enter image description hereenter image description hereenter image description here This is a close up of outside of chipped off piece, and inside alsoenter image description hereI found this rock in Co. Donegal Ireland on a river bank buired under the dirt.

It weighs 16.4lbs- 7.4kg, has a hardness of 7 and measures 8''x5 1/2''x 6 1/2''

It has a greasy kind of feel to it, it contains very small amount of pink feldspar, the river contained pink redish granite this rock really stood out as being different having the contrasting green color.

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I include a photo of where I found this rock, on the right side of this river.There was Quartzite Granite and white Quartz, but mainly the redish granite.This green rock is opaque.It reacts ever ever so slightly to a magnet on string, very weak.

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    $\begingroup$ I have checked it a bit. It can be chlorite not epidote as an alteration product of the granites. The location you provided is too broad. Can you give coordinates of the river where you found it, so I can make a search for local geology? (A link to google maps is ok). $\endgroup$ – Universal_learner May 28 at 13:27
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    $\begingroup$ The location would be the edge of the Blue Stack mountains closer to Donegal Town, I believe the area is called Barnesmore Gap along the N15 road mapcarta.com/18277862 $\endgroup$ – Dave May 28 at 13:51
  • $\begingroup$ Chlorite has a Moh's hardness of only 2 to 3 whereas Epidote has a hardness of 6 to 7. I used a piece of sharp Nephrite to test this green rock, and Nephrite does not scratch it. $\endgroup$ – Dave May 28 at 14:17
  • $\begingroup$ Thank you for your comments Universal_learner I guess Quartz does scratch this rock so its probably around 6.5 moh's hardness $\endgroup$ – Dave May 28 at 15:06
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    $\begingroup$ Mohos scale is for minerals not for rocks. There are metamorphic layers at the zone but a river can transport materials long away. My candidates are epidote and chlorite for the green minerals of the rock. I guess it is a metamorphic rock, being the protolithe igneous. $\endgroup$ – Universal_learner May 28 at 17:25

enter image description hereI will start by saying I am not a geologist so hopefully someone has a better answer. Initially, your sample does look like massive epidote, but the local geology doesn't support that identification well.

Using the geological map found here, Geological Survey Ireland Spatial Resources, there are a few distinct types of bedrock geology in the Barnesmore Gap, two are ancient granites, two are a conglomerate type rock with pale green chloritic matrix, there are also shale, limestone, marble containing areas west of Barnesmore. The interesting thing to note is that the area, particularly associated with the granite zones, has a number of dolerite dykes, intrusives of mafic magmas, these bring with them the possibilities of pyroxenes, with diopside and jadeite having the right hardness, specific gravity, and perhaps lustre. So the green material could be from the dykes in the form of pyroxene or if the lava was more mafic even olivine.

Alternatively, epidote is often associated with hydrothermal events and marble, there doesn't seem to be any dykes in the marble containing areas but that doesn't mean there couldn't be some hydrothermal alteration due to the recent dykes in the area. So massive epidote is a possibility, but overall there doesn't seem to be much in the way of other hydrothermal alteration in the area so this option is not 100% either.

Overall the rock looks like like it contains epidote but it could also be a number of other minerals. I would suggest flaking a small crystal of the green mineral off so that cleavage and transparency can be assessed.

  • $\begingroup$ That's very interesting, thank you Friddy $\endgroup$ – Dave May 28 at 20:41
  • $\begingroup$ I split the rock where it had a fault line, then I broke a small chunk off and did a density test on the chunk that weighed 117grams, the displaced volume in water was 37 grams which gave a density of 3.16. Its a brittle rock. The color difference in the last photo is not as accurate as the first set $\endgroup$ – Dave May 30 at 11:34
  • $\begingroup$ Under the magnifying glass it looks so like rough Nephrite, but it does not have the same toughness as nephrite, it seems a bit like weathered Nephrite. It has the slightest translucence on small flakes that came off. $\endgroup$ – Dave May 30 at 12:13
  • $\begingroup$ I've done more density testing and it appears to be in the 3.22 range overall, which would put it in the Chlorite range, but a bit low for Epidote which starts at 3.30. But Pyroxene is also a possibility, its density is within this specimen's range. $\endgroup$ – Dave Jun 2 at 9:28
  • $\begingroup$ I have to eliminate Chlorite because of its hardness, I think Pyroxene minerals is more likely because of its density and hardness $\endgroup$ – Dave Jun 2 at 9:33

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