资料来源： 实验室的艾伦 · 莱斯特-科罗拉多大学博尔德分校
矿物的物理性质包括可衡量和可辨的各种属性，包括颜色、 条纹、 磁性、 硬度、 晶体生长形态和水晶劈裂。每个这些属性是特定的矿物，和他们都从根本上关系到特定的矿物的化学组成和原子结构。
本实验考察主要源于对称重复的基本、 结构的原子集团，称为单元内晶格、 晶体生长形态和水晶劈裂的两个属性。
晶体生长形式是原子级对称性，生成的单元 （分子积木的矿物） 添加越来越多的晶格的自然生长过程的宏观表现。快速单位细胞加区成为平面曲面，即面孔的晶体间的边缘。
它是重要的是认识到岩石的矿物颗粒聚集。大多数岩石都是矿物 （多种矿物颗粒） 但有些有效单矿物 （由单种矿石组成）。因为岩石的矿物组合、 岩石不统称为具有晶形。在某些情况下，地质学家指岩石具有一般的解理，但在这里一词只是用来指重复的断裂表面，不是原子晶体结构的反映。所以，一般情况下，条款晶体形式和晶体解理被用来指矿物标本和不岩石样品。
所有矿物都具有物理属性，但在个别的晶体不始终表示具体和容易辨认的特征与属性相关联。例如，石英晶体有六角形状特性，但如果晶体生长发生在环境其他矿物阻止或侵犯自然增长形状 （这通常是在绝大多数岩石中） 然后六角形状的位置并不会形成。所以，牢记这一点，很重要要小心地选择一组合适的样品为晶体生长或晶体断裂分析，因为并非所有样本都显示这些关键的功能。
此外，虽然晶解理可以相对容易地测试 — — 由样品用锤子敲碎 — — 不同矿物展示的劈裂质量范围这样打破平面曲面可能得衣衫褴褛，粗糙 （称为”穷理”） 或极光滑 （称为”好的”优秀解理”)。在某些情况下 （如石英），晶体学债券的优点是在所有方向上均匀，这会导致一种矿物，缺乏的可识别的解理面。
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!
