How to determine the magnetism of a mineral?

The most important diagnostic features of minerals include morphological features that characterize the shape of mineral deposits; optical properties: transparency, mineral color, streak color, luster; mechanical properties: cleavage, fracture, hardness, brittleness, elasticity, ductility, flexibility; other physical properties: specific gravity (density), taste, smell, magnetism, etc.

1. Morphological features

Most often, minerals are found in nature in the form of irregularly shaped grains. Well-formed crystals are rarer; their shape is usually a characteristic diagnostic feature. Variety of existing crystal shapes can be divided into three types. Isometric – having similar sizes in all directions: cubes (galena, pyrite), tetrahedra (sphalerite), octahedra (magnetite, pyrochlore), bipyramids (zircon, cassiterite), rhombic dodecahedrons (garnet), rhombohedrons (calcite), etc., as well as various combinations of these simple forms. Extended in one direction – prismatic, columnar, columnar, needle-shaped, fibrous crystals (tourmaline, beryl, pyroxene, amphibole, rutile, etc.). Elongated in two directions (flattened) – tabular, lamellar, leafy, scaly crystals (mica, chlorites, molybdenite, graphite, etc.). As a result of the process of metasomatic replacement or dissolution with subsequent filling of voids, crystalline forms belonging to one mineral turn out to be represented by another mineral; such formations are called pseudomorphoses. Hatching. In addition to the shape of the crystal, a characteristic property of the mineral that helps in its diagnosis is the shading on the faces: transverse parallel (quartz), longitudinal parallel (tourmaline, epidote) or intersecting (magnetite). In nature, it is not single crystals of a mineral that are more widespread, but their various accretion, or aggregates. Many minerals are characterized by oriented regular twin intergrowths of two or more crystals in a certain way. The most widespread specific forms of mineral aggregates, intergrowths and secretions, which have received special names, are given below. Granular aggregates. Depending on the shape of the constituent grains, granular aggregates proper (consisting of isometric grains), as well as lamellar, leafy, scaly, fibrous, needle-shaped, columnar and other aggregates are distinguished. According to the size of the grains, there are coarse-grained aggregates – more than 5 mm in diameter; medium-grained – from 1 to 5 mm and fine-grained – with grains less than 1 mm. In particular, most igneous and metamorphic rocks, as well as many sedimentary rocks, some types of sulfide ores, etc., are composed of granular aggregates. Druze – intergrowths of regular, well-formed crystals of minerals on the walls of voids of various shapes (cracks, caverns, “cellars”, “gnarly holes”, “caves”, etc.). In morphological terms, they are very diverse: “brushes” of crystals, “crystalline crusts” (small closely intergrown crystals, completely covering the walls of narrow cracks), “comb” intergrowths, etc. Crystal druses are typical of pegmatites, some types of hydrothermal veins and alpine-type veins . Secretions – execution of voids of an isometric, often round shape, distinguished by a concentric-zonal structure. The outer zones of secretions are often made of amorphous or cryptocrystalline minerals, and in their inner part there is a cavity, on the walls of which druses of crystals or sinter aggregates of minerals grow. Small secretions found in erupted rocks and tuffs are called tonsils, large, especially characteristic of pegmatites and alpine veins, – geodes. Concretions – spherical or irregularly shaped nodules and nodules formed in loose sedimentary rocks (silts, clays, sands, etc.). Unlike secretions, nodules grow from some center (clastic grain, organic residue, etc.), around which a clot of colloidal substance is formed, subsequently crystallized. Concretions are characteristic of phosphorites, siderites, marcasites and other types of ores of sedimentary origin. Oolites like nodules, they have a spherical shape, but their size is much smaller: from tenths of a millimeter to several millimeters. They are formed by the layering of colloidal material on grains of sand and organic debris that are suspended in mobile aqueous media. Oolites are very characteristic of some limestones, sedimentary iron and manganese ores, and bauxites. Sinter forms mineral deposits form on the walls of various voids and cavities during the slow drainage of solutions. These include calcareous and ice stalactites and stalagmites of caves, similar in shape to ordinary ice icicles, kidney-shaped, cluster-shaped mineral deposits in zones of oxidation and weathering of ore deposits, etc. The sizes and shapes of sinter formations can be very diverse: from fractions of a millimeter to huge pillars (in large caves). Sintered forms of sediments are characteristic of many supergene and low-temperature hydrothermal minerals: calcite, aragonite, malachite, hematite, hydroxides of iron, manganese, opal, gypsum, some sulfides, smithsonite, etc. Earthy masses – loose, soft, mealy aggregates of an amorphous or cryptocrystalline structure, sooty (black) or ocher (yellow, brown and other bright colors). Most often they are formed during chemical weathering of rocks and in the oxidation zone of ores (for example, manganese ores). Plaques and lubricants – thin films of various secondary minerals covering the surface of crystals or rocks. Such are films of limonite on rock crystals, smears of copper green on cracks in rocks containing sulfide deposits with copper minerals, etc. Fading – periodically appearing (in dry weather) and disappearing (in rainy periods) loose crusts, films, deposits, often fluffy or mossy, on the surface of dry soils, ores and rocks and along cracks in them. These formations are most often composed of easily soluble aqueous chlorides, sulfates of various metals, or other water-soluble salts.

