GRANITE

( Fr- granit; Ger- Granit; Nor- granitt; Rus- )

GRANITE

A. Granite. Pin with granite pendant (greatest dimension - ~ 3.5 cm ) that is a tumbled piece of granite from Barre, Vermont region. F.S. Dietrich collection. (© photo by Dick Dietrich)

B. Granite from vicinity of St. Cloud, Minnesota (width - 5 cm), that exhibits a typical granitic texture. (© photo by B.J. Skinner)

C. Granodiorite bookend (height - 15 cm) presented to Adolph Knopf by his students upon his retirement as Sterling Professor of Petrology at Yale University. R.V. Dietrich collection. (© photo by Dick Dietrich)

DESCRIPTION: Granite, is defined by most professional geologists as a phaneritic igneous rock that consists of 20 or more percent quartz, more alkali feldspar than plagioclase feldspar, and between 5 and 20 percent varietal minerals (typically biotite + muscovite + hornblende). A few geologists, who think that at least some granites are not of igneous origin, substitute "igneous or igneous-appearing" for igneous in the just reiterated definition. Properties of the chief minerals, as they occur in granitic rocks, follow: Quartz -- colorless, milky white or smoky (gray-tan); H. 7; S.G. 2.65; transparent to translucent; vitreous luster; conchoidal fracture. Alkali feldspar (which includes microcline, orthoclase and perthite) -- off-white, light tan, salmon-pink to nearly brick-red; H. 6-6½; S.G. 2.55-2.63; subtranslucent to opaque; pearly luster; two good cleavages at or near 90 degrees to each other. Plagioclase feldspar (which in granitic rocks is typically oligoclase) -- off-white to buff; H. 6-6½; S.G. 2.61-2.64; subtranslucent to opaque; pearly luster; two cleavages a few degrees off 90 degrees to each other; polysynthetic twinning -- which is expressed by closely spaced parallel lines that can be seen when certain cleavage surfaces are viewed through a simple (e.g., 10x) hand lens -- is common. As a consequence of the mineral constitutuents, granites have the following overall properties:
    Colors - typically overall gray or salmon pink to nearly brick red
    H. (effective hardness) 7
    S.G. 2.64-2.65
    Light transmission - opaque
    Luster - ranges from dull to grainy with sporadic parts pearly and vitreous
    Breakage - irregular
    Miscellany - In the marketplace, the term granite is frequently extended to include virtually all relatively light colored igneous and igneous-appearing rocks that roughly resemble granite, sensu stricto. -- Most of the other rocks that are so-designated in the market are monzonites and granodiorites, but a few are diorites and syenites according to accepted petrographic nomenclature. A well known example of these marketplace "granites" is the Mount Airy Granite of North Carolina, which in professional petrographic literature should be called a leuco-granodiorite.   Definitions of the names applied by petrographers to these other "granites" are given in the GLOSSARY and general relationships among these rocks can be seen in books such as "Rocks and Rock Minerals" by Dietrich and Skinner (1979).

Herein, marketplace usage is followed -- i.e., granite is used to include all relatively light colored (overall nearly white, gray, tan, pink and reddish) phaneritic igneous and igneous-appearing rocks that have a typical granularity. All consist largely of one or two feldspars and quartz plus or minus some dark-colored mineral such as biotite (=black mica); H. >6 (= effective hardness); S.G. ~ 2.65. Also, it seems prudent to note that several of these rocks are gneissic.

OTHER NAMES: Granite, granodiorite, etc. are widely accepted petrographic designations. Several granite (etc.) masses around the world have been named on the basis of their locations and are shown on geologic maps and referred to in geological reports by those binomial names -- e.g., the Barre Granite of central Vermont, the St. Cloud Granite of eastern Minnesota, and the Clancy Granodiorite, which was named by Adolph Knopf, to whose memory this compilation is dedicated, for exposures in Kain quarry on Clancy Creek, Lewis and Clark County, Montana. Several of the geographically based names of granites (etc.) are also used in the marketplace.

