Just wondering what this cool stone might be. It was found laying around in a field.
Still busy collecting, I see
You have found a lump of feldspar porphyry.
It is a type of igneous rock. It is composed of light-coloured crystals of feldspar in a dark matrix of much finer-grained igneous rock. In this setting, the large crystals are called phenocrysts. There are other types of feldspar, distinguished by the mineral comprising the phenocrysts. I can tell the phenocrysts in your specimen are feldspar from their shapes.
Porphyry forms when a magma that has begun to crystallize (the large crystals) quickly gets cooled (for instance through being ejected from a volcano or dyke) - the chilling causing the remaining liquid rock to crystallize quickly, which results in small crystals.
Porphyry can be described in terms of its phenocrysts and/or its matrix, eg.:
- andesite porphyry (the matrix is andesite)
- quartz porphyry (the phenocrysts are quartz)
- olivine dolerite porpyhry (olivine phenocrysts in dolerite matrix).
Such names are a matter of available information and choice.
- introduction - http://www.google.co.uk/url?sa=t&rct=j&q=porphyry&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FPorphyry_(geology)&ei=3jE_UbT6LMjxOrjxgdAL&usg=AFQjCNH_sSbhT1TJgWIOKwdxJ3_04Uk5XQ&bvm=bv.43287494,d.ZWU
- phenocryst gallery - http://geology.about.com/od/more_igrocks/ig/phenocrysts/
Thanks for the speedy reply.
Yes :). I found loads of interesting including some more of that mystery metamorphic mineral.
This specimen was smaller with smaller black bits but of the same shape and pattern. I also found another part of that pipe. And now my little 5 year old sister is finding many fossils on the gravel.
:D so thanks a lot for your help.
On the current matter:
How come there are circular shapes on only one side of this stone? Are they crystals in cross-section?
...Well, they are not actually circular; we'd call them equant (as opposed to elongate).
In 3-dimensions, the phenocrysts are oriented somewhat in one direction, and the side of the specimen that shows them as equant cross-sections happens to be across that direction (as you thought). Such non-random orientation can be referred to as a fabric. Fabrics can have various causes. In this case it might reflect flow in the melt just before it solidified.
Great that you have a sub-contractor fossil hunting with you