Fonte: Laboratorio di Alan Lester – Università del Colorado Boulder
Le proprietà fisiche dei minerali comprendono vari attributi misurabili e distinguibili, tra cui colore, striscia, proprietà magnetiche, durezza, forma di crescita del cristallo e scissione del cristallo. Ognuna di queste proprietà è minerale-specifica e sono fondamentalmente correlate alla composizione chimica e alla struttura atomica di un particolare minerale.
Questo esperimento esamina due proprietà che derivano principalmente dalla ripetizione simmetrica di raggruppamenti atomici strutturali fondamentali, chiamati cellule unitarie, all’interno di un reticolo cristallino, una forma di crescita cristallina e una scissione cristallina.
La forma di crescita dei cristalli è l’espressione macroscopica della simmetria a livello atomico, generata dal processo di crescita naturale dell’aggiunta di cellule unitarie (i mattoni molecolari dei minerali) a un reticolo cristallino in crescita. Le zone di rapida addizione unità-cella diventano i bordi tra le superfici planari, cioè le facce, del cristallo.
È importante riconoscere che le rocce sono aggregati di grani minerali. La maggior parte delle rocce sono polimineraliche (più tipi di grani minerali), ma alcune sono effettivamente monomineraliche (composte da un singolo minerale). Poiché le rocce sono combinazioni di minerali, le rocce non sono indicate come aventi forma cristallina. In alcuni casi, i geologi si riferiscono alle rocce come aventi una scissione generale, ma qui il termine è semplicemente usato per riferirsi a superfici di rottura ripetitive e non è un riflesso della struttura cristallina atomica. Quindi, in generale, i termini forma cristallina e scissione del cristallo sono usati in riferimento a campioni minerali e non a campioni di roccia.
Tutti i minerali possiedono proprietà fisiche, ma le caratteristiche specifiche e facilmente riconoscibili associate alle proprietà non sono sempre espresse in un singolo cristallo. Ad esempio, i cristalli di quarzo hanno una caratteristica forma esagonale, ma se la crescita dei cristalli avviene in un ambiente in cui altri minerali bloccano o ostacolano la forma di crescita naturale (che è comunemente il caso nella maggior parte delle rocce), allora la forma esagonale non si forma. Quindi, con questo in mente, è importante selezionare attentamente un gruppo adatto di campioni per la crescita dei cristalli o l’analisi della scissione del cristallo, poiché non tutti i campioni mostrano queste caratteristiche chiave.
Inoltre, sebbene la scissione del cristallo sia relativamente facile da testare – rompendo un campione con un martello – diversi minerali dimostrano una gamma di qualità di scissione, in modo tale che le superfici planari generate dalla rottura possono essere lacere e ruvide (definita “scarsa scissione”) o estremamente liscia (definita “buona” o “eccellente- scissione”). In alcuni casi(ad esempio il quarzo), le forze di legame cristallografico sono uniformi in tutte le direzioni e questo si traduce in un minerale con una mancanza di piani di scissione riconoscibili.
Historically, evaluating the physical properties of minerals has been a key first step in mineral identification. Even today, when lacking microscopic and modern analytical instrumentation (e.g. petrographic microscopy, x-ray diffraction, x-ray fluorescence, and electron microprobe techniques), observed physical properties are still quite useful as diagnostic tools for mineral identification. This is particularly the case in field geologic studies.
Evaluating and observing the physical properties of minerals is an excellent means to demonstrate the critical dependence of macroscopic features on atomic-level structure and arrangement.
The key physical properties of minerals are not always expressed in specific samples. Therefore, actually being able to recognize and use these properties as diagnostic tools requires a combination of science, experience, and craft. Often, the geologist must utilize a hand lens to evaluate relatively small mineral crystals or grains within the matrix of a larger rock. In such cases, it can become a distinct challenge to identify the useful aspects of crystal form and crystal cleavage.
In an academic or teaching setting, the evaluation of minerals via hand sample analysis is an exercise that demonstrates how repetitive patterns and characteristics are imposed by the physical chemistry of natural materials. In other words, for any specific mineral, there are certain crystallographic features (e.g. crystal morphology) and physical properties (e.g. color, hardness, streak) that are imposed by chemical composition and atomic structure.
In the realm of mineral resources and exploration geology, the identification of minerals via hand sample is a key component of fieldwork, aimed at locating potential ores and economically useful deposits. For example, the identification of various metal sulfides (pyrite, sphalerite, galena) in association with hydrothermal iron oxy-hydroxides (hematite, goethite, limonite) can be indicative of potential Au- and Ag-rich veins and regions.
In the context of historical geology (deciphering the deep temporal history of a region), mineral identification can set the stage for interpretations of ancient conditions. For example, certain metamorphic minerals (e.g. the Al2SiO5 polymorphs, kyanite, andalusite, and sillimanite) are markers of particular pressure and temperature conditions in the ancient crust.
Minerals are inorganic substances found in the Earth, with unique properties that aid in identification and analysis.
Many minerals exhibit crystalline structure. These crystalline materials have highly ordered atomic arrangements, made up of repeating atomic groupings, called unit cells. Because unit cells are identical within a crystal, they are responsible for the symmetry of the crystal on the micro- and macro-scale.
