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Identity HelpActinolite?

11th Jan 2020 17:35 UTCBrian Fussell

I'm guessing based on visual observation, and if so would the rock be considered an amphibolite? I found it in Langlade Co WI, likely glacialy deposit rock.

11th Jan 2020 17:35 UTCBrian Fussell


11th Jan 2020 17:36 UTCBrian Fussell


11th Jan 2020 17:36 UTCBrian Fussell


11th Jan 2020 18:37 UTCNoah Lunster

I'd say amphibolite or gneiss. So the black one is most likely a mineral from the group of amphiboles.

11th Jan 2020 19:34 UTCFrank K. Mazdab Manager

certainly looks like a classic amphibolite and the dark crystals do appear to be an amphibole (in your last photo one can even see the two cleavage directions alternatively lit and darkened from the lighting).

Determining what species of amphibole is more challenging and as is being discussed in another thread, may not be easy to determine by home testing without additional chemical analysis.

But as a rule of thumb, as the rock increases in metamorphic grade, amphiboles tend to contain more Al. So among rocks with also a decent amount of calcium (if much of your pale mineral is plagioclase), then actinolite (with no Al) might be present in fairly low grade rocks, one of the lower-to-intermediate-Al "hornblendes" is most likely in medium grade rocks (something like "hornblende" (sensu lato) or maybe one of the pargasite or edenite varieties), and in the highest grade grade rocks, then sometimes accompanied by diopside, one of the pargasite or tschermakite (with quite a bit of Al) varieties may be present.

Amphiboles are quite diverse and occur in a variety of metamorphic rocks (and of course igneous and sedimentary rocks too), so there are roughly parallel trends for other starting bulk rock compositions (e.g. rocks without a lot of Ca might make a suite of Mg-rich amphiboles; higher pressure rocks might make a suite with more Na present, rocks rich in Mn may show simillar sequences but with Mn as the dominant M2+ cation, and so on).  One of my favorite mineral groups... too bad the IMA keeps making them more difficult than they have to be... grrr... lol.

13th Jan 2020 04:41 UTCBrian Fussell

Thx Noah and Frank. Very informative reply Frank, thank you, I cant determine if the pale mineral is plagioclase or not, maybe if I broke it to see a fresh surface I could. I am starting to really like the amphibole group myself as I learn more about them.

13th Jan 2020 05:15 UTCFrank K. Mazdab Manager

yes, on a fresh surface, you can look for cleavages in the pale mineral (feldspars have two like amphiboles, but the angle between them in feldspar is ~90°) with a hand lens.  Also with a hand lens, if it's plagioclase, it may or may not be possible to see faint, finely-spaced parallel striations on some of the faces if you look closely; these striations represent the surface traces of the "albite" twinning planes, but they can be tough to see.

13th Jan 2020 15:37 UTCBrian Fussell

Frank, do have a link to the discussion on amphibole identification you mentioned? I didn't see it.

13th Jan 2020 18:09 UTCFrank K. Mazdab Manager

someone had an asbestiform mineral of probable amphibole, and was interested in trying to narrow down what variety it could be:

13th Jan 2020 20:00 UTCHarold Moritz Expert

This is an amphibole (or hornblende) gneiss. An amphibolite is composed of just amphibole and albite - you have way too much quartz in it to be an amphibolite. Amphibolites are typically very dark, they started life as basalt, which has essentially no quartz.

14th Jan 2020 00:35 UTCFrank K. Mazdab Manager

Hi Harold,

agree that the protolith for this rock was almost certainly not a basalt, and similarly agree that names for metamorphic rocks that emphasize observational features like combined mineralogy and texture (like hornblende schist or hornblende gneiss) are much more useful than names that rely on presumed interpretational features.

But by that same token, your interpretational definition of amphibolite is much too narrow. Amphibolites need not have started their lives as basalts; even from among igneous rocks, a variety of bulk compositions, including more quartz-rich compositions like quartz diorite or quartz monzodiorite have appropriate chemistries to become metamorphic amphibolite under the right P-T conditions; at the other end of the spectrum, the suspected protolith for some of the Madagascar amphibolite-hosted rubies is gabbroic anorthosite and so quite feldspar-rich. And even in the case of basalt, while a fresh basalt wouldn't normally have much quartz, an altered basalt can have quite a bit as a consequence of silica-releasing hydration reactions. All of these rocks too can become amphibolites on metamorphism.  And additionally, some amphibolites started their lives as graywackes or volcaniclastic rocks, and it's even possible to make an amphibolite from entirely sedimentary rocks, for example in the thin fluid-assisted metasomatic reaction zone between a carbonate unit and a pelite. From just a hand sample, a rock from such an occurrence may not be easily distinguishable from one derived from an igneous protolith, without knowing the field relationships and having additional petrographic information.

Lastly, unrelated to petrogenesis, it's not unequivocal what the two dominant non-amphibole minerals actually are in Brian's photos.  Very likely one is plagioclase, but the other might not be quartz. While it does look there's quartz in this rock, photos alone are not always definitive.

14th Jan 2020 18:09 UTCBrian Fussell

I did break the rock in half, I could not find any evidence of plagioclase or any feldspar for that matter, (I am thinking most of the clear/grey mineral is quartz) while scanning the surface with a loupe. There does seem to be two quartz veins running through the rock, some iron staining on one side and plenty of the amphibole mineral.

14th Jan 2020 18:09 UTCBrian Fussell

pic 2

15th Jan 2020 21:48 UTCPaul Brandes Manager

Looks like a gneiss to me. Whether it is amphibole or possibly biotite would be easier determined if in hand.

16th Jan 2020 20:58 UTCHarold Moritz Expert

I know that there are plenty of other ways to make amphibolite, but regardless, it needs to be overwhelmingly composed of amphibole. Based on mapping of metamorphic rocks in Connecticut (around here, they are all interpreted as metavolcanic), above is a "hornblende gneiss", split along the layering so it looks like there is much more amphibole in it. As far as other grains, as I've mentioned in other threads, a thin-section is really the best way to tell, especially when fine-grained. Such work on the rocks depicted here (by others) says plagioclase and minor quartz. Notice how it is easier to see the amphibole grains on the weathered surfaces than a freshly broken one, due to differential weathering of the various grains. Folks bring weathered ones like Brian's a lot to mineral club meetings for ID, but not broken ones!
Below is an amphibolite:

Here's a finer-grained version of amphibolite (with scheelite):

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