Saturday, November 21, 2009

Minerals from the State of Delaware

Delaware surely must yield the least variety of mineral specimens of any other state in the US. That's because a coastal plain consisting of sand and sediment underlies most of it. Delaware's relatively few collectible minerals confine themselves to a diminutive tract of Appalchian Piedmont in the far northwest part of the state.

Delaware's State Mineral is sillimanite. The specimen shown beneath our title was collected at Brandywine Springs. A polymorph of kyanite and andalusite, gemologists refer to sillimanite when gemmy and transparent as fibrolite. I've also heard it called Delaware's State Gem. Give Delaware credit. Half the states in the US don't have a state mineral. Recognize them as well for its first class mineralogical society and the wonderful mineralogical museum at the University of Delaware.

Although the university of Delaware Mineralogical Museum has yet to exhibit Delaware minerals, next door at 257 Academy Street in the Delaware Geological Survey, a cabinet on the second floor features several impressive Delaware specimens. At top left are apatite crystals from Dixon's Quarry in Woodale. Below it are almandine garnet crystals from a no longer accessible locality beneath the homes of a Newark neighborhood known as West Branch. The beryl at top right is also from West Branch; so are the curved schorl tourmaline crystals pictured beneath it. In addition to showing me these minerals and allowing me to photograph them, Dr. Fitzgerald accommodated me similarly with other Delaware material in storage at the Museum.

With the West Branch locality extinct, Wilmington's Brandywine Quarry---not to be confused with the Brandywine Springs sillimanite locality---probably has Delaware's most extensive variety of minerals with 18 entries including 14 valid minerals noted in Mindat. The orange chabazite pictured at left especially impressed me among Brandywine Quarry minerals. Chabazite is one of four zeolites known to occur here along with stilbite, laumontite, and natrolite.

The stilbite pictured at left attracted my interest. It came from a road cut along I-95 near Naaman's Road. And the historic magnetite at right in a sheet of muscovite from Chandler's Hollow (Beaver Valley) in Newcastle County fascinated me. To see images of the the Delaware minerals shown in this post and other Delaware minerals that I photographed during my recent visit to the University of Delaware Mineralogical Museum, please follow the link to this set on my Flickr Site.


  1. Sillimanite should really be Connecticut's state mineral because it is named after Benjamin Silliman a famous geology professor from Yale University in New Haven, Connesticut during the 19th century. I am very pleased to see that Delaware has adopted sillimanite as its state mineral. Chemically sillimanite is anhydrous aluminium silicate that forms a solid solution series with andalusite, kyanite and mullite. it is used commonly to determine the metamorphic grade found in rocks containing the mineral and its cousins.

  2. How about the Cape Henlopen Diamonds, found on the State Park Beaches?