This symmetry causes mineral crystals to break, or cleave, in a predictable way. Cleavage is the tendency of a crystal to break along weak structural planes. Thus, the way a mineral cleaves provides insight into its crystal structure.
This video will demonstrate the analysis of macro-scale mineral crystal forms by breaking mineral samples and observing their cleavage.
Crystalline solids contain atoms organized in a repeated pattern, whereas amorphous solids have no order. For example, carbon can be found in many forms. The atoms in amorphous carbon are randomly organized, whereas the atoms in diamond are arranged in an ordered crystal.
A crystal is an array of repeating, identical unit cells, which are defined by the length of the unit cell edges and the angles between them. These repeated structures extend infinitely in three spatial directions, and define the uniformity and properties of the crystal.
There are seven basic unit cells. The simplest unit cell, the cube, has equal edge lengths, and an atom at each corner. Variations include tetragonal and orthorhombic, which possess different edge lengths.
Rhombohedral crystal structures possess similar parallel face geometry without right angles. Monoclinic and triclinic are similar in shape, but with varied angles and edge lengths. Finally, the hexagonal structure is composed of two parallel hexagonal faces, with six rectangular faces.
Variations in these structures arise when additional atoms are contained in the crystal face, called face-centered, or in the crystal body, called body centered.
When crystals are broken, they tend to cleave along structurally weak crystal planes. The cleavage quality depends on the strength of the bonds in and across the plane. Good cleavage occurs when the strength of the bonds within the place are stronger than those across the plane. Poor cleavage can occur when the bond strength is strong across the crystal plane. Crystals may cleave in one direction, called basal cleavage, resulting in two cleaved faces. This results from strong atomic bonds within the plane, but weak bonds between the planes.
Similarly, crystals may cleave in two directions, due to two weak planes, resulting in four cleaved faces and two fractured faces. Cubic and rhombohedral forms result from cleavage in three directions. Octahedral and dodecahedral forms arise from four and six fracture planes, respectively.
Some minerals don’t cleave along a crystal plane at all, due to strong bonds in all directions, and instead result in irregular fracture.
Now that we’ve covered the basics of crystal structure, and the different types of crystal cleavage, let’s look at these properties in real mineral samples.
To analyze crystal forms, first collect a group of mineral samples, such as quartz, halite, calcite, garnet, biotite, and muscovite.
Place the sample on the observation surface. Rotate the sample in order to observe all sides. Look for crystal faces, crystal edges, and crystal vertices.
Where possible, measure the interfacial angles using a goniometer. To do so, lay one side of the goniometer on a particular crystal face, and the other side of the goniometer on an adjoining face. Then read the angle.
Compare the observations to the set of characteristic crystalline polyhedra. Repeat these steps for other minerals, and note the differences.
Quartz samples have a hexagonal dipyramidal crystal form, as indicated by the 6 sides.
The calcite material, exhibits scalenohedron form, as shown by the 8 faces of the twinned pyramid structure.
Halite, shows characteristic cubic structure, with 90° angles.
Garnet has angled surfaces with 12 sides, indicative of its dodecahedron form.
Finally, biotite can show an apparent hexagonal form.
Next, to observe crystal cleavage, first put on eye protection.
Place a piece of quartz on the breaking surface. Using a hammer, break the piece of quartz. Using a hand lens, observe the broken piece of quartz for cleavage surfaces. Notice that quartz has none.
The unit cells in the quartz crystal lattice have comparably equal bond strengths in all directions, resulting in a crystal with no preferred breaking planes, called conchoidal fracture.
Next, repeat this breaking step for other specimens. Use a hand lens to evaluate different cleavage qualities.
When there is a dramatic difference in bond strengths in a particular orientation, such as between sheets of silicate groupings in the case of mica, a nearly perfect cleavage is generated between these sheets, called basal cleavage.
Biotite and muscovite each display basal cleavage, with a single break plane.
Halite displays cubic cleavage, resulting from three cleavage planes at 90°.
Calcite displays rhombohedral cleavage, resulting from three cleavage planes at 120 and 60°.
The analysis of crystal structure is important to understanding the types of minerals found in the field.
The quantitative analysis of crystal structure can be performed using X-ray diffraction, or XRD.
In this example, the crystal structure of an iron oxide was synthesized from a mixture of hematite and iron at high temperature and pressure in a diamond anvil cell. The XRD scattering pattern was analyzed throughout the reaction to determine the crystal structure.
The results showed smooth or spotty Debye rings, which indicate crystallinity. The location of each ring elucidates the crystal structure, as each ring corresponds to a crystal plane.
Due to its planar cleavage property, and therefore atomically flat surface, mica is frequently used as a substrate for small molecule imaging.
In this example, mica was used as a substrate for the imaging of photoreceptor molecules using atomic force microscopy, or AFM. The protein sample was adsorbed to a freshly cleaved mica sheet, and then rinsed with buffer.
The sample was then imaged using a fluid cell. The mica substrate enabled high resolution imaging of the protein sample due to its atomically flat surface.
You’ve just watched JoVE’s introduction to physical properties of minerals. You should now understand the basics of crystal unit cells, and how to determine crystal cleavage planes. Thanks for watching!
