Harrisite
Baracca quarry, Mattarana, Carrodano, La Spezia Province, Liguria, Italy

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minID: HHC-RXJ

Harrisite

Locality: Baracca quarry, Mattarana, Carrodano, La Spezia Province, Liguria, Italy

Field of View: 10 cm

Harrisitic structure of black olivine and white plagioclase in troctolite at the Baracca quarry.
Photo in situ

This Photo was Mindat.org Photo of the Day - 19th Jun 2021

This photo has been shown 890 times

Photo added:	18th Apr 2019
Dimensions:	4032x2774px (11.18 megapixels)

Data Identifiers

Mindat Photo ID:	950404 📋 (quote this with any query about this photo)
Long-form Identifier:	mindat:1:4:950404:9 📋
GUID:	8c15f494-2cba-4f9c-a17b-14e72594b031 📋
Specimen MinID	HHC-RXJ (note: this is not unique to this photo, it is unique to the specimen)

View Andrea Oppicelli's Photos View Harrisite Gallery

Discuss this Photo

PhotosPOTD - harrisite - mineral or rock ?

19th Jun 2021 03:45 UTCHerwig Pelckmans

see POTD https://www.mindat.org/photo-950404.html

Certainly of interest for rock lovers, maybe not as much for mineral collectors.

However, that is not the point I want to make.

What I do feel that is missing on the Mindat page about harrisite, is that harrisite is not a mineral (name), but some specific kind of rock.

I know that there are many indications that this is the case, but for someone who is not very familiar with Mindat, I think this basic info should be clearly stated at the beginning of the harrisite page.

Just my two eurocents, of course.

Cheers, Herwig

19th Jun 2021 05:04 UTCKeith Compton 🌟 Manager

Herwig

When you look at the Harrisite page the second set of data regarding "Classification of Harrisite" details that Harrisite is a rock.

I certainly think it is clear enough as it is.

Cheers

19th Jun 2021 05:43 UTCFrank K. Mazdab 🌟 Manager

It's not a mineral name or a rock name... the rock name is troctolite; harrisite is just one of our thousands of varietal names. What's written about it on its dedicated (why?) page is already more than it needs... it really ought to just be a sentence or two on the troctolite page... ± an accompanying photo.

19th Jun 2021 09:40 UTCErik Vercammen Expert

I should say it is the nme for a certain texture of the (troctolite) rock, just like graphic granite.

19th Jun 2021 22:53 UTCRalph S Bottrill 🌟 Manager

I quite agree Erik, though we list harrisite and graphic granite as rock types they really are just relatively small scale textural features related to pegmatoidal segregations, not really rocks in their own right. Pegmatite itself is not a formal rock type but is applied to any coarse grained variant of an igneous rock, even basalts. We need the terms described on separate pages here with photos but we should probably use the formal rock names as well as the varietal or textural names in photo captions.

19th Jun 2021 10:21 UTCAmir C. Akhavan Expert

I think the photo description is fine.
But how come quartz-free troctolite is in field 10 on the QAPF diagram instead of 10' ?

19th Jun 2021 11:00 UTCFrank K. Mazdab 🌟 Manager

Because 10' field rocks are feldspathoid-normative rocks, and troctolite is typically quartz-normative even if there's no modal quartz present (for example, if there's any orthopyroxene present it has to be quartz-normative... and up to 10% opx [and/or cpx, up to a total of 10% combined pyroxenes] is allowed before troctolite grades into olivine norite [with opx>cpx] or olivine gabbro [with cpx>opx]). I suppose theoretically one could have a nepheline-bearing troctolite and so populate a portion of field 10' too, but I'm not aware such a rock has been described.

19th Jun 2021 12:41 UTCAmir C. Akhavan Expert

I see.
But if the IUGS classification hasn't changed, it is ultramafic anyway, problem solved, doesn't belong into QAPF diagram at all.
Or do I (and for example Philpotts and Ague) misinterpret Fig 2.6, p. 25 of Le Maitre (2002) ?

19th Jun 2021 12:53 UTCAmir C. Akhavan Expert

O.k., forget it, in fact I did misinterpret that figure (and Philpotts and ague didn't)

19th Jun 2021 18:58 UTCFrank K. Mazdab 🌟 Manager

yeah, although it's always made me scratch my head a bit, a rock can have up to 90% mafic minerals (that is, down to only 10% feldspar ± either quartz or feldspathoid), and still be considered appropriate for plotting on the QAPF diagram. As such, a troctolite (with say 70% olivine and 30% plag, perhaps not too much different than the photo) wouldn't be considered an "ultramafic" rock... the same as a gabbro with similar ratios of cpx and plag content also wouldn't be.

