Week2 D2 Flashcards
Study of crystal bodies and the laws that dictate parameters such as growth, shape and geometry
Crystallography
A term that denotes the ordered arrangement of atoms in an object’s structure
Crystalline
Subtances with microscopic level of crystalinity
Cryptocrystalline
Those without any crystalline structure
Amorphous
minute particles that are arranged in a 3D pattern that keeps repeating almost indefinitely
lattice
A powerful method in determining the structural constrains of minerals. It utilizes monochromatic radiation and that is beamed on a crystal rotated about its major axis
Rotation method
Smooth planar surfaces resulting from cleavage breaks.
Cleavage planes
An instrument used to measure interfacial crystal angles
Goniometers
Considered to be a guarantee of the chemical homogeneity of a mineral
A. Crystal B. Color C. Streak D. Chemical composition
A. Crystal
Classification of crystal classes/systems is based mainly on
symmetry
crystal is said to have a ___ if an imaginary line can be passed from any point on its surface through its center and a similar point is found on the line at an equal distance beyond the center.
Center of Symmetry
How many crysal classes are possible
32
Symmetry of a Hexoctahedral crystal class
C, 3A4, 4A3, 6A2, 9P
Symmetry of a Dihexagonal-Dipyramidal crystal class
C, 1A6, 6A2, 7P
The only crystal class with not symmetry
Pedial
Definition of a mineral
A mineral is an object that is naturally occuring, stable at room temperature, formed by inorganic process, represented by a chemical formula, and has an ordered atomic arrangement
Three prominent modes of crystallization
Solution, Fusion, Vapor
What is a unit cell
smallest portion of a crystal that still exhbit its properties
What is X-ray diffraction
The patterned interference of incident x-ray beams when passing through a crystal due to their uniform spacing.
Briefly explain the Laue photograph
A photographic record that shows the pattern of xray beams that diffracts when passing through a mineral. The dark spots indicate the positoins of the crystal lattice and the arrangement is a manifestation of a crystal’s symmetry.
Define cleavage
Tendency of a crystal to split into perfect planes due to inherent weaknesses in a crystal lattice
The outward form of a mineral depends on what factors?
The unit shape and the environment in which they are formed
External influences found in a mineral’s environment include
temperature, pressure, nature of solution, speed of crystal growth, surface tension, and direction of the movement of the solution.
Face positions of a crystal is dependent on
which direction through the internal structure that has the greatest number of crystal units
Fundamental symmetry operations
- Rotation about an axis 2. reflection in a plane 3. combined rotation and reflection
Define symmetry plane
imaginary plane which divides a crystal into halves, each of which, in a perfectly developed crystal, is the mirror image of the other.
Define symmetry axis
an imaginary line through a crystal about which the crystal may be revolved and repeat itself in appearance two or more times during a complete revolution.
What is a binary symmetry.
A crystal is repeated twice in appearance during a complete revolution
What are the possible axes that a crystal can manifest
2-, 3-, 4-, and 6- fold axes
What would be the symmetry notation of a crystal with a center, 3 two fold symmetry, four 3 fold symmetry, and 3 planes
C, 3A2, 4A3, 3P
Crystal classes belonging to Monoclinic
Prismatic, Spheroidal, Domatic
Crystal classes belonging to Triclinic
Pinacoidal, Pedial
Enumerate the six crystal systems
Isometric, hexagonal, tetragonal, orthohombic, monoclinic, triclinic
Describe the isometric system
has three mutually perpendicular axes with same lengths
Describe the hexagonal system
has 4 axes, three equal horizontal axes intersect at 60 degrees, fourth axes has different length and intersects perpendicular to the other three axes
How does a solution based crystallization happen and give examples?
A brine solution (common salt + water) becomes supersaturated where it comes to a point that no solute can be held and salt must be precipirated. Under the right conditions (slow evaporation of water), salt separates from water and crystals will eventurally form.
How does temp and pressure relate to crystallization?
Lowering temperature and/or pressure can result in the formation of crystals. Solutions with higher PT conditions can slightly dissolve more. Lowering those conditions would lower its capacity and thus making it supersaturated enough to encourage crystal formation.
How does a fusion based crystallization happen and give examples?
A change in temperature would cause atoms to be fixed in their position and arrange themselved in order to build a crystalline mass. Ice for example is fused water then temperatures drop. Similarly, magmas fused into crystalline masses under certain temperatures.
How does a vapor based crystallization happen and give examples?
A solid with a crystal structure is formed from the cooling of gas. The formation of sulfur crystals from fumaroles is an example.
Can a unit cell be small as an atom?
No. The interrelation of atoms are an important aspect in determining the properties of a specific substance and so a subdivision smaller than the unit cell wouldn’t have the properties of a mineral. Thus, a Na and a Cl atom separately has different properties when compared to the mineral Halite (NaCl)
How is Xray diffraction relevant in a crystallography
X ray diffraction results when the gaps in the atomic structure filters x-rays in such a way that geometrical patterns can be seen when they fall on a photographic plate. Such patterns are used in studying the internal strucutre of crystals
In a nutshell, what are the steps involved in the powdered method.
The powdered method is another way of analysing the internal structure of a crystal. In this manner, the sample is grinded into a powder and is then subjected to x-ray beam. Diffracted radiation is recorded on a film in which the structure can be calculated.
Describe the cleavage of halite, relate it to its crystal structure
Cleaves well on three planes at right angles to each other. Cleavages are always parallel to the external facets of the crytal strucutre.
Glass does not produce optical properties (such as double refraction) in contrast to other similarly transparent objects such as calcite. Why is this the case?
Glass lacks the definite internal structure that can distort light passing through unlike calcite.
Why is it that flourite can exhibit cubic and octahedral crystal habits even if the building blocks have the same internal structure?
A difference in the external factors of a mineral’s environment is a cause of the variations growth rates in different directions. Thus making the external expressions different despite having the same internal structure.
Explain and Illustrate the Law of the constancy of interfacial angles
The law states that the angles are the same in corresponding faces on crystals of the same substance.
Explain and Illustrate the Axis of rotary Reflection
combines a rotation about an axis with reflection across a plane at right angles to the axis.
Explain the dual nature of light
Light can be described as both a particle and a wave. Light as explained in the particle theory is composed of subatomic particles called photons which are produced when valence electrons of an excited ion relases energy. The wave theory meanwhile described light as a transverse wave that has elctrical and magnetic properties.
Why is the electric vector of the EM wave important in optical mineralogy.
The interaction between the electric vector and the electrical character of atoms and chemical bonds of a mineral greatly affects the behviour of light. These behaviour is extensively studied in optical mineralogy.
Describe and Illustrate the light wave.
A lightwave can be mathematically described as f=V/lamda where f is frequecy, V is velocity and lamda is the wavelength. The velocity of light is a constant, so if f changes, so does V.
