An alkali gabbro primarily composed of plagioclase, hornblende, biotite, and titanaugite [Titanian Augite], with lesser amounts of alkali feldspar and nepheline. Essexite grades into theralite with a decrease in potassium feldspar and an increase in the feldspathoid minerals. Its name is derived from Essex County, MA.
In order to produce a magma composition suitable for forming essexite the partial melting of the source rocks must be restricted, generally to less than 10% partial melting. The source melts of essexites contain more aluminium and alkali ions than available silica tetrahedra, which is why essexites crystallise nepheline instead of plagicolase. Higher than normal potassium favors the production of orthoclase, which is usually absent from most mafic igneous rocks.
*NH Geologic Survey report received June 17, 2006 after sending a 1/2 inch slice for analysis in early May of 2006.
Based on what I have found out to date, [you are] partially right...it is an "alien" rock. Based on what we have found to date, it appears as though the "rock" is a chunk of Essexite Gabbro. (Wally Bothner and Peter Thompson, UNH Geology.)
This gabbro outcrops on Mt. Monadnock, just north of the CN River in Lemington VT. . (See pg. 44. Geology of the VT Portion of the Averill Quad, Meyers, Paul Benton, Jr. VT GS Bull #27. 1964) A piece (the boulder that was found) that large, most likely was carried by the glacial ice and left in the glacial deposits along what is now Cathederal Road in Lancaster.
The magnetism of this rock is due to the mineral magnetite, one of the minerals in Essexite. The rounded shape of the rock is probably due to it transport by the glacial ice and weathering. In fact, weathering of some of the rocks minerals has created the "pits" that we now see on its surface.
Further "proof" that this "unknown rock"is an erratic (transported glacial boulder) of Essexite would be:
- Establishing a Boulder Train from Mt. Monadnock (Lemington VT) to the Lancaster (NH) find site. In other words, search the Lancaster area (stone walls are a good place to start looking) southward from Mt. Monadnock, for other pieces of the "unknown rock". Plotting any "finds" on a map of the area, should create a fan-shaped pattern of distribution, with the apex of the fan at the Essexite outcrop on Mt. Monadnock.
- Finding glacial striations (scratches in the bedrock) at or near the Lancaster find site confirming local ice direction. (Note: J.W. Goldthwaite’s Surficial Geology Map of NH, 1950, shows nearly a N-S striation direction for Lancaster, NH. Mt. Monadnock is nearly due north from Cathederal Road/Lancaster.)
- Obtain several samples of Essexite and have it tested along with the "unknown rock" to see if they have a similiar mineral content. Mineral Optic Laboratory in Wilder, VT., does such testing. Their phone number is 802.295-9373.
I want to thank you for "this project". It has been fun and informative. But...if you are still unsure of my findings, you should pursue the steps outlined above.
Public Outreach Coordinator
NH Geological Survey
29 Hazen Drive, PO Box 95
Concord, NH 03302-0095
External Links about Essexite
The Nith Lodge Stone Axe
Here is my on-the-spot impression;
The specimen makes a 'ringing' rather than a 'plink' sound when tapped with a hammer. You can feel the calcite on your fingers after you rub the specimen. Small bits and pieces easily flake off the surface, though the specimen, as a whole is very solid. Every bit of the specimen, even the flakes are attracted to magnets.