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

KYANITE, SILLIMANITE, AND ANDALUSITE DEPOSITS OF THE SOUTHEASTERN
STATES
By GILBERT H. ESPENSHADE and DONALD B. POTTER
ABSTRACT
Deposits of the Al2SiO5 minerals kyanite, sillimanite, and
andalusite in the Southeast were investigated during 1950
to 1953. These minerals, particularly kyanite, are useful in
the manufacture of high-alumina refractory materials. Deposits
in the Southeast have been important sources of kyanite
since the 1920's.
These deposits occur in the large area of metamorphic
rocks in the Blue Ridge and Piedmont provinces that extends
for nearly 700 miles from the Potomac River to central Alabama.
The AUSiOs minerals are found here in four major
types of deposits: (a) in quartzose rocks, (b) in micaceous
schists and gneisses, (c) in quartz veins and pegmatites, and
(d) in residual soils and placers. The quartzose deposits
generally have a higher content of ALSiOs minerals than the
other deposits; kyanite-quartz deposits have been the most
productive in the region.
Kyanite-quartz rock, sillimanite-quartz rock, and andalusitepyrophyllite-quartz
rock are the main varieties of quartzose
deposits in the region; they seem to be more numerous in the
Southeast than in other parts of the country. Although these
varieties differ in their mineral content and other geologic
features, they are similar in chemical composition, consisting
principally of SiO2 and ALO3.
Deposits of kyanite-quartz rock are the largest and most
numerous of the group. Kyanite and quartz are the predominant
minerals; kyanite generally constitutes 10 to 30
percent of the rock; muscovite is generally present, and rutile
is a widespread but minor constituent, generally in amounts
between % and 1 percent; pyrite is also common; other minor
constituents in some deposits are pyrophyllite, lazulite, topaz,
and clay minerals. The deposits have a great range in size.
The largest and most persistent are in the Farmville districtr
Virginia, the Kings Mountain district, North Carolina-South
Carolina, and at Graves Mountain, Ga. The rock is extremely
resistant to weathering and erosion, and the larger deposits
form prominent ridges or monadnocks several hundred feet
high and a mile or more long. Sillimanite quartzite deposits
are small, few in number, and have been found only in the
Kings Mountain district, North Carolina-South Carolina.
Sillimanite makes up from 10 to 35 percent of this rock;
andalusite and kyanite are also present in places. Other
aluminous minerals occurring locally in amounts of a few
percent are white mica, chloritoid, diaspore, topaz, and lazulite;
rutile is a widespread accessory mineral. Significant
amounts of andalusite, generally accompanied by a little
diaspore, are present in some of the North Carolina pyrophyllite
deposits; small amounts of kyanite, topaz, corundum,
and lazulite are also present locally; rutile is a persistent
accessory mineral. A new deposit of this type was discovered
at Boles Mountain, S.C., during the present investigation.
Very fine grained topaz is the most abundant aluminous
mineral in parts of the Brewer deposit, South Carolina;
andalusite, kyanite, and pyrophyllite are also present.
Kyanite occurs in micaceous schist and gneiss in extensive
areas of the Blue Ridge of western North Carolina and
northern Georgia. The kyanite content of large bodies of
rock ranges from a few percent to about 10 percent, though
some individual layers contain 15 percent or more. Several
deposits of this type have been mined in North Carolina and
Georgia. Sillimanite schist is rather widespread in a belt in
the western Piedmont that extends from North Carolina to
eastern Georgia, and in another belt in the Blue Ridge of
North Carolina and Georgia. Sillimanite usually is in very
fine crystals that are intimately intergrown with mica; it
generally forms less than 10 percent of the rock. Sillimanite
schists have been explored rather extensively, but none have
been mined. Large crystals of kyanite are common in quartz
veins and pegmatites in the kyanite schist areas of the Blue
Ridge. Such deposits are small and irregular, and attempts
to mine them have not been successful. In the kyanite schist
belt of Habersham and Rabun Counties, Ga., kyanite has
accumulated in the residual soil and in stream placers; these
deposits have been mined at several places. Kyanite and
sillimanite are among the heavy minerals in sandy formations
of the Atlantic Coastal Plain.
We believe that the kyanite and sillimanite in the micaceous
schists and gneisses were formed by the metamorphism of
.aluminous sedimentary rocks. Some geologists have thought
that the alumina was brought in by hydrothermal solutions,
possibly from igneous sources, and regard the kyanite-bearing
quartz veins and pegmatites that occur in kyanite schist and
gneiss as having been the agents that introduced alumina.
However, this is quite unlikely, because kyanite in the quartz
veins and pegmatites appears to be younger than kyanite in
the country rock; alumina probably was transferred from the
country rock to the quartz veins and pegmatites.
On the other hand, the quartzose deposits of AlzSiOs minerals
seem to have been formed by two distinct modes of
origin: metamorphism of aluminous sedimentary rocks and by
hydrothermal activity. Both types occur with, or in the vicinity
of, volcanic rocks. Kyanite quartzite in the Farmville
district, Virginia, and kyanite quartzite and sillimanite
quartzite in the Kings Mountain district, North Carolina-
South Carolina, are folded concordantly with the other metamorphic
rocks and have the distribution patterns of stratigraphic
units. There seem to be only two possible modes of
origin for the high-alumina quartzite deposits in these districts:
either by the metamorphism of clay-sand sediments or
by the introduction of alumina selectively into sandstone beds.
There is no evidence for large-scale introduction of alumina,