2. Physical properties

Optical properties. Transparency – the property of a substance to transmit light. Depending on the degree of transparency, all minerals are divided into the following groups: transparent – rock crystal, Iceland spar, topaz, etc.; translucent – sphalerite, cinnabar, etc.; opaque – pyrite, magnetite, graphite, etc. Many minerals that appear opaque in large crystals are translucent in thin fragments or grain edges. Mineral color – the most important diagnostic sign. In many cases, it is due to the internal properties of the mineral (idiochromatic colors) and is associated with the inclusion of chromophoric elements (Fe, Cr, Mn, Ni, Co, etc.) in its composition. For example, the presence of chromium determines the green color of uvarovite and emerald, the presence of manganese determines the pink or lilac color of lepidolite, tourmaline or sparrowite. The nature of the coloring of other minerals (smoky quartz, amethyst, morion, etc.) lies in the violation of the homogeneity of the structure of their crystal lattices, in the occurrence of various defects in them. In some cases, the color of a mineral can be caused by the presence of the finest scattered mechanical impurities (allochromatic colors) – jasper, agate, aventurine, etc. To indicate color in mineralogy, a common method is comparison with the color of well-known objects or substances, which is reflected in the names of colors: bloody- red, azure blue, lemon yellow, apple green, chocolate brown, etc. The names of the colors of the following minerals can be considered standards: violet – amethyst, blue – azurite, green – malachite, yellow – orpiment, red – cinnabar, brown – limonite, lead-gray – molybdenite, iron-black – magnetite, tin-white – arsenopyrite, brass-yellow – chalcopyrite, metallic-gold – gold. Line color – the color of a fine mineral powder. A mineral trait can be obtained by passing the test mineral across the matte unglazed surface of a porcelain plate (biscuit) or a fragment of the same surface of a porcelain chemical vessel. This sign is more permanent compared to coloring. In some cases, the color of the line coincides with the color of the mineral itself, but sometimes a sharp difference is observed: for example, steel-gray hematite leaves a cherry-red line, brass-yellow pyrite leaves a black line, etc. Brilliance depends on the refractive index of the mineral, i.e. a quantity that characterizes the difference in the speed of light when it passes from air to a crystalline medium. It has been practically established that minerals with a refractive index of 1,3–1,9 have glass luster (quartz, fluorite, calcite, corundum, garnet, etc.), with an index of 1,9–2,6 – diamond shine (zircon, cassiterite, sphalerite, diamond, rutile, etc.). Semi-metallic luster corresponds to minerals with a refractive index of 2,6–3,0 (cuprite, cinnabar, hematite) and metal – above 3,0 (molybdenite, stibnite, pyrite, galena, arsenopyrite, etc.). The brilliance of a mineral also depends on the nature of the surface. Thus, in minerals with a parallel-fibrous structure, silky luster (asbestos), translucent “layered” and lamellar minerals often have pearl luster (calcite, albite), opaque or translucent minerals, amorphous or characterized by a disturbed crystal lattice structure (metamictic minerals) differ resinous shine (pyrochlore). Mechanical properties. Cleavage – the property of crystals to split in certain crystallographic directions, due to the structure of their crystal lattices. Thus, calcite crystals, regardless of their external shape, always split along their cleavage into rhombohedrons, and cubic fluorite crystals into octahedra. The degree of perfection of cleavage varies according to the following accepted scale: Cleavage very perfect – the crystal easily splits into thin sheets (mica, chlorite, molybdenite, etc.). Cleavage perfect – when struck with a hammer, cleavage marks are obtained; It is difficult to obtain a fracture in other directions (calcite, galena, fluorite). Cleavage average – a fracture can be obtained in all directions, but on mineral fragments, along with an uneven fracture, smooth shiny cleavage planes (pyroxenes, scapolite) are clearly observed. Cleavage imperfect or no. The grains of such minerals are confined to irregular surfaces, except at the edges of their crystals. Often differently oriented cleavage planes in the same mineral differ in degree of perfection. Thus, gypsum has three directions of cleavage: in one direction the cleavage is very perfect, in the other – average and in the third – imperfect. Cracks separately, unlike cleavage, are rougher and not completely flat; most often oriented transversely to the mineral elongation. Kink. In minerals with imperfect cleavage, fracture plays a significant role in diagnosis – conchoidal (quartz, pyrochlore), splintery (for native metals), small-shelled (pyrite, chalcopyrite, bornite), earthy (kaolinite), uneven and more Hardness, or the degree of resistance of a mineral