Do you know what causes these speckles in a mafic lava rock? These rock samples are from La Palma (Canary Islands) and appear to be basanite or olivine-phyric basalt. This is no quiz question. I don’t know the answer but hopefully someone with more experience has seen such rocks before.
Width of sample approximately 30 cm.
Width of sample 8 cm.
My first thought was leucite, but they don’t really appear to be discrete crystals.
Leucite is interesting offer. However, they really do not resemble crystals. They are like some embryonic crystals or crystallization centers or something like that. Really confusing for me. I did take a look with a microscope but it did not reveal much. Just a gray patch which slightly seemed like an airy light-weight material but the internal texture was really too fine to say anything for sure.
I’d hypothesize that this is some sort of glomerocrystic texture, possibly with plagioclase crystals forming the glomerocrysts. I’ve seen glomeroporphyritic textures in basalt before, and I’ve seen a similar “spotted” texture in amphibolites, but this is a first for me, too. Ideally I’d like to see a thin section to test my hypothesis.
Glomerocryst is a new word for me but it seems to me now that it may be the answer indeed. Thanks, Ron!
Definitely think a thin section is in order. If these are indeed glomerocrysts, it’ll be obvious under a petrog scope.
I will take thin section into my plans.
I”ll bet the hypothesis of `glomerocrystic’ texture is right; that sounds pretty plausible. They also remind me of devitrification features, such as the `snowflakes’ in snowflake obsidian. I think your blog is very readable and informative, BTW – I really enjoy reading it!
Thanks, Marek! I will do my best to keep it readable and informative.
This rock is in my opinion a basanite with a clear erosion appearance called “Sonnenbrenner”. Many alkali-olivinbasalts show this phenomenon. The main cause is the sparsely occuring analcime in this rock which is attracting water. After several years this dark rock with little spots of olivine ( forsterite) – you can see this bleaching green mineral in the photograph below – is falling down into little fragments in order to become a believed topsoil.
Yes, it is indeed quite fragmental. I discovered it when I tried to prepare it for a thin section. I cut a slice of it with a rock saw and to my disappointment discovered that it is too friable for a thin section. However, I have not given up yet. There are some solutions for such rocks like epoxy to keep it together.