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Identity HelpAndesite? Mineral identification newbie

9th Sep 2017 15:54 BSTPitvars

07914090015651854858262.jpg
Hello all, I'm new to Mindat and am trying to refresh my geological knowledge from a school course many decades ago as I've inherited a collection of minerals and am working my way through identifying them. I've identified almost all but came across the attached and was intrigued enough to send it off to be cut and polished. I think my granda collected it from Tenerife as it was in a box of volcanic material/lava and I know he visited the canaries.


From having doing some reasearch I think it might be Andesite but am interested in whether I've got it right and also what the minerals may be in the mix. The matrix is very fine dark brown material and as you can see there are voids. The voids have a white lining [maybe zeolite] and there are also very thin needle like black crystals [maybe hornblende] some larger roughly rectangular black crystals [maybe pieces of obsidian] then some red/brown cyrstals [maybe garnet or andesine]


00665380015654868757416.jpg

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Any pointers or suggestions gratefully received.

9th Sep 2017 17:19 BSTRob Woodside Manager

Welcome to Mindat Pitvars!


Your first photo looks like a basalt with vesicles probably containing a white zeolite. However, I'm troubled by the large number of barren vesicles in the cut section that suggest a slag??? I'm not sure one can tell from a photo, but perhaps others might recognize the material.

9th Sep 2017 18:04 BSTPitvars

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Thanks Rob, I've attached the exterior photo too in case it helps.

9th Sep 2017 18:40 BSTDoug Schonewald

Ptvars,


This looks like much of the vesicle 'basalt' from my area of central Washington comprised of the Columbia Flood Basalts. Most of these flows are tholeiitic basalt. Where your piece was found (Canary Islands) is known for its variety of rock types. Probably a thin section and chemical analysis will be required to determine what type it is. Someone who has collected extensively in the Canary Islands may have some insight on what this might be. To me it looks like the whitish vesicles are probably lined with a light colored clay and the darker ones lined with a high-iron hyalite opal. That is typical of what I find in my region and this rock looks so similar it could have come from the Columbia Basin.

9th Sep 2017 20:37 BSTBecky Coulson Expert

Hi Pitvars,

Though I'm certainly no expert, I've collected volcanics and zeolites extensively in the Canaries - and yours looks typical of the amygdaloidal basalt from several islands. (Rob, the filling of some vesicles but not others is common there.) The dark minerals could be augite, hornblende, kaersutite, etc. so that would need analysis - all are found on each island. I cannot see if there are any surviving zeolites - too small - but calcite and clay commonly replace them in older specimens. It also looks like there are some plates of nepheline or feldspar (3rd photo) - also typical of the region. It really looks nice cut and polished! Becky

9th Sep 2017 20:44 BSTDoug Daniels

I too would vote for a vesicular basalt, porphyritic at that. Can't say I've ever seen a vesicular andesite; I suppose it is possible, just haven't seen any.

9th Sep 2017 22:01 BSTPitvars

Thanks very much all, I was pleased with how it came back from being cut and polished - it revealed minerals and structures that couldn't be seen from the exterior. It was the red/brown minerals that led me to having it cut as I couldn't tell from the exterior whether they were oxidized/degraded.


Becky Coulson Wrote:

-------------------------------------------------------

Hi Pitvars,

Though I'm certainly no expert, I've collected volcanics and zeolites extensively in the Canaries - and yours looks typical of the amygdaloidal basalt from several islands. (Rob, the filling of some vesicles but not others is common there.)

The dark minerals could be augite, hornblende, kaersutite, etc. so that would need analysis - all are found on each island. I cannot see if there are any surviving zeolites - too small - but calcite and clay commonly replace them in older

specimens. It also looks like there are some plates of nepheline or feldspar (3rd photo) - also typical of the region. It really looks nice cut and polished! Becky


There were some areas that made me wonder about calcite Becky. I have both halves cut and polished so if you'd like one half to have a closer look at and to say thank you for the help in identification just yell. Peter

9th Sep 2017 22:07 BSTPaul Brandes Manager

Welcome to Mindat, Pitvars!


I would agree that it is a vesicular basalt with zeolites filling the void spaces. As Becky mentioned, it is not uncommon to find vesicular basalt with empty vesicles; we find them all the time in the Keweenaw basalts. To truly know what all minerals you have, an analysis will be needed.


Though rare, I have seen vesicular andesite on the Sierra Grande shield volcano in the Raton-Clayton Volcanic Field of northeastern New Mexico. I would imagine it is possible to have such andesites anywhere there is a gassy eruption....

10th Sep 2017 07:26 BSTPitvars

Thank you to everyone for the welcome - I've always maintained a general interest in geology whilst out hiking and walking but cataloguing the collection has certainly renewed my interest - luckily many of the specimens appear straight forward and were collected in the UK so I won't have to trouble this section of mindat too often... cheers, Peter

10th Sep 2017 10:37 BSTPeter Nancarrow Expert

Hi Peter,


The Canaries are "hot spot" oceanic volcanoes (similar in character to Hawaii), not related to any subduction at a convergent plate margin such as along the W coast of S. America, Japan, Indonesia, The Aegean arc, etc. It is in the latter sort of environments that intermediate lavas such as andesites and dacites usually occur, formed by the mixing of subducted low-Si oceanic crustal material with rocks from the overlying more silicic continental crust. Most of the Canary island lavas are therfore generally basaltic in character.


Your specimen with mostly empty vesicles (i.e. gas bubbles) suggests that it is a fairly recently erupted basalt. Infilling of the vesicles by calcite, zeolites, chlorite etc, as commonly seen in lavas such as those from the Tertiary volanic rocks of The Hebrides and the Faroe islands (and also in some of the older parts of the Canary Islands) hasn't had time to develop significantly beyond the thin fine-grained coatings visible in some parts of your specimen.


The black crystals are probably pyroxene phenocrysts (obsidian fragments are very unlikely to be found in a basalt) and similarly, the reddish crystals could be iron-stained feldspar, or maybe altered olivine (although that is not much more than a guess, based just on a photo), but it is very unlikely that they are garnet.


Pete N.

10th Sep 2017 16:12 BSTRob Woodside Manager

Thank you all This has been a learning experience!

11th Sep 2017 01:23 BSTTony Peterson Expert

Let's finish by making one refinement - the mineralogy given by Becky is that of a nephelinite, not a basalt. Nephelinites are richer in Na, K and others and depleted in Si relative to basalts. Poke Becky again and see if she'll talk about the phonolites, carbonatites etc. that are also prominent on the Canaries :)


Tony

11th Sep 2017 08:58 BSTBecky Coulson Expert

Ouch, Tony - someone poked me! Yes, nephelinites are present on the Canaries, as are carbonatites (but very restricted in distribution), andesites, phonolites (Mt. Teide), trachytes (Mt. Tindaya), basanites, etc. - a very complex geology, with some lavas typical of low melting points of magma. Certainly, most recent lavas tend to be basaltic (e.g. Timanfaya). Although a mantle plume is believed to be involved in the formation of the islands, their ages and composition do not show the clear succession seen in Hawaii - other factors (like plate margins?) are probably involved. This sample does look like weathered basalt to me (note exterior in photo 4), and the more common phenocrysts could be augite and feldspars with bits of olivine - but without thin sections, we are guessing, of course. No more poking unless you make thin sections for us!! I'll search again next spring...
 
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