In addition, granites (etc.) of certain overall colors or exhibiting certain relatively distinct appearances have been given names that relate to those characteristics, and those names are used along with or instead of the formal geographically based names. And, a few of these rocks have been and continue to be used as gemrocks. Two examples are:

Orbicular granite - Typical orbicular granites contain orbicules -- i.e., spheroidal masses the components of which are arranged in roughly concentric layers -- surrounded by a continuum of "normal" granite. The layers consist of concentrations of the major light versus dark components of the rocks. In some orbicular granites, at least some of the orbicules have cores of the "normal" rock. (Figures D & E).
Rapakivi granite (also called "Baltic brown") - Typical Rapakivi granites consist of ovoid masses, with chiefly alkali feldspar cores and relatively thin chiefly off-white plagioclase feldspar mantles, within a so-to-speak continuum of "normal" granite. The alkali feldspars in most of Rapakivi granite are salmon colored orthoclase whereas the plagioclases tend to be off-white oligclase; the other common constituents are biotite and/or hornblende. Some petrographers consider these rocks to be porphyries with the feldspar ovoid masses the phenocrysts and the surrounding "normal" granite the groundmass. (Figure F).

D. Granite. Orbicular granite boulder (partly sheathed hammer head gives scale) in glacial debris "a few miles to the north of Granite Harbour in South Victoria Land, Antarctica. The dark coronas of these orbicules are recorded as hornblende. (© photo by Bernie Gunn in Geology, Glaciology, Geography & Wildlife of the Ross Sea Region, Antarctic -- www.RossSea.info -- reproduced here by permission)

E & F. Granites. Orbicular granite (top) and Rapakivi granite (below) as shown on two stamps of a set of three that depict rocks from famous localities in Finland This set of stamps, the third of which is shown in the MIGMATITE entry, was issued in February 1986. (© photos of stamps by Richard Busch, http://mineralstamps.rbnet.net)

USES: Stones on bolo cord slides and pendants, watch and clock faces, paperweights, bookends, lazy susan turntables, etc.

OCCURRENCES: As relatively large igneous or igneoid masses.

NOTEWORTHY LOCALITIES: Barre, Vermont; St. Cloud, Minnesota.

REMARKS: Origin of the name granite traces back to the Italian granito, which is the past participle of granire, which means to make grainy; Granire, in turn, is from grano (grain) from the Latin grānum, said to have Indo-European roots. Its use as a rock name dates back to at least the early 17th century.

Three major uses of rocks widely marketed as granites are for ashlar (for both exterior and interior facings), tombstones and other monuments and sculptures.

In the literature, several rocks other those mentioned under OTHER NAMES have been called granite, usually with some modifying adjective. Among those I have seen so-labeled are rocks included in the GNEISS and AMPHIBOLITE entries in this book and even BASALT (called black granite), none of which is a granite.

So far as the use of granites as decorative stones, the following quotation from the preface to The Marbles and Granites of the World (Grant, 1955) seems noteworthy.

"The author was led [to compile the information published in this book because of his] ... realisation of the artistry of the patterns and colouration of almost all the rocks of the earth. [He believes] ... a humble stone will delight the cultured observer even more than the polychrome of far more brilliant objects. [And, consequently] these Marbles and Granites, then, may be regarded ... as pictures, or at least as colour-schemes designed at the dawn of the world. [So,] they are thus not as incompatible as might be thought with Art, the subject of his previous volumes, but an interesting, and to many perhaps a novel aspect thereof."

An especially interesting, remarkably detailed ancient sculpture is illustrated in the July/August issue of "Saudi Aramco World" (page 16): It is described as sculpted by "a royal architect ... , made of diorite about 2090 BC, ... [and] found at Girsu, Mesopotamia."

Granite (though not as a gemrock) is the state rock of New Hampshire(?) and North Carolina, and one of three rocks designated as the state rocks of Vermont; blue granite is the official state stone of South Carolina; and red granite (also not as a gemrock) is the state stone of Wisconsin. In addition, granite is the state building rock, as contrasted to Plymouth Rock being the state historical rock of Massachusetts

SIMULANTS:

***Graniteware (enamelware) - enamel-coated metal (usually iron or steel, rarely tin). All sorts of kitchenware, ranging from cookware and bakeware to tableware (and even bedpans) were made from the mid 1800s well into the 1900s. The enamel was speckled and baked on the metal. Some of the dark gray graniteware items that were speckled with white roughly resemble granite; although other colors, of which there were several, did notl, they too were called graniteware in many quarters. Of the dark gray and white ones that I have seen, those labeled as made by Kemp Mfg. Co. of Toronto, Ontario, Canada resembled granite most closely. None, so far as I have been able to determine was fashioned into anything marketed as granite per se. - [Appearance suffices.].

***Wedgewood china - one variety has been made that resembles granite - [Appearance - china is a so-to-speak single phase material.].

REFERENCES: No general reference. Grant, 1955.

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