An interesting Slate Axe


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Apr 15, 2009
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In my relic collection, I have over 150 axes and celts, found over many years. Of these less than 15 are made of slate. Of these slate relic, only 4 are considered a complete relic, with only minor plow or use damage. Only 2 of these slate axes or celts are made from black slate. The remainder made of green and banded slate. Both of the black slate axes are made from a "slab" of slate cleaved along flat planes.

Slate is derived from shale-type sedimentary rock of clay or volcanic ash that underwent low-grade regional metamorphism. It is mainly composed of quartz and muscovite or illite.

Illite is a group of closely related non-expanding clay minerals. Some minerals like biotite, chlorite, hematite, and pyrite are also usually present in slate. Apatite, graphite, kaolinite, magnetite, tourmaline, and feldspar are sometimes present as well, although less frequently. Slates vary greatly in color from black through various shades of gray to greens, reds, and purples of different hues. Slates divide themselves naturally into (1) those derived from aqueous sediments and (2) those of igneous origin.

Slate is considered as the finest grained foliated metamorphic rock. Metamorphic rocks are formed from the change in form of existing rocks, a process called metamorphism. Slate arises from the repetitive layering or foliation of metamorphic rocks, particularly through the low-grade metamorphism of shale or mudstone. When shale or mudstone is exposed to heavy pressure and heat from a tectonic plate activity, its clay mineral components metamorphose into mica minerals. Mica minerals such as biotite, chlorite, and muscovite, are the main components of slate. One unique characteristic of slate is that it is formed through the process of foliation, which refers to the repetitive lamination of metamorphic rocks caused by shearing forces or differential pressure. Layers of rocks are then formed perpendicular to the direction of the pressure of metamorphic compression. This gives slate its ability to cleave along flat planes.

Black slates owe their blackness to carbonaceous matter, probably derived from the decomposition of marine organisms on the sea floor.

In Indiana, I have found many types of slate. Banded slate which exhibit green slate with darker gray and black bands, also pinkish red and purple slate. While black slate is found in Arkansas, Maine, New York, Pennsylvania, Tennessee, Vermont and, Virginia. I believe most if not all of the glacial slate found in my area of Indiana is a green banded slate brought down from the Canadian Shield.

In my experience and view, finding a complete slate axe or celt is relatively rare. While finding one broken is scarce compared to other hardstone axes or celt.

These photos show the ripple of the Layers between the planes.








1914 United States Geological Survey SLATE IN THE UNITED STATES
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