These two images are 1.15 mm wide. Both types of magnetic particles have been extracted from river sand- the rounded ones from construction sites and more irregular ones from dried-up riverbeds. The rounded-grained samples have been subject to mechanical smoothing action pre- and post-extraction, while more irregular-shaped magnetic particles have not undergone any severe mechanical erosion. Both sand samples are need not be chemically the same. The rounded magnetic particle samples are likely to be magnetite as they are strongly magnetic. But the more irregular-shaped grains are weakly but certainly magnetic. What they can be - chromite, ilmenite, zircon, garnet, amphibole, pyroxene, or any other mineral?
Even if the exact mineral name cannot be said, can the mineral family be identified by observing its fracture and cleavage?
Yes, it is possible to identify the individual components of the heavy mineral concentrate.
The mineral grains must be embedded in epoxy resin, and a polished section of them must be manufactured. With a vertically reflected light microscope, the internal structure of the individual components can then be precisely identified and determined (see the example attached).
The distribution of the mineral species can only be seen in a polished section through each individual grain. In the example shown here, you can see magnetite grains (crème-brown) that are homogeneous (in the center and top middle), width of image 1 mm. In the surrounding grains, oriented intergrowths network of magnetite (crème-brown) with hematite (bright) can be clearly seen. In those magnetite grains in which the proportion of hematite lamellae is very large, the intensity of the magnetism is correspondingly lower. The higher the hematite content, the lower the magnetism.
This examination method is indispensable for the clarification of your questions. For further information please find enclosed the link to our publication "Preparation methods ...
You have two options to use, field or laboratory methods.
In the field there are only handmagnets (Type Wilke) for different field strength values with adjustible working distances (for separation of Fe oxides, Fe-(Mg) silicates) , etching with HCl and and alkaline solutions such as (KOH) mainly for alteration zones and a couple of pycnometer and balance to determine the specific gravity of minerals which covers a rather wide range. These field method which needs some experience are good for grouping of mineral groups (ferromagnetic-diamagnetic..), solubilities and density. I have a so-called "Emergency kit for applied geosciences" which contains all these items and more, e.g., (hand) lenses of larger magnification Moh´s hardness set, UV lamp, diamond tester (useful also for topaz, corundum varieties etc,). I have built up the kit over a long period of time along with increasing practical experience gathered in the field mainly for heavy mineral exploration and soft rocks.
It may be enough for exploration but it is insufficient for a precise identification which can only been done in the lab using polished sections - see Dr. Grundmann´s suggestions-, XRD, SEM-EDX/WDX, and EMPA.
HGD
Don't forget that cassiterite can also be magnetic and is often overlooked as a possibility especiall where there are highly fractionated granites in the headwaters.
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