The platy mineral appears glassy, but can be scratched by steel blade. The matrix of the dike is probably chlorite rich. A close-up of the platy minerals in last picture.
I'm guessing staurolite. possible kyanite but not as good a speculation.
Photography not good enough. Need to see terminations better. Tourmaline maybe possible but it has basal cleavage...i see none. kyanite has two hardnesses and is a high pressure pure aluminosilicate check. Very diagnostic.
Staurolite is more garbage...it is often twinned. Possible penetration twinning visible at apical vertexes and a 60 deg. twin angle seems visible.
This stuff is probably retrograded to some vague iron bearing aluminosilicate. it does not look right for any actinolite or other hbl minerals but sitting in chlorite...maybe.
The high aluminum content of the granitic pegmatite supports its being an aluminosilicate but surrounded by chlorite-like stuff with fe-mg ...count on its not being nice.
The setting is pretty good for staurolite. The chlorite carries iron too so we have reason to speculate a remnant paragenetic association.
I say staurolite.
Thank you Steve Johnson for your thorough investigation.
I was puzzled by the bladed appearance of the crystals (kyanite can be bladed according to Wikipedia).
I found one instance with two crystals at right angles like a cross (stauros in Greek), see photo.
I guessed the chlorite in the matrix, it maybe something else, but it is soft and greenish.
It is a bit of a coincidence that I found this glacial boulder outside a private chapel with a big cross outside it. The lady owner that lent me the rock would be delighted that these crystals are probably of staurolite (cross-rock)!
It’s in dike rock float. Who knows what. Chlorite looks like an altered lithic fragment...probably country rock. There are signs of 60 and 90 deg. Penetrative staurolite twinning.. plagioclase would look blocky and bladed. Staurolite only bladed. You can almost see two prismatic faces parallel to long axis.
Given it is a mafic to ultramafic dyke and the mineral can be scratched using a steel nail the common silicates such as lath-shaped crystals of labradorite or amphibole (in lamprophyres) can be ruled out. Even staurolite can be excluded due to its Mohs´hardness number of 7 and its Al contents. In view of the data presented, only pyrrhotite with a hardness number of 3.5 to 4.0 and hexagonal platy XX would comply with the dataset mentioned. Its hardness is reduced as it is subjected to weathering because it is the least resistant ore mineral / sulfide known within the supergene alteration zone. It seems to be a typical weathering texture of a dolerite or so showing rounded bodies undergoing exfolation.
H.G.Dill
Dear all: ir looks like the mafic dike rock is cut by a HT vein of some sort. I’ve seen altered basalt crossed by veins of different widths containing platy-lenticular minerals as shown in the pictures. I rule out any silicate such as kyanite, staurolite, and even amphiboles of tremolite-actinolite series. So, my best guess, considering the lack of información, is siderite or ankerite, which hardness is around 4. Regards,, Sebastián Grande.
Feldspar in Dolerite porphyry makes sense. Is there twinning? Can’t rely on hardness 6-7 too vague. The laths weathered out. Didn’t see “primary” fe-mg minerals and figured they constituted the chloritic meta-mass material. No signs of polysynthetic twinning. A “dolerite” could have lots of platy plagioclase ca-silicate however.
Thank you Steve Johnson Sebastián Grande Harald G. Dill Beda A. Hofmann
for your valuable contributions.The bladed crystals were not magnetic. I managed to break out what it looks to be a double blade (new picture shows cleavage along a line) and maybe vague twinning on cleavage surfaces which are very thin. It can be scratched hard with a steel screwdriver, so it is 6.
I am grateful for everyone's input. I learned a lot. By posting a question one hopes someone might have seen such a peculiar rock before, but one needs to get as many features as possible to make identification easier.
Dear Michael: Definitely you should analyze this mineral with DRX, its crystals are large and scattered, so it will be real easy to obtain a powder sample and so to establish certainly what species it is. Also, a petrographic this section is a lot cheaper to make and will help to identify most species easily. Regards, Sebastián.
Sorry Michael: two mistakes, it's XRD in English, and also thin section...
Dear Dr. Issigonis,
it is correct that pyrrhotite is ferrimagnetic but with a lowering of its intensity on alteration which may lead to sulfides like bravoite, marcasite or even “limonite” .
H.G.Dill
Thank you Sebastián Grande for your valuable input.
I understand that XRD will do the trick as well as a thin section, because it is relatively easy to dig one of the crystals out of the rock. Unfortunately, right now I have no access to either, but my client is thinking of buying a portable XRD, if he can find a suitable price.
Still it is very comforting to discuss visual (simple pictures or magnifications) observations with colleagues in RG, such as yourself. Sometimes, others point out things that one observer by himself often misses.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Metals
- Alloys
- Steel