

Mica inclusions occur as thin gold to brown lines and streaks, and they also occur with quartz in clouds of gray mixed with brown.

Typical feldspar grains are from 20 to 80 micrometers in length

Mica inclusions in the marble at the Lincoln Memorial do not have any distinctive weathering features. On a small scale, the area around inclusion streaks may be rougher with more relief in the calcite because of the loss of small flakes of mica. However, for the most part, the weathering of the mica inclusions at the Lincoln Memorial is not as noticeable as the weathering of other, larger, more isolated inclusion phases such as feldspar and quartz. Feldspar inclusions are present in the Colorado Golden Vein samples. They are inter grown in clusters with muscovite, quartz, and sphene but they are not major constituents of these samples.

Typical feldspar grains in the polished sections studied are from 20 to 80 micrometers in length; however, there are some rather large (1.5–3.5 cm) feldspar inclusions at the Lincoln Memorial. Potassium-, sodium-, and calcium-bearing feldspars have all been found in the samples.

Weathered feldspar is quite noticeable

Calcium feldspar occurs as larger grains and appears to be slightly more abundant than the other two feldspars, but this relative abundance has not been determined statistically. In addition to its size, weathered feldspar is quite noticeable in some areas of the Lincoln Memorial because it is typically white to gray and is raised relative to the surrounding calcite.

Weathered feldspar appears blocky, and gray feldspar grains are less translucent than quartz inclusions. Although some feldspar occurrences stand out, feldspar is a less common inclusion than quartz in the Lincoln Memorial stone.

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MINERALOGIC AND PHYSICAL CHARACTERISTICS

Inclusions are small (typically about 10 micrometers)

Pyrite, sphene, apatite, rutile, zircon, and sphalerite all occur as minor inclusions in the marble. With the exception of pyrite, which ranges in size from about 20 to 160 micrometers, these inclusions are small (typically about 10 micrometers).

Rutile, zircon, and apatite are minor inclusions visible in polished sections of the marble but not noticeable on weathered surfaces in buildings.

Apatite is the most common of these minor inclusions; it occurs as part of the inclusion-rich clusters in the Colorado Golden Vein samples, along with quartz, mica, and occasional feldspar. Sphene, which is mostly noticeable in thin sections of the marble, also occurs in the clusters of quartz, mica, and feldspar in the Golden Vein samples.

Inclusions are raised compared to the surrounding calcite

Typically, pyrite and sphalerite inclusions occur in small groups of a few isolated grains. The composition of the pyrite in the samples studied is variable but within the usual range for this sulfide.

At the Lincoln Memorial, where they have weathered, sphalerite and pyrite are quite noticeable compared to the surrounding white calcite. The exposed sphalerite surfaces have a reddish-brown rusty appearance, the pyrite has a golden-metallic appearance, and both of these inclusions are raised compared to the surrounding calcite.

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An investigation of differences in the durability of the Colorado Yule marble, a widely used building stone. By Elaine S. McGee.

U.S. GEOLOGICAL SURVEY BULLETIN 2162

Building Stone of the Lincoln Memorial

Abridged and Presented for your enjoyment by Martin Cooney

PART 3) GRAIN SIZE, TEXTURE, MINERAL PHASES AND COMPOSITIONS of the MARBLE

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Next

PART 4) SELECTION of YULE MARBLE for the LINCOLN MEMORIAL … Click Here

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The Story of Colorado Yule Marble at martincooney.com

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