19th Jun 2021 22:20 UTCRalph S Bottrill 🌟 Manager

That’s always confused me also Frank, that mafic contents <90% are unimportant to the classification of most igneous rocks except as an adjectival modifier. Eg. if you have a coarse grained igneous rock with 80-90% biotite and minor quartz and orthoclase then its still classified as granite! And I have seen rocks described accordingly, though I would consider they are probably metasomatised wallrocks or enclaves.

19th Jun 2021 23:11 UTCAmir C. Akhavan Expert

It's a consequence of treating everything the same way. On Mineralienatlas someone asked how much mafic content is allowed in granite. Correct answer is 90%, of course, but some wouldn't believe it. The point is: nobody has to call a rock with, for example, 70% mafic minerals a granite, you may, if you wish to classify according to QAPF, but you don't have to, you can chose another classification scheme.

Talking about this: in IUGS classification mineral composition take precedence, so one cannot argue with modal content if the mineral content is known, so troctolite belongs to 10', and 10 is wrong.
But, as pointed out below, there should be a ternary diagram for gabbroid rocks, not a QAPF diagram.

20th Jun 2021 11:56 UTCRalph S Bottrill 🌟 Manager

There is a quaternary diagram for gabbroic rocks with plag, olivine, opx and cpx , but hornblende should also be there!

“...if you wish to classify according to QAPF, but you don't have to, you can chose another classification scheme.”

I don’t know of any other classification scheme for plutonic rocks of such compositions but am happy to hear about them?

20th Jun 2021 12:56 UTCFrank K. Mazdab 🌟 Manager

Ralph Bottrill 🌟 Manager ✉️

There is a ternary diagram for gabbroic rocks with plag, olivine, opx and , but hornblende should also be there!

There are actually two diagrams appropriate for gabbroic rocks:

plag-olivine-[total pyroxene]

plag-opx-cpx

Is primary hornblende sufficiently abundant in true gabbroic rocks to warrant anything more than an adjectival modifier? I presume at typical abundances, one would initially ignore the amphibole content (the same as one would ignore the probably also fairly abundant oxides), name the rock based only on the ternary mineralogy of one of the two diagrams above, and then just add "hornblende-bearing" or just "hornblende" as a modifier.

Hornblende-plag rocks would typically be relegated to diorite, although I guess technically it's the plag composition ("IMA" anorthite for gabbro/norite, and "IMA" albite for diorite; determined most easily petrographically) that defines the name.

Are there examples of plutonic rocks with significant igneous amphibole, little to no pyroxene, and either labradorite or bytownite as the average plag composition? If so, what would the name of that rock be?

It doesn't seem that it should be called either a gabbro (cpx + "IMA" anorthite) or a norite (opx + "IMA" anorthite), because those are so precisely defined by their pyroxene. It also doesn't seem like diorite is appropriate because diorite should have "IMA" albite, but the necessary mafic mineral (whether hornblende, opx, cpx) doesn't appear to be specified. It strikes me a bit odd that the kind of pyroxene is so important to whether a gabbroid is a gabbro proper or a norite, but once we move over to diorite by simply making the plag a bit more sodic, suddenly not only does the kind of pyroxene seem unimportant, but even whether it's a pyroxene at all apparently seems unimportant. I recognize that in gabbroic rocks the kind of pyroxene has important petrogenetic implications (because of the fo + SiO2 = opx reaction, which doesn't have a common analog in cpx), but one would think that distinction would carry over to diorites too. Maybe as "hornblende" probably is only sufficiently common in gabbroids to perhaps warrant just a modifier, maybe by the same token "cpx" and/or "opx" are similarly subordinate in diorites to only warrant modifiers there too? I don't know...

I know this discussion has taken a bit of an off-topic turn from the original topic of the description of "harrisite", but petrology is so rarely discussed here on mindat that I thought it'd be worthwhile nurturing this diversion.

21st Jun 2021 21:26 UTCRalph S Bottrill 🌟 Manager

I’m no expert on true gabbros, most I have seen are at least moderately amphibolitised. But there are hornblende gabbros defined and shown on ternary diagrams for Pl-Px-Hbd, eg. https://www.scirp.org/html/5-1210373_59253.htm.

The gabbro page here needs a bit of work: we have lots of subdivisions of gabbro, including hornblende gabbro, norite, troctolite, etc, that should probably come under gabbroids rather than gabbro? And the definition for clinopyroxene-gabbro is the same as gabbro!

21st Jun 2021 23:14 UTCFrank K. Mazdab 🌟 Manager

I checked it that paper, but there are all sorts of issues with it. The author's "amphibole analyses" (Table 4b) don't look like hornblende to me (they don't have any Ca). Since I would hope he did his petrography correctly (amphibole is pretty easy to ID in thin section), I assume the missing Ca is a typo? It's also not clear from the low quality image (Figure 1e) whether his "interstitial" hornblende is really magmatic or if it's just amphibolitized pyroxene (as an aside, B&W thin section photos are not very useful).

