Limestone cut by crystal-fiber calcite vein and minute crack-seal veins, northern Appalachians, Vermont

&lt;p&gt;The limestone was compressed during thrust faulting and folding in the northern part of the Appalachian Mountains. &amp;nbsp;Even in this small rock sample such as this, the effects of compressional shortening are seen. &amp;nbsp;The vein is calcite, and it is evident that the calicite was progressively precipitated as fibers as the fracture dilated. &amp;nbsp;The vein formation was ‘syntectonic.’ &amp;nbsp;The fibers are essentially perpendicular to the fracture trace, indicating that there was no oblique slip. &amp;nbsp;There are faint white lines parallel to the trace of the vein, and these are expressions of minute calcite veins. &amp;nbsp;The rock was progressively buried during crustal thickening (by thrust faulting), and in the presence of overpressured water would ‘crack’ along fractures. &amp;nbsp;There was a rhythm to it: &amp;nbsp;build-up of fluid pressure; creation of a local tensional environment; tensional fracturing. &amp;nbsp;As soon as a crack would form, it would heal. &amp;nbsp;The healing worked this way: &amp;nbsp;the release of pressure upon cracking would cause calcium carbonate to precipitate from solution, thus filling (healing) the crack; then the pressure buildup would begin again. &amp;nbsp;The cracking and sealing has the effect to spall off some of the wall rock as thin slivers, which can be seen along the margins of the thickest vein in this specimen. &amp;nbsp;What was the source of the calcite? &amp;nbsp;Pressure solution. &amp;nbsp;Under the influence of directed stress, even under non-metamorphic conditions, calcite can be forced into solution, for it is more soluble under directed stress. &amp;nbsp;The source of the calcite thus is in the very (close) neighborhood of the veins. &amp;nbsp;In one direction, the calcite dissolves; at a right-angle direction of tension it precipitates. &amp;nbsp;Closed system!&amp;nbsp;&lt;/p&gt;

Whereabouts

I found this rock on a little island near Lone Rock Point near Burlington, Vermont, in 1991. On that day (a sunny afternoon) I was canoeing with a friend, Drew Ives. Of course I gave Drew (trained as a lawyer) a lecture on pressure solution. I like the fact that this one specimen displays pressure solution, tiny crack-seal veins, and a thick vein (once a tiny crack-seal vein) that ‘took off,’ growing in ways that created crystal fibers tracking the opening process.

Age & Formation

Limestone (and dolomite) of Cambrian age, ~520 Ma.