Hi Titus, that is a good question. In the case of intrusions you might encounter felsic ones containing mafic enclaves. In this case, I would sample and document them individually in order to elucidate their petrological evolution. Another case would be intrusions containing different xenoliths. In that case, I would only collect samples of the host intrusion without any xenoliths. You can describe the xenoliths petrographically, but usually they are too small for assaying anyway. In the case of metamorphic rocks, such as migmatitic gneisses, I recommend to collect a large sample (>4 kg) for whole-rock assays in order to obtain representative data. Cheers Daniel

Geologists must have the experience and the courage to draw the line between two types of rocks in the field during mapping, to the best of their knowledge and conscience. The following petrographic study and chemical analyses (at home) then show very clearly that you were realistic (or not) about your decision and approach on the determination of rock types in the field.

A second subsequent controlled field work with mapping then will show where still needs to be corrected.

The most important requirements for a representative sampling and analysis are 1) the geological overview and/or detailed mapping with the determination of the rock types and the borders between two distinct rock types.

The best way of mapping is the detailed profile perpendicular to the foliation and/or layering (banding).

The geologist determines the so-called homogeneous areas. A homogeneous area shows most of the conformity and similarities in color, texture, structure and mineral content.

For the texture/structure, the following criteria are important: Grain size and grain shape of the mineral grains, as well as the distribution of the particle sizes, (relative grain size). The spatial arrangement, distribution of the rock components: I) orientation of the minerals (= orientation of the forming forces) Example: primary flow structure, foliation, folding, etc. II) Distribution of the minerals in space => (homogeneous or heterogeneous) Example => material change, grain size change, layering, banding, stratification) III) The degree of space filling (porous or compact)

Best regards,

Guenter Grundmann

Either for petrography or for geochemistry the rock should be devoid of mineral alterations, deformation, brecciation/ veination or visible encrustations/ vug fillings in the first step

you have to 2-5 samples from the same outcrop to estimate the overall mineralogy/ chemistry in the preliminary stage and more number of samples in detailed study stage.

megascopic observation is important to select fresh samples without any visible anomoly in rock structure/ texture/ mineralogy...which will give the actual components of the rock in terms of minerals/ elements.

The answers provided by D Muller, G. Grundmann and A Latha are relevant and useful. The purpose of your petrological or geological investigation will decide the type of sampling methodology to be followed or designed as per requirement. Assuming that you have carefully mapped, identified and documented the samples collected over each outcrop, all you need for preliminary work is to size a sample (break to hand-specimen size) for megascopic examination and later thin sectioning with or without chemical analysis. The petrographic correspondence of hand specimen and thin section examination is a rough confirmation that you have taken a representative sample. The best confirmation is the reproducibility of similar petrographic results in your samples collected over an outcrop or several outcrops. The chemical analyses of the samples also needs to correspond to the modal mineralogy of your samples. For ore samples we may need to compare the tenor to establish them as representative samples. Always keep backups of your samples. There may be some outliers due to sampling bias and errors.

Diffe from another study. According to the type of rocks (sediment - fire - mutant) and the purpose of the study (general - to draw a map - to track the process of mineralization - structural - ... each has its own way.

The collected sample should be freshest possible. The sample must represent one rock type i. e. Homogenous petrologically. Always it's recommended samecsample should be divided into two halves; one for petrography and the other for geochemistry. Remember the sample must be devoid of alteration, weathering, crustal contamination etc. In case of kimberlites and rated rocks thus may seldom be possible. However, in other igneous rock types fresh rock samples are, easy to collect from the outcrops. Always keep some spare sample for repeat analysis, also keep a photograph of the sample sent to geochemistry lab. Thorough understanding of local lithology is of prime importance while sampling.

Hi Titus., hope you got the answers to your question.

Dear Titus,

You got some good suggestions. There are descriptions of sampling techniques in the literature and you should benefit from reading them. Several colleagues here (e.g., D Muller, G Grundmann, J Manickam) have given useful suggestions, including the purpose of your study, and the importance of careful field observations and mapping for delineating rock-types. The most important care is that samples (not just one) should be taken from fresh parts of the rock-type. If the rock is foliated, you would need to make thin sections along and against foliation/shistosity. Avoid enclaves, amygdules, schlieren, and shears.

Keep the grain-size of the rock in mind during collection; a fist size sample may be OK in the case of medium- and fine-grained rocks, but you would need a much larger sample for coarse-grained rocks.

But these are generalities for homogeneous rocks; I have not touched the issue of heterogeneity because you are not explaining it sufficiently. What sort of heterogeneity do you have? Is it fine scale, i.e., mm-scale, or cm-scale, m-scale or even coarser and mappable (the last depends on the scale of the map). The coarse banding would be easy to collect, the cm-scale may be purified by using cutting saw/grinding, but the mm sacle would be a problem, not in petrography, but for geochemistry.

As per above answers given by our experts, I just would like to add few sentences,

1. Before going to field, you have to clear about the geological map and what are the analysis you will want to do.

2. Samples should be fresh, unweathered and unaltered.

3. For Petrography study and Mineral Chemistry analysis chips of rocks are sufficient for examine the mineralogy.

4. If you want to go for Isotopic analyses (Geochronology) then you have to take huge amount of samples like 1 kg in weight. Because if you go for Zircon dating then procedure is very long and they crushed rocks to get good amount of zircon crystals.

5. For XRF and ICPMS analyses 2 or 3 gm of powder is sufficient.

I hope it will help you ....

Thanks

If the purpose of study is to examine the features in the actual rock, and not the weathering effects, utmost care should be taken while sample preperation to avoid fractures and fissures which would have happen to the specimen during the manual chipping itslef. Initial megascopic observations helps in expeling areas with foreign intrusions like xenoliths (which can be any other rock particle within the actual rock of interest) or larger phenocrysts. For chemical analysis using XRD, even though ony 2 grams of powdered sample is needed, it is recommended to take a uniform rock portion, powder it thoroughly and pass all of the powder through 75 microns or 45 microns sieve as per the need and then take 2 grams out of it. Otherwise some of the minerals which can't be powdered as easily as others in the rock will be missing in the test result.

Most suggestions given by our colleagues above are very germane. I will therefore avoid repeating their very relevant inputs but add that:

1. Since geological field work is at a cost, it is more profitable that all geoscientists in an organization such as the Geological Surveys in most countries will take the issue of sampling as part of capacity building training project . This way, all such trained personnel involved in field geological mapping will see their assignment in a bigger scope; know the possibilities of the samples being used for several purposes as research findings dictate. The size and quality of samples taken can be enhanced and funds conserved.

2. The above notwithstanding, for isotope/geochronological studies that require sample characterization and adequate knowledge of the structural setting and geochemistry (Major, trace elements, etc,) of the terrains under study, there is no alternative to being involved in the field unless detailed observations and reports are made available for unambiguous interpretation.