Greigite acquires a strong GRM and RRM, which you can measure without heating the samples. Check out this paper:

https://www.researchgate.net/publication/229905346_A_large_gyromagnetic_effect_in_greigite?ev=prf_pub

Perhaps you could also read more here:

MAGNETIC PROPERTIES OF SEDIMENTARY GREIGITE (Fe3S4): AN UPDATE

Author(s): Roberts, Andrew P.; Chang, Liao; Rowan, Christopher J.; et al.

Source: REVIEWS OF GEOPHYSICS Volume: 49 Article Number: RG1002 DOI: 10.1029/2010RG000336 Published: JAN 29 2011

Article A large gyromagnetic effect in greigite

Another point would be to use the TH treatment using an Argon atmosphere during the heating. This is not always working but, if you are lucky ,you will see the different unblocking temperatures .

Mössbauer-spectroscopy would also be an option. It is a non-destructive technique in the way that it does not require any heating - and in backscattering mode can be used on either a powder-sample or a solid sample.

I think that studies of isothermal remanent magnetization could help you. From the acquisition IRM measurements you can try to separate magnetic phases (there are numerical and experimental methods that you can use), and therefore estimate the different magnetic contributions. As detailed in Peters and Dekkers (2003), mean remanent coercivities of greigites are higher than MD magnetites. Perhaps, you can obtain a successful separation!

You can estimate remanence carried by multi-domain magnetite in your samples by cooling them below the Verwey transition temperature. Measure the NRM, put the sample in liquid nitrogen for few minutes, let it warm up to room temperature in zero field (i.e., in a shielded room or in a mu-metal box), and measure the NRM again. Remanence lost is the NRM residing in multi-domain magnetite.

Why do you think - you have MD grains& May be these are SD particled of bacterial magnetic? May be you could try use coercivity spectra (IRM spectra). You can send me one sample for probe. But very important - samples should be freeze dryed (non oxidated). For example you could look papers:

8. D. K. Nourgaliev, P. G. Yasonov, and L. R. Kosareva, A. Yu. Kazanskii, A. P. Fedotov The origin of magnetic minerals in the Lake Khubsugul sediments (Mongolia)/ Russian Journal of Earth Sciences, 2005, v.7, 3, 1-6.

13. Enkin, Randolph J.; Baker, Judith; Nourgaliev, Danis; Iassonov, Pavel; Ham-ilton, Tark S. Magnetic hysteresis parameters and Day plot analysis to charac-terize diagenetic alteration in gas hydrate-bearing sediments J. Geophys. Res., Vol. 112, No. B6, B06S90 10.1029/2006JB004638

Methodological very important paper - Physics and Chemistry of the Earth

Volume 29, Issue 13-14 SPEC. ISS., 2004, Pages 851-867

Characterization of individual rock magnetic components by analysis of remanence curves. 2. Fundamental properties of coercivity distributions

Egli, R.

Physics and Chemistry of the Earth

Volume 29, Issue 13-14 SPEC. ISS., 2004, Pages 869-884

Characterization of individual rock magnetic components by analysis of remanence curves. 3. Bacterial magnetite and natural processes in lakes

Egli, R.

It seems that you are dealing with sedimentary samples and as other colleagues mentioned, in these samples greigite is made by bacterial activity in lake and in this way, magnetite also can be produced in the same way, but in this case this magnetite will be SD not MD. Now, if you are sure about the type of your magnetite, then the source should be verified. Regarding recognition between magnetite and greigite as a main magnetic carrier of your sample, to me, the best way is conducting IRM and measuring Hc. MD magnetite will have have lower Hc.

I agree with Dr Madsen that Moessbauer spectroscopy will be of immense help in identifying the contributions of individual mineral to the magnetism and their coordination numbers.

The SIRM/X ratio will discriminate between MD magnetite and diagenetic greigite, even if the latter is MD or SD. It requires that the magnetic susceptibility is dominated by the ferrimagnetic components.