Preliminary identification service for minerals, gemstones and meteorites

DESCRIPTION

The Preliminary Identification Service for geological samples provides initial information on specimens received at the Museum to be explicated.

For this, purpose, we make use of simple and fast instrumental techniques, such as magnifying glass, refractometer, magnet, acids, conductivity meter, polariscope, optical spectrometer, hydrostatic balance, diamond disc cutting, etc.

After the sample examination, we will make plain between the convenience of: (1) considering this first identifying diagnosis sufficient and very clarifying, (2) continuing with non-destructive analysis in other Museum Laboratories, for example, valuable samples (gems, archaeological samples or forensic, etc..), or, (3) propose new micro-destructive analyses for other types of samples that allow extracting a small fragment.

 

TYPES OF SAMPLES AND ANALYSES

The accumulated experience of the last 30 years on consultations of external samples in the Museum shown a great majority of solids arrived to verify their meteorite nature, obviously, many of them were terrestrial iron ores, foundry slags, basalts, glasses, etc.

For these last cases, the preliminary informative report is very useful because in many cases it eliminates the meteorite possibility.

 

 

 

 

The separation among terrestrial slags and meteorites is usually relatively easy and the preliminary separation is very fast and reliable. In some other rare cases of potential meteorites, e.g. siderites having abundance of iron, the MNCN Preliminary Identification will recommend analysing the nickel content using the electron microscope of another laboratory of the Museum itself, but this operation would have extra analytical costs.

Meteorites have several distinguishing characteristics that make them different from terrestrial rocks, such as melting crusts, gas outlet craters, flight erosion shapes, sets of many different mineral groups, such as elements, sulphides, oxides, phosphates, silicates, etc. ...

In their metallic parts they could exhibit silicate phenocrysts, Widmanstädten patterns due to the intergrowth of taenite and kamacite bands during slow coolings, etc ...

 

 

 

 

For a large number of gemstones and minerals, simple analysing techniques, such the naked eye, could be sufficient for their identification. For example, some opaque stones as rhodochrosite, lapislazuli, turquoise, quiastolite, chrysocol, unakite, tiger's eye, charoite, malachite, azurite, etc., These gemstone types with very distinctive qualities and colours allow to perform quite correct diagnoses.

 

 

 

 

In the transparent cut gems cases, the situation is more complicated because chromophore colours, texture patterns and crystallographic habits vanish and their external aspects are not specific. It is common to see colourless gems with similar size, shape and feature; in these cases, we can still compare their brightness or fire (refractive index), feel their thermal conductivity on our fingers, etc. From there, a 10X magnifying glass allows to see some internal inclusions, external damages related to hardness, colour nuances, reflections, etc., which greatly expand the examining possibilities.

 

 

 

 

When we reach a point in where is not possible to identify the stone, we perhaps could measure refractive indices with the refractometer, specific weights with a hydrostatic balance or dense liquids, reflectivity with gem-tester equipment, some polarization properties, and to observe internal inclusions at higher magnification in binocular loupes or optical microscopes. etc.

 

 

 

 

 

These gemmological kits are composed by non-destructive apparatus to avoid physical damage to valuable stones. They are very simple tools of effective monitoring and, if properly used, can be of great help in that respect to differentiate clean, transparent faceted gems of the same colour and very similar to each other.

 

 

 

 

In many cases the informative opinion will correctly guide to the type of stone and it will not be necessary further analyses. In other cases, the identification will be complex and it will be recommended additional measurements by determining the molecular structure using Raman spectroscopy, the crystalline structure by X-ray point micro-diffraction, the chemical composition by X-ray dispersive energy analysis or the photoluminescence and cathodoluminescence spectra. All the necessary equipment for these purposes is also located in the Non-Destructive Techniques Laboratories of the National Museum of Natural Sciences (MNCN).

For other types of rocky geological samples with special morphologies, textures or structures, supplementary information will be given on the physicochemical nature of the rock together with consultation paths towards specialists’ palaeontologists (fossils), archaeologists (archaeological pieces), petrologists (rocks), materials science (ceramics) or biologists and organic chemists (organic and hydrocarbon biomaterials).

For accurate classifications of rock types, it is necessary to perform mineral counts and specific chemical analyses, for which it is necessary to make petrographic sections and to use other sophisticated analytical equipment above the Preliminary Identification Service of the MNCN as a previous step for the rock identifications. This MNCN service will provide alternative suggestions on other more convenient additional analytics.

LIMITATIONS OF THE TECHNIQUES AND THE TECHNICIANS

Both in the Preliminary Identification Service of external geological samples, as in the other analytical services, e.g., Microscopic and Non-destructive Spectroscopic Techniques, also in the Museum, kind analytical results will be provided. That is, pure scientific results and measurements without genetic hypotheses, uncertain assumptions, or subjective evaluations, such as, geographic origins, types of synthesis, or sample formation mechanisms. For this reason, NO OFFICIAL CERTIFICATES OR FINANCIAL VALUATIONS WILL BE PROVIDED, but ALL THE EXTERNAL SAMPLES WILL BE ANALYSED. In addition, if necessary, the most appropriate analytical facility for each sample entering to the laboratory will be suggested. Laboratory Technicians and Museum Researchers analyse and obtain scientific results in an absolutely serious and rigorous way, but these results do not always satisfy some claims of the users and sample owners. It is very common to reduce expectations of some users about imagined meteorites or illusory gold and diamond specimens towards common minerals and rocks.

RESEARCH FACILITIES

Backlit optical, binocular and handheld loupes.

Rayner Solids Refractometer with Contact Liquid

Gem-tester reflectometer

Polariscope

Hydrostatic balance

Dense liquids

Chelsea filter

Optical spectrometer

Dicroscope

Diamond blade cutters

Magnets

Office automation, international databases, Origin and Corel Software, etc..