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Crystal formExplain and Demonstrate
- Cleavage and fracture
- Color and streak
- Miscellaneous physical properties
Points to Emphasize
- Observations of crystal growth and crystal form indicate minerals have a definite geometric internal structure. A given mineral will have the same interfacial angles. Refer to Figures 2.12–2.16 (pp. 18–24). The value of these photographs depends on how they are used. Insist that students study the photographs carefully. A successful technique is to require students to label the most diagnostic physical properties observable in these photographs. This will ensure that they observe and recognize the significant properties rather than the size, shape, and color of specimens. Also, have the students compare a mineral specimen with its corresponding photograph and determine which properties are fundamental in mineral identification.
- Cleavage results from weakness in internal structure and should not be confused with crystal faces. Refer to Figure 2.2 (p. 11 in Lab Man.).
- All physical properties of a mineral are constant (within specific limits), and many may serve as diagnostic features. The student should learn the diagnostic properties of each mineral.
More than 95% of the minerals the students will see in the field will be varieties of species shown on pages 18–24.
Emphasize that although these minerals appear in various forms, sizes, and colors, each species has definite diagnostic physical properties.
The color photographs are to be used only as a reference, not as a key.
This is important, because most students have a strong tendency simply to compare the specimens with the photos and determine the names of the minerals.
Insist that each student record the observable physical properties of the specimen he works with.
This lab work is largely memorization. Remind the students that repetition is the key to learning this material but that there is system and order in the mineral classification chart (Figure 2.11). Suggest that they study the material in the manual outside of class and review the samples in the display cases as often as possible. Answers to Problems, p. 25
- A number of physical properties, such as hardness and cleavage, are determined only from mineral specimens, not from a photograph. Other properties, such as color, size, and shape, which are most obvious on the photographs, may not be diagnostic.
- The angles between similar crystal faces of a specific mineral will be the same regardless of where or when the mineral was formed, even though the overall size or shape of the mineral may vary.
- Crystal form, cleavage, fracture, hardness, density, and streak are generally considered the most important types of physical properties of minerals when studied in hand specimens.
- A mineral is a naturally occurring, inorganic substance with an orderly internal atomic arrangement and definite chemical composition that varies only within certain limits.
- Solid solution is the substitution or replacement of one element for another in the crystal structure of a mineral. Solid solution changes the composition of a mineral but not its internal structure.
- Solid solution is important in feldspars, olivine, pyroxenes, amphiboles, garnets, and micas.
- A crystal face is the external expression of a mineral’s internal atomic structure. A cleavage plane is a plane of weakness in the crystalline structure along which the crystal will break.
- Hardness is the degree of resistance of a mineral to abrasion.
- Calcite is the only common mineral that reacts with HCl.
- The typical crystal form of quartz is six-sided prismatic.
- Specimens of microcrystalline quartz are aggregates of microscopic crystals.
- Chert occurs most commonly as nodules in limestone.
- Color, which commonly results from the presence of impurities or inclusions, does not constitute a fundamental difference in the varieties of the mineral quartz.
- The most diagnostic physical properties of feldspars are: (1) two directions of cleavage nearly at right angles, (2) hardness of 6, and (3) porcelain luster.
- The two main subgroups of the feldspars are plagioclase feldspar and potassium feldspar. Plagioclase feldspars represent a Ca to Na solid solution series of aluminum silicates, whereas potassium feldspars have few or no elements substituted for K.
- For general identification, the K-feldspars can be recognized by their pink or green color. Plagioclase is commonly white to greenish black. Specific identification requires the use of chemical, optical, or X-ray tests.
- The most important physical properties of calcite are three directions of cleavage not at right angles, and hardness of 3.
- Microcrystalline calcite can be recognized in hand specimens by its reaction to HCl.
- The diagnostic properties of olivine are green color and aggregates of small, glassy grains.
- Olivine is important in basic igneous rocks, especially basalt, gabbro, and peridotite.
- The distinction between amphibole and pyroxene is made primarily on crystal form and cleavage.
- The diagnostic properties of halite are taste, cleavage in three directions at right angles, and hardness.
- The diagnostic properties of gypsum are hardness of 2, feel, and cleavage.
- A hand specimen of kaolinite is an aggregate of many submicroscopic crystals.
- The most important rock-forming minerals are quartz, calcite, feldspars, olivine, pyroxene, amphibole, and mica.
- Tests for physical properties are essential for mineral identification. They cannot be made from inspection of a photograph. Misidentification from photo inspection is also likely because of emphasis on color, shape, and form of the specimen.