to external mechanical influence. The simplest way to determine it is by scratching one mineral with another. To assess the relative hardness, it is taken Mohs scale, represented by 10 minerals, of which each subsequent one scratches all the previous ones. The following minerals are accepted as hardness standards: talc – 1, gypsum – 2, calcite – 3, fluorite – 4, apatite – 5, orthoclase – 6, quartz – 7, topaz – 8, corundum – 9, diamond – 10. When diagnosing, very It is also convenient to use for scratching such objects as a copper (hardness 3,0–3,5) and steel (5,5–6,0) needle, knife (5,5–6,0), glass (5,0) . Soft minerals can be scratched with a fingernail (2,5). Fragility, malleability, elasticity. Under fragility in mineralogical practice, the property of a mineral to crumble when drawing a line with a knife or needle is implied. The opposite property – a smooth shiny mark from a needle (knife) – indicates the ability of the mineral to deform plastically. Malleable minerals are flattened by a hammer into a thin plate, elastic are able to restore their shape after removing the load (mica, asbestos). Other properties. Specific weight (density) can be accurately measured in laboratory conditions by various methods; An approximate judgment of the specific gravity of a mineral can be obtained by comparing it with common minerals, the specific gravity of which is taken as a standard. All minerals can be divided by specific gravity into three groups: lungs – with a specific gravity less than or equal to 2,9 (gypsum, muscovite, sulfur, chalcedony, amber, etc.); average – with a specific gravity of about 2,9–5,0 (apatite, biotite, sphalerite, topaz, fluorite, etc.); heavy – with a specific gravity greater than 5,0 (arsenopyrite, galena, cassiterite, cinnabar, etc.). Magneticity. Some minerals are characterized by pronounced ferromagnetic properties, i.e. attract small iron objects – sawdust, pins (magnetite, nickel iron). Less magnetic minerals (paramagnetic) are attracted by a magnet (pyrrhotite) or an electromagnet; Finally, there are minerals that are repelled by a magnet – diamagnetic (native bismuth). The magnetic test is carried out using a freely rotating magnetic needle, to the ends of which the test sample is brought. Since the number of minerals with distinct magnetic properties is small, this feature has important diagnostic value for some minerals (for example, magnetite). Radioactivity. All minerals containing radioactive elements – uranium or thorium – are characterized by the ability to spontaneous α-, β-, γ-radiation. In the rock, radioactive minerals are often surrounded by red or brown rims, and radial cracks radiate from the grains of such minerals included in quartz, feldspar, etc. Radioactive radiation affects photographic paper. Other properties. For diagnostics in field conditions are important solubility minerals in water (chlorides) or acids and alkalis, private chemical reactions into individual elements flame coloring (for example, minerals containing strontium color the flame red, sodium – yellow). Some minerals emit noise when struck or broken. smell (for example, arsenopyrite and native arsenic emit a characteristic garlic odor), etc. Individual minerals are determined to the touch (for example, talc feels greasy to the touch). Table salt and other salt minerals are easily recognized to taste. Quite regularly, posts like “help me identify a mineral”, “find”, “stone”, etc. appear on Pikabu. For example – one, two, three and so on. I will try to create something like a simple manual on definition. First, a little theory (where would we be without it). A mineral is a natural compound of chemical elements formed as a result of geological activity. A simple example is quartz. This is a mineral with the formula SiO2. Quartz is part of rocks, they are monomineral (a significant predominance of one mineral) – quartz sands or granite – there is a lot of quartz, but there are also other minerals, so this is a polymineral rock. Minerals have a more or less constant chemical composition and set of properties, and these same properties are used to determine minerals, as well as the rocks that these minerals make up. Actually, most of the time a geologist does this, trying to understand what exactly he found and where to fit his find into the existing geological model. Most often, what is found will be a rock consisting of several different minerals, a couple of these minerals are the main ones, and the rest are impurities and most often do not play a special role. The brief introductory part is over. Let’s move on directly to what we have at hand and how to identify the found pebble. Lyrical digression – a geologist is usually a mobile creature, take more and carry further. Therefore, they cannot particularly operate with tricky laboratory reagents and other intricacies, using the simplest rules and tools (in other words, you have to walk far, and the carrying capacity is limited, given that you need to carry