July 14, 2011
Geology Word of the Week: G is for Glomeroporphyritic
Posted by Evelyn Mervine
A textural term used to describe igneous rocks that contain clusters of phenocrysts, which are large crystals in a finer-grained matrix or groundmass.
Glomeroporphyritic is one of my favorite geology words. I love this word because it’s such a funny, complicated-looking word, and it’s so much fun to say. Sometimes, I manage to say “glomeroporphyritic” perfectly. Other times, I stumble over the tongue-twister of the word, saying something like “glomeroporphaglomeroporphyryyitic” before taking a breath and slowly, deliberately saying, “glomeroporphyritic.”
The word “glomeroporphyritic” is a modification of the word “porphyritic,” which is a textural term used to describe igneous rocks that have large crystals in a finer-grained groundmass. How do porphyritic igneous rocks form?
Well, to quote an earlier blog post of mine,
How to porphyritic igneous rocks form? They generally form when magma that has been slowly cooling for a long time, possibly in a magma chamber, is suddenly erupted to Earth’s surface. Deeper in the Earth where magmas can cool more slowly, large crystals have time to form and grow. Magma takes awhile to crystallize completely, so sometimes partially-crystallized magmas are brought to Earth’s surface and erupted as lavas. When these partially-crystallized lavas are erupted, the rest of the molten rock cools quickly, and there is no time for large crystals to form.
Here are a couple of pictures of regular porphyritic rocks:
Unfortunately, “glomeroporphyritic” is not in the Oxford English Dictionary, so I’m not sure when the word was first adopted into the English language (anyone know?). The word “porphyritic” was first used in English in the late 1700s and comes from the Latin word “porphyrites,” which comes from the Greek word for purple and was used by ancient Romans to describe a purple-colored (and apparently porphyritic!) stone found in Egypt and used as a building stone.
The prefix “glomero-” is also of Latin origin, coming from the Latin “glomerat-“, which is the participial stem of the verb “glomerare,” which means “to form into a ball or mass” or “to collect.” So, a glomeroporphyritic rock is a rock which contains balls or collections of phenocrysts. As an aside, I have just discovered that “glomerate” is a real word in English, and I think it’s my new favorite verb. I’m actually headed off to a meeting tomorrow, and I think I’m going to say to people, “Hey, shall we glomerate over there?” just for fun. It’s a nerdy meeting, so I think people will like the word “glomerate.”
There are two other geology words which are related to the word “glomeroporphyritic.” The first word is “glomeroporphyroclast,” which describes a cluster or collection of phenocrysts. Although the word contains the singular “clast,” the term really describes a collection of crystals. Actually, the “-clast” suffix is a misnomer. Technically, a “clast” is a rock fragment found in a sedimentary rocks. Glomeroporphyroclasts don’t really contain clasts at all– they contain phenocrysts. So, I think the term should really be “glomeroporphyrocryst,” but as far as I can tell no one uses that. The second word is “glomeroporphyroblast,” which describes a collection of “blasts,” which are large crystals found in a finer-grained matrix in a metamorphic rock. For more on crysts, clasts, and blasts, see this “About Geology” article.
You might be wondering– do geologists actually use these fun but complicated words? Absolutely. A quick search of the GeoRef database returns 52 papers when I search with the term “glomeroporphyritic.” If you google the term “glomeroporphyritic,” you will find numerous scientific papers and reports which include the word as a textural description for rocks. I suppose geologists could just say something like “clumps of crystals” or “globs of phenocrysts,” but “glomeroporphyritic” is such a fun word! I’ve even used the word in my own research to describe globs of plagioclase phenocrysts in basalts.
For example, below are some pictures of a geologic thin section which I described as having glomeroporphyroclasts of plagioclase phenocrysts:
The thin section pictured above came from a porphyritic basalt collected along the Ninetyeast Ridge in the Indian Ocean. In hand sample, the basalt looked something like this (not the same rock, but similar):
Finally, back in May I identified (with the help of some fellow geologists!) a “Mystery Rock” with gorgeous plagioclase phenocrysts. This mystery rock has slight glomeroporphyritic texture– some of the phenocrysts look as if they are clumping together. The owner of this mystery rock actually decided to name her rock “Glomer.” You can see some more photographs of Glomer the rock here.
***Note: Thanks to Ron Schott and Ian Stimpson for pictures for this week’s geology word of the week.***
“glomerate, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.
“porphyritic, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.
“porphyrite, n.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 13 July 2011.
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Interesting post here, with some fine photos. How would you say the genesis of these clusters of phenocrysts differs from that of zenoliths? Can a phenocryst also be a zenolith?
Or, alternately, could the phenocrysts form in situ due to differential cooling rates within the magma?
It is generally the refractory minerals which are xenocrysts, because they can endure high temperatures without being melted. One of these minerals is Garnet. During the ascent of xenocrysts alongside magma some textures signifying this process is imparted on them. Because of the pressure decrease during the magma ascent small blobs of material can become molten inside xenocrysts and when this molten material cools rapidly small blobs of isotopic glass are formed inside the mineral. These minute blobs imparts the so-called sieve texture to the mineral. In addition, when the xenocrysts are heated they usually start to melt beginning from the crystal outlines. This gives them a rounded shape instead of an angular and sharp outline.
I stumbled across your blog through your article on the natural nuclear reactors in the Francevilles uranium deposit. I liked that article as well as your blog.
I am doing my M.Sc. and my thesis is about uranium deposits. I will follow your blog from now on.
By the way, the photo you have taken of the plagioclase glomeroporphyroclast in plane polarized light shows mostly a dark color in the groundmass. If it was glass it would be light, not dark. Is it due to small crystals of opaque minerals scattered in the groundmass?
Hi Meisam– So glad you liked my article and that you’ll be following my blog!
I’m not sure about the groundmass- I’d have to take another look at the thin section again. There very well could be opaque minerals in the groundmass, though.
Evelyn, I found your blog some time ago and drop in for a revel in your writings from time to time (hence reading this article only now!). Thank you for taking the time to share your knowledge with us!
If you are truly curious about the origin of glomeroporphyritic, you could try contacting Michael Quinion on World Wide Words (http://www.worldwidewords.org/). See his Contact me page for how to submit such a query.
Hi Evelyn. Nice blog !
The luster of the matrix of your Mystery Rock seems to be glassy. If so, it is possible a nice sample of vitrophyre. We do have nice outcrops of this kind of rocks in the Paraná Igneous Province, just in the base of the rhyodacite of the Guarapuava – Chapeco type, close to the contact with the basalts.
Hello Evelyn. Very nice background on a term I use quite frequently in logging a coarse grained granodiorite/tonalite in northern Ontario. I was beginning to think I was the only geologist I knew of who used the term and, yes I agree, it is a fun term to use.