Geological State Symbols Across America           Geology of the National Parks Through Pictures

Delaware State Geological Symbols
Year Established
State Mineral
State Fossil

Belemnitella americana




State Mineral: Sillimanite

Sillimanite Phase Diagram

Kyanite-Andalusite-Sillimanite metamorphic phase diagram (


Sillimanite from Brandywine Springs, Delaware (

Sillimanite is an aluminum silicate mineral with the formula Al2SiO5. It is actually one of three metamorphic minerals with the same chemical formula. These three minerals, kyanite, andalusite, and sillimanite, form what is known as a metamorphic series. Each of the three minerals, which are metamorphosed clay minerals from shales, form at different pressures and temperatures as seen in the phase diagram to the right, with sillimanite representing the highest temperature version of the minerals. The size of the minerals is directly tied to how long the metamorphic rocks were subjected to those specific temperatures and pressures. The larger the minerals, the longer they were at those specific conditions. These three minerals are therefore good indicator minerals that tells scientists about what temperature and pressure was reached during metamorphism. Sillimanite can be brown, pale-green, grey, or white in color and it can be found in a variety of different forms including fibrous, radiating crystals, columnar, massive, and rounded crystals. Sillimanite is found within the metamorphic rocks schist or gneiss, and has a fairly high hardness; 6.5-7.5 on Moh's Hardness Scale. Sillimanite is named in honor of Benjamin Silliman (1779-1864), the earliest professor to teach mineralogy at Yale University. All three of the almunia-silicate minerals are mainly (95%) used as a refractory minerals, meaning they make bricks out the mineral for use in high temperature ovens; typically associated with iron and steel manufacturing.


Cat's Eye Sillimanite

Cat's eye sillimanite cabochon from Madagascar (

Within Delaware, sillimanite is found within the Hoopes Reservoir and Brandywine Springs areas. Around Brandywine Springs, sillimanite occurs as remarkably pure boulder sized pieces, and can be found as stream-rounded boulders in the area. The large size of the sillimanite crystals indicates that these rocks underwent high temperature metamorphism for a long period of time. The sillimanite here is ~30% the fibrous variety, giving the mineral a wood-grain texture when broken. Within Delaware, sillimanite is not mined for refractories, instead it is can be cut into cabochon gems that show a "cat's eye" effect, when is it collected at all. The sillimanite from Delaware can be found within the Delaware Gneiss, which formed 1.5 to 1.2 billion years ago. The Delaware Gneiss is part of a larger structure called the Delaware Piedmont, a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. The piedmont formed from a series of tectonic events that raised the Appalachian Mountains between 470 and 440 million years ago.

State Fossil: Belemnitella americana (Belminite)

Belmnite Reconstruction

Reconstruction of an extinct belemnitids, the animal from which a belemnite comes from (Klug et al., 2016).


Belemnites are the skeletal remains of an extinct group of cephalopods known as belemnitids. The belemnite is a cigar shaped, calcitic, chambered internal shell, which differs from the external shell of the closely related ammonites. Rarely does anything beyond this internal skeleton remain from the belemnites, although some soft tissue remains have been discovered giving scientists important clues into the morphology of these animals. The fossil is the part of the tail called the rostrum that was covered with a leathery skin. The rostrum was composed of calcite crystals that grew outwards producing growth rings, similar to tree rings. The rostrum likely acted as a counterbalance to the weight of the rest of the body, allowing easier maneuverability within the water column. Belemnites lived during the Jurassic and the Cretaceous, going extinct along with the non-avian dinosaurs at the end of the Cretaceous. These fossils are extremely useful in paleontological studies. If found in abundance, which they often are, the streamlined profile of the fossil allows scientists to determine current direction, by measuring the alignment of the fossils. Belemnites are also fantastic indicators of paleo-oceanic sea temperatures by measuring the oxygen isotopic ratios preserved within the fossils themselves.



Belemnitella americana examples (

Belemnites are found in abundance in many parts of the globe including Europe and North America. Within Delaware, belemnites can be found in the Mount Laurel Formation, a 70 million year old Cretaceous shallow-sea, sandstone deposit. Fossil hunters are able to collect these fossils from the dredge spoil piles along the Chesapeake and Delaware Canals west of St. George. Different genera and species of belemnites can be identified based on the shape of its nib, the pointed end of the fossil, and the shape of its guard, the rest of the fossil. Belemnitella americana is a prolific species that can be found across eastern North America and Europe. It has a wide, flat nib and a moderately wide guard. Although they have a wide range of possible colors (red, brown, orange, blue, purple, violet or yellow and translucent) they also fluoresce under ultraviolet light.


Klug, C., Schweigert, G., Fuchs, D., Kruta, I., & Tischlinger, H., 2016, Adaptations to squid-style high-speed swimming in Jurassic belemnitids: Biology Letters, v. 12, p. 20150877

Geology of Delaware's National Parks

Through Pictures

(at least the one's I have been to)