It isn't a lamprophyre or alnoite, but an olivine mellilitite from southern Germany. But it's melilite, as far as I can tell. The paper is about some odd rocks from Nyiragongo, and also contains a melilite picture, albeit in B&W. I am sure there are better pictures around, but it's a start...

Dear Jayabalan,

These are examples of Melilitite (see PDF below)

a) from the 'Atlas of igneous rocks and their textures' by Mackenzie, Donaldson and Guilford (1982) Longman Group Limited, ISBN 0-582-30082-7 and

b) from the 'Atlas de Asociaciones Minerales en lamina Delgada' by Joan Carles Melgarejo (1997), Universitat Edicions de Barcelona, Fundació Folch, ISBN 84-89829-24-1

Maybe it helps a bit further.

Best regards

Guenter

Dear Dr. Manickam,

I can only refer to the various textbooks and full-picture books of thin sections also mentioned above. But my question arises simply from the approach taken. Have you already some chemical analysis or even a whole-rock XRD, which is a rough but sometimes helpful method if the mineral makes up a great deal of the rock in question? I do not deal with melilite-bearing lamprophyres but the common group of kersantite, minette.... A question which also should be raised is, what one can we expect in the enviroment.

Best regards

Harald G.Dill

Thank you for your replies and suggestions Prof. Marlina, Prof. Grundmann and Prof. H G Dill. We have sent the samples for chemical analysis. The dike is associated with coarse dolerite dikes very near a regional shear. The investigated dike has been intruded in the host biotite gneiss.

Dear Jaybalann

I also work with different rocks, but both Atlas mentioned by Guenter bring good photos you can compare with. I am sure the chemical analyses also suggested will help you in your decision.

Best regards,

Zenaide Silva

Thank you Prof Silva.

My first port of call for a question like this would be the BGS (British Geological Survey) collection of geological photographs, including thin sections in plane- and X-polars.

Unfortunately, it's down when I write this (Sunday evening), but with any luck it'll be back up when you try.

http://www.bgs.ac.uk/data/britrocks.html

Thank you for the information and link Dr Aidan.

                            M.Jayabalan.

It's Mr Aidan, not Dr. I work for my geological living, not lounge around in Ivory towers!

Mmmm, interesting idea some people have about working at a university....

Dear Mr Aidan,

           Thanks for your reply. Happy to know that you work in Industry and have an active interest in geology. 

                                    M.Jayabalan.

What is the RI  (relief ) of the mineral in question?  Lozenge shape and straight extinction? Can you provide a photomicrograph of the same both in PPL and Xed nicols-though it is not advisable to identify a mineral from a photomicrograph - just academic curiosity.  KNRAO

Dear N R Kesavaraju,

                 I will make arrangements for the upload. Thank you for your interest.

                                            Regards,

                                            M.Jayabalan.

Melilite occurs typically in association with olivine,nepheline and other feldspathoidal minerals in undersaturated alkaline rocks.so the paragenesis in the rock can guide you.secondly the birefringence of melilite is typically berlin bluish.Does this find of yours from an alkaline suite of rocks? Ref., also a paper by S.F.Foley et., al., on a melilite-bearing lamprophyre of E.Antarctica in Min.Petrol.,(74), 2002. Good luck.

Dear D Vasudevan,

 Thank you for your interest. We are yet to get the results of the chemical analyses. The rock is in the form of a thin dyke associated spatially with dolerite dykes. 

                                                   M.Jayabalan.

These photographs were taken using iPhone 4.

The first photomicrograph of the rock indeed shows meilite  laths; can you attempt a quantitative estimation of minerals distributed in these thin sections and work out a mineral paragenesis that will be useful to tell more about this interesting dyke.

Dear D Vasudevan,

             We are at present preparing more thin sections for point counting. Samples have been sent for whole rock geochemistry. Thank you for your interest and response. 

                                M.Jayabalan

please feel free to contact me for any help in petrochemistry,petrology and archaean geology.

=Dr.D.Vasudevan, Dy.D.G, G.S.I (Retd.,)

Dear Dr Vasudevan,

                           Thank you.

                                 M.Jayabalan.

The lath in the first two photographs -based on low relief (melilite will have fairly high relief than feldspars) appears to be that of feldspar (probably K-feldspar-in the absence of lamellar twinning)) and there appear to be quite a few such grains in the groundmass The 2nd photomicrograph taken under plane polarised light clearly shows low relief of the mineral. What are other minerals noticed in the rock besides suspected melilite?  I feel  naming  the rock from chemical analyses should be attempted if we fail to identify the minerals in thin section and if the rock is extremely fine grained making it difficult to identify the minerals. Mineralogy, texture and grain size (and of course,  mode of occurrence and knowledge of field association) should help us to name the rock and in my opinion,  whole rock chemistry should be the last resort for naming the rock. K.N. RAO.

We also suspected the laths to be K feldspar,but it displays straight extinction. Some smaller laths and lozenge shaped grains display distinct reaction rims which stands out in XN condition. We are awaiting the results of the chemical analyses. Thank you for your response.

                                   M.Jayabalan.

Hi all,

did you have a final response from the chem about this issue?

Best regards,

Francesco

Our analyses are pending as the lab has been closed due to covid.

I think that chemical analysis alone will not solve the problem with certainty. I also think that it will not be possible without powder diffraction analysis. It was already proposed here by prof. Dill. Performing XRD analyzes from rock samples is no shame. In conjunction with Rietveld evaluation, it is a relatively objective way to verify what is seen under a microscope. The melilities, that I know from slag and basaltoid rocks look a little different.

Dear Prof Dalibor Matysek,

Thank you for your feedback.

Thank you all for your reply. However, I was wondering wether melilite might resemble k feld even for what concerns interference colours. Is it always blueish in appearance or might have pinkish to white colours? I m dealin with ultra-K rocks from the roman province (italy) where it is expected to be, but day by day I m realizin that what I m lookin at is simply sanidine..

(Probably it s sanidine, don't really know)

Dear Francesco Palozza,

Thank you for your feedback.

DEAR JAYABALAN

GOOD AFTERNOON. ALNOITES FROM THE TYPE LOCALITY OF ALNO ISLAND IN SWEDEN ARE DEFINED AS FOLLOWS (TAPPE ET AL. 2005): "Alnoites are melilite-bearing UML, characterized by olivine, phlogopite and clinopyroxene macrocrysts and/ or phenocrysts, and groundmass melilite, clinopyroxene, phlogopite, spinel, ilmenite, perovskite, Ti-rich garnet, apatite and minor primary carbonate. Monticellite may occur in rare instances.". AS ONE CAN SEE ALNOITES DO NOT CONTAIN MELILITE PHENOCRYSTS BUT MELILITE IS CONFINED IN THE GROUNDMASS. IF THERE ARE MELILITE PHENOCRYSTS THEN YOU HAVE A MELILITITE, NOT AN ULTRAMAFIC LAMPROPHYRE. I SEND YOU ATTACHED A CHARACTERISTIC PHOTO OF ALNOITE FROM THE TYPE LOCALITY (PHOTOMICROGRAPH BY ALESSANDRO DA MOMMIO).

BEST REGARDS

IOANNIS

IOANNIS

Dear Ioannis,

Thank you for your interest and upload.

M.Jayabalan.

Can it be Rhomb Porphyry? The logenze or wedge shaped feldspars are typical of Rhomb Porphyry. Those can be phenocrysts of Anorthoclase. Sanidine is also a commin constituent of Rhomb Porphyry. These are found in Continental Rifts.