Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

REGIONAL GEOLOGY
metamorphic rocks of the Piedmont and Blue Ridge
provinces that extends for nearly 700 miles from
the Potomac River to central Alabama. The rolling
surface of the Piedmont province slopes gently eastward
from altitudes of 1,000 to 1,500 feet at the
base of the Blue Ridge to between 200 and 300 feet
at the Fall Line, the west edge of the Coastal Plain.
Low mountains are common in the western part
of the Piedmont province; only a few isolated
monadnocks are found farther east. The rocks of
the Piedmont province are overlapped on the east
and on the south by the Coastal Plain sedimentary
rocks of Cretaceous and Tertiary age that dip gently
seaward. The Blue Ridge province is a rugged,
mountainous region that is bordered on the west
by the folded sedimentary rocks of Paleozoic age
of the Appalachian Valley and Ridge province. The
mountains are 5 to 10 miles wide in Virginia, but
they become wider to the south and are over 60
miles across in southwestern North Carolina.
The region is well populated, and good highways
and railroads provide excellent transportation. The
climate is moderate; even in the Blue Ridge long
periods of severe winter weather are rare.
Two general types of metamorphic rocks predominate
in the region: hornblende schists and gneisses
and micaceous siliceous schists and gneisses. These
rocks have been considered to be Precambrian in
age in some areas and Paleozoic in other places;
metamorphosed rocks of both ages are probably
present. Much of the hornblende schist and gneiss
was probably igneous rock originally, either intrusive
rocks or flows of intermediate to basic composition.
Some hornblende schist and gneiss appear
to be metamorphosed sedimentary rocks. The hornblende-bearing
rocks have been called the Roan
gneiss in the southern part of the region. Many of
the micaceous siliceous schists and gneisses were
originally sedimentary beds; some show relict volcanic
structures, and others may have been intrusive
igneous rocks. In the northern part of the
region, large areas of these rocks have been called
the Wissahickon schist; in the southern part, similar
rocks have been called the Carolina gneiss.
Kyanite, sillimanite, and andalusite occur chiefly in
the different varieties of micaceous siliceous gneisses
and in associated quartzose rocks.
Large granite bodies occur in parts of the region,
and pegmatites are abundant in certain areas. Gabbro
and diorite are common; small bodies of ultrabasic
rocks occur mostly in the Blue Ridge area. A
large area in central North Carolina is underlain by
volcanic flows, tuffs, and slates, some of which are
but slightly metamorphosed; this area has become
known as the Carolina slate belt. These rocks extend
southwest across South Carolina into Georgia,
where they have been called the Little River series.
Isolated basins of unmetamorphosed sedimentary
rocks of Triassic age occur in the Piedmont of
Virginia and North Carolina. Widely distributed
diabase dikes cut the Triassic sedimentary rocks
and the metamorphic rocks.
The distribution of some of these major geologic
units the belts of volcanic rocks, the late Paleozoic
granitic bodies, pegmatites, and basins of Triassic
sedimentary rocks is shown on the map of the
Southeastern States (pi. 1); the map also shows
the location of the kyanite, sillimanite, and andalusite
deposits. Current knowledge of, and ideas about,
the geology of the central part of the Appalachians
have recently been discussed in a review article and
shown on a geologic map by King (1955).
The geologic structure of the region is highly
complex. The rocks generally are strongly folded,
and have a foliation which is some places completely
obscures the original bedding. As a rule, the geologic
units trend toward the northeast; the mountains
and ridges of the region also have the same
trend. The degree of regional metamorphism varies
from place to place; metamorphic grade ranges from
slightly deformed and metamorphosed shaly beds
in the Carolina slate belt to sillimanite schists associated
with granite and pegmatite.
The rocks in the region have undergone deep
weathering for long periods of time, and exposures
of fresh rock are rare in many places. Most rocks
have been decomposed by chemical weathering. The
less resistant minerals, such as feldspar and hornblende,
are altered to clay; the more resistant minerals,
such as quartz, kyanite, sillimanite, and
muscovite, are little altered. The original structure
of the rock is generally well preserved in the rotted
rock. This mantle of decomposed rock, known as
saprolite, extends to depths of 50 to 100 feet in
some places. Fresh rock is found most commonly
along streams where the saprolite has been removed
by erosion. Rocks that are made up principally of
resistant minerals are generally only slightly decomposed.
Kyanite-quartz rock is very resistant and
forms some of the prominent isolated knobs and
ridges in the Piedmont province.
GEOLOGY OF THE DEPOSITS
Kyanite, sillimanite, and andalusite have the same
chemical composition Al2SiO 5 but different physical
properties. These minerals characteristically oc-