Sillimanite

OFR54 Bedrock Geologic Map of the Delaware Piedmont

The Piedmont rock units in Delaware, and bedrock geologic map of Schenck et al. (2000) are revised in this report based on new rock geochemistry, geochronometric data, petrography, and recent detailed mapping. Major revisions include:

A.I.duPont Students see geology of the Delaware Piedmont

Date

William "Sandy" Schenck lead a field trip through the Delaware Piedmont for the A.I. duPont High School Earth Science Class. The trip made use of the Wilmington-Western Railroad and everyone rode the railroad's "Doodle Bug." Activities included up close examinations of rock and mineral features and even "Panning for Garnets" at Brandywine Springs Park.

Deal Formation

It is a clayey, calcareous, shelly, glauconitic (10-20 percent) silt. Its colors range from greenish-gray and gray-green to brownish-gray and light gray. It is rich in calcareous and siliceous microfossils. The matrix mineralogy shows a high calcite component, except in the lower part of the formation which is within a calcite dissolution interval. In the lower half of the formation quartz is predominant.

Setters Formation

In Delaware, predominantly an impure quartzite and garnet-sillimanite-biotite-microcline schist. Major minerals include microcline, quartz, and biotite with minor plagioclase, and garnet. Muscovite and sillimanite vary with metamorphic grade. Accessory minerals are iron-titanium oxides, zircon, sphene, and apatite. Microcline is an essential constituent of the quartzites and schists and serves to distinguish the Setters rocks from the plagioclase-rich schists and gneisses of the Wissahickon Formation.

Wissahickon Formation

Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to 30 feet thick or as large massive bodies, are in sharp contact with the psammitic and pelitic gneisses. An attempt has been made to show some of the amphibolites mappable at the scale of the map. Granitic pegmatite is ubiquitous and occurs at all scales. Pyroxene bearing quartzite with garnet occurs locally near the contact with the Wilmington Complex. An ultramafic lens composed of cumulus layers of serpentinized peridotite, metapyroxenite, and metagabbro occurs near Hoopes Reservoir. The ultramafic lens may be correlative with the Baltimore Mafic Complex.

Delaware State Mineral - Sillimanite

In 1977, the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical Society, established sillimanite as the Delaware State Mineral. This act recognizes the geological and mineralogical significance of the large masses of this mineral found as boulders at Brandywine Springs, an occurrence that was recognized as important in the 6th (1892) edition of Dana's System of Mineralogy. The Brandywine Springs boulders are remarkable for their size and purity. The sillimanite has a fibrous texture reminiscent of wood and could potentially be cut into cabochon gems showing a chatoyant ("cat's eye") effect. Sillimanite is not mined as an ore or raw material in Delaware.