Indeed, there are numerous inconsistencies related to the analyses (some look like serious typos... for example, the normalized Al for spot #3, Table 4c, or what's going on with 3 rows of different normalized Al in Tables 4a and 4b). The feldspar data is also problematic... the CaO and Na₂O numbers suggest Ca-rich plag, but why are all the reported normalized Na greater than normalized Ca?

Given that hornblendites presumably exist, and diorites certainly exist, I suppose a magmatic rock with hornblende and calcic plag should too. But there are so many typos/mistakes/omissions in that particular paper that it's hard to separate any good observations from, well, the mess that's been published. I don't think this paper can be used as an example that true magmatic hornblende "gabbros" exist.

I do agree that gabbro-like rocks (I guess these would be rocks that don't plot too close to a corner on a [plag]+[two mafic minerals] ternary diagram) should probably be called gabbroids and not gabbros; "clinopyroxene gabbro"... LOL... but are we sure we don't also have an "olivine dunite" or a "nepheline nepheline syenite"? We do love our redundant names here...

22nd Jun 2021 21:49 UTCRalph S Bottrill 🌟 Manager

Agreed that paper has a lot of issues, Open source journals can be a worry with their common lack of editing and peer review. And yes it’s not possible to see the relations between hornblende and pyroxenes in the photos. Hopefully there are more convincing examples about, but I haven’t had time to follow this up. I have always worried about whether hornblendeites are true igneous rocks also, some rocks I have seen verging on that classification look more metamorphic or metasomatic.

Yes the rock classifications here were thrown together in a bit of a rush in the hope that some experts would come along and fix it all up. I periodically fix a few bits here and there, adding better descriptions and tweaking classifications but it’s a huge job and it’s easy to find yourself outside your area of expertise!

22nd Jun 2021 22:13 UTCFrank K. Mazdab 🌟 Manager

no worries, Ralph... the conversation here has been interesting and informative. While I like pretty crystals as much as the next collector, it's still always nice to engage in conversation with you and others who also share this additional interest in the typically less showy and often overlooked (on this site at least) petrologic materials that host those pretty crystals everyone is seeking.

23rd Jun 2021 22:32 UTCRalph S Bottrill 🌟 Manager

Yes Frank there is a world of wonder within even the ugliest of rocks, when you look at them close enough!

19th Jun 2021 19:09 UTCAmir C. Akhavan Expert

It is odd to show a QAPF diagram for troctolite, there's a truckload of rocks in field 10.
There are ternary diagrams like the one in the mentioned papers that would be a better choice. Which is probably true for a lot of other rocks. I guess that the QAPF diagram is shown is just a consequence of this urge to put everything into a single hierarchy tree ("hierarchitis").
One could produce a few more like these
https://www.mindat.org/photo-471844.html

19th Jun 2021 20:57 UTCFrank K. Mazdab 🌟 Manager

I don't mind that troctolite (or for that matter gabbros, norites, etc.) can be initially plotted on QAPF (in some cases literally exactly on the "P" corner), but I do agree that anything that nominally plots in field 10 on QAPF should then be shown expanded (or maybe a better term would be "detailed") with an additional plot specifically on the olivine-plag-pyroxene ternary diagram (or if olivine is absent or insignificant, then on the alternative and even deeper down the hierarchy plag-orthopyroxene-clinopyroxene ternary diagram).

I don't think the hierarchical approach is bad, and I think it helps students appreciate that these rocks, some with perhaps strange and unfamiliar names to students, don't all just exist independent of one another. I think the utility comes not from deciding which is the better diagram to show, but rather comes from opting to show how one individual composition plots in different degrees of detail on all the applicable diagrams.

19th Jun 2021 21:36 UTCAmir C. Akhavan Expert

The problem is the attempt to put everything in a single hierarchy ("something is either this or that").
This does not work.
Rocks are defined in many ways, arbitrarily, as people working on something see fit, and not according to a predefined parameter set.

19th Jun 2021 23:06 UTCRalph S Bottrill 🌟 Manager

Probably most scientists see it their duty to classify things so they can make sense to everyone, else it would all be total chaos. A simple classification like QAPF is a great start, then you can go to other classifications for more detail, eg there are several such diagrams covering gabbroic rocks, which we really should add to the respective pages here if we could work out how.

Of course classifications can vary and volcanic rocks can be a nightmare which you start using the various chemical variation diagrams along with modal mineral classifications. Eg I’m currently trying to classify a basalt which can be lumped with alkaline basalts, transitional basalts, trachybasalts, hawaiites, alkali olivine basalts etc depending which classification scheme you prefer!

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HarrisiteBaracca quarry, Mattarana, Carrodano, La Spezia Province, Liguria, Italy