Metamorphic rock: CR-PG-2016

Item

Title

Metamorphic rock: CR-PG-2016

Description

This rock came from a Chestnut ridge. There is a park on top of this ridge overlooking the Delaware River. This river is the border between Pennsylvania and New Jersey. The rock was found on the uphill side of the path, atop leaves and grass. The coordinates for this rock are N40.71984, W075.19255. Well-developed crystals are abundant in this rock. Micas give the rock a shiny look to it. There are layers of black crystals towards the top and the bottom, with a layer of clearer reddish crystals in the middle. The shiny and black combination is likely biotite, but that will be easier to see in thin section. I suspect this rock is primarily quartz and hornblende, with biotite representing a small portion of the rock. There is also the long and thin hornblende crystals present. The clarity of these crystals points towards them being quartz. The entire rock is orange or white, with crystals throughout. The rock is porphyritic; there are crystals present, but they do not take up the entire rock. The density of this rock is rather average. The alteration of this rock is important. The orange layer looks as though it has been altered. Parts of it are darker than others, and it has a “rusty” look to it.
In thin section, this rock looks very much out of equilibrium. There are cracks through all of the crystals. Crystals that were once euhedral, look as though they have changed. Their are a few muscovite/biotite crystals, but this rock is made up dominantly of quartz, feldspar, and amphiboles. It is an aphanitic rock, that likely cooled slowly, and then was taken out of equilibrium by increased heat and pressure.

Creator

3681

Type

Metamorphic Rock

Mediator

Tamara Carley

Media

https://exhibits.lafayette.edu/geology/files/original/5a46d9f7e3b892ccfbb5b40e8ee0b742.jpg

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