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Mineral Identification and Classification

Exploring the Mohs Hardness Scale and its role in mineral identification, this overview covers the scale's range from talc (1) to diamond (10), and other minerals in between. Additional hardness tests like Vickers and Rockwell, and properties such as lustre, diaphaneity, color, streak, cleavage, and specific gravity are discussed, providing a comprehensive guide to mineralogy.

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1

Mohs Scale Originator and Year

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Developed by Friedrich Mohs, 1812.

2

Mohs Scale Range

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Ordinal scale from 1 (softest) to 10 (hardest).

3

Mohs Scale Examples: Gypsum, Quartz, Diamond

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Gypsum rated 2, Quartz rated 7, Diamond rated 10.

4

The ______ hardness test differs by using a spring-loaded device to determine a mineral's resistance to indentation.

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Shore

5

Unlike the Mohs scale, these hardness tests provide ______ values, which are essential for material selection in ______ contexts.

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numerical industrial

6

Types of Lustre

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Metallic, sub-metallic, non-metallic; further divided into vitreous, pearly, resinous, silky.

7

Example of Metallic Lustre

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Galena exhibits a metallic lustre, reflecting light like metal.

8

Diaphaneity Classifications

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Transparent (e.g., calcite), translucent (e.g., opal), opaque (e.g., pyrite).

9

The inherent hue of idiochromatic minerals, like the ______ hue of malachite, is due to their ______ makeup.

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green chemical

10

______ minerals, in contrast, get their color from ______ or structural differences, as seen in the diverse colors of fluorite.

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Allochromatic impurities

11

The ______ color of a mineral is less influenced by surface flaws or weathering, especially in minerals with a ______ luster.

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streak metallic

12

Cleavage Quality Variance

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Ranges from perfect (mica) to absent (quartz).

13

Parting vs Cleavage

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Parting is breakage due to stress or defects, not crystal structure.

14

Types of Fracture

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Includes conchoidal, fibrous, hackly; describes breakage without cleavage.

15

The property of specific gravity is ______ and aids in identifying minerals.

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dimensionless

16

Minerals with high specific gravity contain ______ elements, like lead in galena.

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heavy

17

A mineral's specific gravity is found by weighing it in air and ______.

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water

18

Minerals that are porous or have a well-structured crystal lattice tend to have a ______ specific gravity.

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low

19

The difference in weight when a mineral is suspended in water versus air reveals its ______.

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density

20

Acid reaction test for minerals

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Identifies carbonate minerals by effervescence when in contact with hydrochloric acid.

21

Magnetism in mineral identification

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Used to recognize magnetic minerals like magnetite.

22

Radioactivity detection in minerals

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Identifies minerals with radioactive elements like uranium using a Geiger counter.

23

The initial efforts to classify minerals date back to ______ by ______.

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315 BCE Theophrastus

24

Foundational work for modern mineral classification was done by ______ and ______.

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Georgius Agricola Carl Linnaeus

25

Today's systems classify minerals by their ______, ______, and ______.

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chemical composition crystal structure physical properties

26

Current classification systems aid in the study of ______ and are used in both ______ and ______ settings.

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mineralogy educational professional

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Exploring the Mohs Hardness Scale for Mineral Identification

The Mohs Hardness Scale is an essential tool in mineralogy, developed by Friedrich Mohs in 1812, to determine the relative hardness of minerals. It is based on the ability of a harder material to scratch a softer one. The scale is ordinal and ranges from 1 (softest) to 10 (hardest). Talc, with a Mohs hardness of 1, is the softest mineral and can be scratched by all other minerals. Diamond, rated as 10, is the hardest and can scratch all other substances. The scale includes other well-known minerals such as gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), and corundum (9). Each mineral can scratch those below it and be scratched by those above it on the scale. This scale, while not precise, is a practical method for geologists and gemologists to quickly estimate the hardness of mineral specimens.
Collection of colorful minerals on dark surface, with polished gems in the center and hand performing striation test.