food, water, a pot, a hammer, etc., so everything is just angrily) . So, a set of characteristic properties for minerals – color, shine, dash color and transparency. These things need to be determined on a fresh chip of the found stone (usually the stone is covered with dirt, weathered, oxidized, etc., so the geologist needs a hammer). Color и transparency, I think there is no need to explain. Line color is the color of the mineral in a fine powder. It sounds complicated, but in reality you just need to run the stone under study (preferably a chip of one mineral) over a chip of porcelain (such as an insulator, a mug-plate, etc.) Why exactly is a chip – the surface of porcelain is usually covered with glaze and there is no point in moving over the glaze. Brilliance There are metallic, semi-metallic and non-metallic – greasy, glass, diamond, etc. Now let’s move on to slightly more complex things. Physical properties of minerals – hardness, density, cleavage, fracture, separation, flexibility, malleability. From this list we take the simplest ones – hardness, density, fracture and cleavage. Hardness – we measure on the Mohs scale (talc-gypsum-calcite-fluorite-apatite-orthoclase-quartz-topaz-corundum-diamond – this series is assigned a hardness from 1 to 10, arbitrary units that have only practical significance). The meaning is simple – if something with a hardness of 5 scratches the sample, then the hardness of the sample is lower than 5. From household scratchers – nail 2-3, nail 4-5, glass 5-6, file -7, sandpaper (if with corundum) 9 . Density – Weighed the sample, put it in water, and found out the volume. We divided one by the other and found the approximate density. Cleavage – ability to split along planes – the best example is mica. It forms plates, because the cleavage is very perfect. There is also perfect and imperfect. Fracture – when we split a sample, we can get different types of fracture – stepped, conchoidal, uneven, etc., the surface of the fracture is a diagnostic sign. Below in the photo is an example of minerals with very perfect cleavage – muscovite and biotite mica: Minerals also have a smell (a classic example is that when you hit arsenopyrite with a hammer, it will smell like garlic), taste and porosity – the degree of adhesion to the tongue – the better it sticks, the more porous the stone. But this is all for an amateur, or maybe, on the contrary, a professional, you can get by with the previous methods.
Another of the simplest is magnetism. To accurately understand whether it is magnetic or not, you need to hang the magnet on a thread and smoothly bring it to the stone; if it deviates from the vertical, it means the rock is magnetic. Below in the picture is the simplest tool for measuring hardness and magnetism. Where the pen has a writing unit, this thing has a steel tip (below hardness 7, quartz does not scratch), and on the back there is a magnet. So this was all descriptive. In short, we take a stone and split it. We look at the color, transparency, shine, type of fracture, what the crystals look like (if they are visible without a magnifying glass), cleavage, etc. In most cases, this is already enough to determine. We check the color of the line, magnetism, hardness. We estimate the density (volumetric weight). We remember or write this down and Google something like “online mineral identifier.” As an example – http://edu.tsu.ru/eor/resourse/803/html/4.html or http://world-of-stones.ru/minerals/filter. You can also use a mineral reference guide, for example, Yubelt R “Minerals Determinant” or even hardcore if “Mineralogy Course” by Betekhtin A.G. We substitute our received data into a table or online identifier, get a list of minerals, scratch our heads at “why did I find this diamond, weighing 4 kg” and happily look for an exchange where it can be quickly sold 🙂 What I mean is that some practical experience is needed and not everything will work out right away, but even if some of the measurements are incorrect, some of the results obtained will correspond to the find and by rejecting completely crazy ideas, you can get a completely adequate list of minerals or rocks , and one of the list (surely) will be what you are looking for. ps This is not a manual for a geological technician, not a pocket mineral determiner, but an exclusively brief description of WHAT needs to be determined and HOW. There may be mistakes in the post, if anyone notices and writes, I will try to correct them. Most likely, this is just a technique for what to do if you find an unusual stone and want to understand what exactly you found, and even if you don’t understand it yourself, you can make a post not just with photographs of the find, but also with a brief description of the properties. And in this case, it will be easier to determine and suggest what exactly was found. To lighten up the dry text, here are a few recent photos: Thermometer on the backpack, 13.5 km covered (according to GPS). It turned out to be such a day.