Flashcards in Mineralogy Deck (29):
What is a mineral?
Any naturally occurring inorganic substance with a definite chemical composition and characteristic crystalline structure, color, and hardness.
How many minerals are there?
There are approximately 4000 varieties of minerals which can be classified into one of eight major mineral classes similar to how biologists classify living organisms into groups such as plants, animals, fungi, and bacteria.
How are minerals classified?
Minerals, however, are classified into groups by their composition and crystal structure. Ninety five percent of the rocks found in Earth’s crust contain minerals known as silicates.
What is hardness?
Hardness is the ability of a material to resist abrasion when a pointed fragment of another substance is drawn across it without sufficient pressure to develop fracture or cleavage.
Why determines the hardness of a mineral?
Differential structure is related to both the crystal and atomic structure of the diamond crystal.
What is differential hardness?
Diamonds have differential hardness which means the hardness differs with the direction of scratching in certain gem materials. This differential hardness is due to the arrangement of the atomic bonding in crystal structures.
What is an impurity
Foreign material, such as elemental additions, within a mineral's structure that are not part of its integral structure. Impurities are often responsible for color changes.
What factors influence color in minerals?
2. Chemical impurities
3. Defects in crystal structures
4. Mechanical mixture of fine impurities
5. Finely spaced structures in the mineral
What is color?
Result of dominant wavelength of light. Minerals are colored because certain wavelengths of incident light are absorbed, and the color we perceive is produced by the remaining wavelengths that were not absorbed.
Why minerals are colorless?
This means that none of the incident light has been absorbed.
Cause of Color Center
Color can also be caused by structural defects in minerals. For example, an excess electron that is unattached to any single atom may be trapped in a structural defect such as a void due to a missing ion. A hole, or the absence of an electron, can have the same effect.
When a mineral sample is split in a direction which does not serve as a plane of perfect or distinct cleavage. A mineral fractures when it is broken or crushed.
Mohs' method relies upon a scratch test to relate the hardness of a mineral specimen to the hardness of one of a set of reference minerals.
The characteristic of tenacity describes the physical behavior of a mineral under stress or deformation.
Most minerals are brittle; metals, in contrast, are malleable, ductile, and sectile. Most mineral species are brittle, and will crumble or fracture under pressure or upon the application of a blow. Such materials break or powder easily.
Is the color of a mineral substance when it has been ground to a fine powder.
Plane of structural weakness along which a mineral is likely to split smoothly.
Cleavage thus refers to the splitting of a crystal between two parallel atomic planes. Cleavage is the result of weaker bond strengths or greater lattice spacing across the plane in question than in other directions within the crystal. Greater lattice spacing tends to accompany weaker bond strength across a plane, because such bonds are unable to maintain a close interatomic spacing.
Quality of Cleavage
Refers to both the ease with which the mineral cleaves and to the character of the exposed cleavage surface. Described by terms such as 'eminent,' 'perfect,' 'distinct,' 'difficult,' 'imperfect,' or 'indistinct.'
Results in a series of smoothly curved concentric rings about the stressed point, generating a shell-like appearance. The familiar ripples of a broken glass bottle demonstrate this type of fracture.
As mass per unit volume:
A unit-less quantity which is defined as the ratio of the weight of a substance to the weight of an equal volume of water at a temperature of 4° Celsius. This ratio is equal to the ratio of the density of the substance to the density of water at 4° Celsius.
Refers to the quantity and quality of the light which is reflected from a mineral's exterior surfaces. Luster provides an assessment of how much the mineral surface 'sparkles'.
Categories of Luster:
Primarily divided into the three categories of metallic, sub-metallic and nonmetallic luster.
Minerals possessing metallic luster are opaque and very reflective, possessing a high absorptive index.
The unpaired electron which produces color by light absorption into excited states does not have to be located on a transition element ion; under certain circumstances it can be located on a nontransition-element impurity ion or on a crystal defect such as a missing ion.
Electron Color Center
If an electron is present at a vacancy, we have an "electron" color center; if an electron is missing from a location where there usually is an electron pair, we have a "hole" color center.