Expanding on Hardness: Additional Testing Methods in Mineralogy

In addition to the Mohs scale, mineralogists employ other methods to measure hardness more quantitatively. The Vickers, Rockwell, and Brinell hardness tests involve the indentation of the mineral surface with a specific force and measuring the size of the impression left, which correlates to hardness. The Shore hardness test, another method, uses a spring-loaded device to gauge the resistance of a mineral to indentation. These tests provide numerical values for hardness, offering a more precise measurement than the comparative Mohs scale. These methods are particularly useful in industrial contexts where exact hardness values are necessary for material selection and processing.

The Role of Lustre and Diaphaneity in Determining Mineral Properties

Lustre describes the quality and intensity of light reflected from a mineral's surface and is a key characteristic in mineral identification. It can be metallic, sub-metallic, or non-metallic, with further subdivisions such as vitreous, pearly, resinous, and silky. For example, galena exhibits a metallic lustre, while quartz typically has a vitreous lustre. Diaphaneity refers to the transparency of a mineral and is classified as transparent, translucent, or opaque. Transparent minerals like calcite allow light to pass through with little to no distortion, while translucent minerals like opal permit light but do not allow for clear image transmission. Opaque minerals, such as pyrite, do not let light pass through. These properties can be influenced by a mineral's thickness and purity, and are valuable for identification and classification.

The Significance of Color and Streak in Mineral Identification

Color is a noticeable but sometimes misleading characteristic of minerals due to the presence of impurities or variations in mineral composition. Idiochromatic minerals have a characteristic color intrinsic to their chemical makeup, such as the green color of malachite. In contrast, allochromatic minerals owe their color to impurities or structural anomalies, like the various colors of fluorite. The streak of a mineral, or the color of its powdered form, is often more consistent and can be observed by rubbing the mineral across a streak plate. This property is particularly useful for identifying minerals with a metallic luster, as the streak color is less likely to be affected by surface irregularities or weathering.

Understanding Cleavage, Parting, Fracture, and Tenacity in Mineralogy

Cleavage is the tendency of a mineral to break along specific planes related to its crystal structure, resulting in smooth, flat surfaces. The quality of cleavage can vary from perfect, as seen in mica, to absent, as in quartz. Parting, often confused with cleavage, is breakage along planes of structural weakness caused by stress or growth defects. Fracture describes the pattern in which a mineral breaks when cleavage is not present, with types including conchoidal, fibrous, or hackly. Tenacity refers to a mineral's resistance to breaking, bending, or deforming and is categorized as brittle, malleable, ductile, sectile, flexible, or elastic. These mechanical properties are critical for mineral identification and understanding a mineral's potential uses.

Specific Gravity and Its Importance in Mineral Identification

Specific gravity is a measure of the density of a mineral relative to the density of water. This property is dimensionless and can be a diagnostic tool in mineral identification, especially for minerals that look similar but have different weights. High specific gravity is indicative of minerals containing heavy elements, such as the lead in galena, while low specific gravity may be characteristic of minerals with a porous or highly structured crystal lattice. Specific gravity is determined by weighing the mineral in air and then suspended in water, with the difference in weight revealing the mineral's density.

Other Diagnostic Properties of Minerals

Mineralogists consider a variety of other properties for mineral identification. The reaction of minerals to hydrochloric acid, for instance, is a quick test for the presence of carbonate minerals, which effervesce upon contact with the acid. Magnetism is a distinctive property of minerals like magnetite. Some minerals have a characteristic taste, such as the salty flavor of halite, or a distinctive smell, such as the sulfur odor of pyrite when struck. Radioactivity is an important property of minerals containing radioactive elements like uranium, and it can be detected with a Geiger counter. These properties, while not always applicable, can provide conclusive identification in certain cases.

Evolution of Mineral Classification Systems

The classification of minerals has progressed significantly since Theophrastus's initial attempts in 315 BCE. Contributions by Georgius Agricola and Carl Linnaeus laid the groundwork for modern classification, which has been refined with advancements in crystallography and mineral chemistry. Today's classification systems are comprehensive, categorizing minerals based on their chemical composition, crystal structure, and physical properties. These systems facilitate the study of mineralogy and provide a framework for identifying and comparing minerals in both educational and professional settings.