Sistem Kristal Isometrik
Sistem ini juga disebut sistem reguler bahkan sering dikenal sebagai sistem kubus atau kubik. Jumlah sumbu kristalnya tiga dan saling tegak lurus satu dengan yang lainnya. Masing-masing sumbu sama panjang.
Sistem kubus ini terbagi menjadi lima kelas, yaitu : 1. kelas hexoctahedral 2. kelas hextetrahedral 3. kelas gyroidal 4. kelas diploidal 5. kelas tetartoidal Kelas Hexoctahedral a. Kelas : ke-32 b. Simetri : 4/m 3bar 2/m
c. Elemen Simetri : merupakan klas yang paling simetri untuk bidang tiga dimensi dengan 4 sumbu putar tiga, 3 sumbu putar dua, dan sumbu putar dua. Dengan 9 bidang utama dan 1 pusat.
d. Garis Sumbu Kristal : tiga garis yang sama disimbolkan dengan a1, a2, dan a3.
e. Sudut : ketiga-tiganya 90o
f. Bentuk Umum : kubik, bidang delapan, bidang dua belas dan trapezium. Dan kadang-kadang trisoktahedron, tetraheksahedron, dan heksotahedron.
g. Mineral yang Umum : flurit, galena, intan, tembaga, besi, timah, platina, perak, emas, halit, bromargyrit, kllorargirit, murdosit, piroklor, kelompok garnet, sebagian besar kelompok spinel, uraninit dan lain-lain.
Kelas Hextetrahedral
a. Kelas : ke-31 b. Simetri : 4bar 3 m
c. Elemen Simetri : ada 4 sumbu putar tiga, 3 sumbu putar empat, dan 6 bidang kaca.
d.Sumbu Kristal : tiga sumbu sama panjang yang disebut a1, a2, dan a3.
e. Sudut : ketiga sudutnya = 90o
f. Bentuk Umum : empat sisi, tristetrahedron, deltoidal dodecahedron, dan hekstetrahedron serta yang jarang kubik, rhombik dodecahedron dan tetraheksahedron.
g. Mineral yang Umum : sodalit, sphalerit, domeykit, hauyne, lazurit, rhodizit, dan lain-lain.
Kelas Giroid
a. Kelas : ke-30 b. Simetri : 4 3 2
c. Elemen Simetri : terdapat 3 sumbu putar empat, 4 sumbu putar tiga, dan 6 sumbu putar dua.
d. Garis Sumbu Kristal : tiga garis yang sama disimbolkan dengan a1, a2, dan a3.
f. Bentuk Umum : kubik, octahedron, dodecahedron, dan trapezohedron, serta yang jarang trisoctahedron dan tetraheksahedron.
g. Mineral yang Umum : cuprit, voltait, dan sal amoniak.
Kelas Diploidal
a. Kelas : ke-29 b. Simetri : 2/m 3bar
c. Elemen Simetri : ada 4 sumbu putar tiga, 3 sumbu putar dua, 3 bidang kaca dan satu pusat. d. Garis Sumbu Kristal : tiga garis yang sama disimbolkan dengan a1, a2, dan a3.
e. Sudut : ketiga-tiganya 90o
f. Bentuk Umum : diploid dan pyritohedron dan juga kubik, octahedron, rhombik dodecahedron, trapezohedron dan yang jarang trisoctahedron.
g.Mineral yang Umum : pyrite, kobaltit, kliffordit, haurit, penrosit, tychit, laurit, dan lain-lain
Kelas Tetartoidal
a. Kelas : ke-28 b. Simetri : 2 3
c. Elemen Simetri : terdapat 4 sumbu putar tiga dan tiga sumbu putar dua.
d.Garis Sumbu Kristal : tiga garis yang sama disimbolkan dengan a1, a2, dan a3.
e. Sudut : ketiga-tiganya 90o
f. Bentuk Umum : tetartoidal yang unik, serta pyritohedron, kubik, deltoidal dodecahedron, pentagonal dodecahedron, rhombik dodecahedron, dan tetrahedron.
g.Mineral yang Umum : changcengit, korderoit, gersdorffit, langbeinit, maghemit, micherenit, pharmacosiderit, ullmanit, dan lain-lain.
Cubic crystal system
A network model of a simple cubic system.
The primitive and cubic close-packed (also known as face-centred cubic) unit cells.
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of these crystals, called simple cubic (sc), body-centered
cubic (bcc), and face-centered cubic (fcc), plus a number of other variants listed below.
Note that although the unit cell in these crystals is conventionally taken to be a cube, the primitive unit cell often is not. This is related to the fact that in most cubic crystal systems, there is more than one atom per cubic unit cell.
The three Bravais lattices which form cubic crystal systems are
Simple cubic (P) Body-centered cubic (I) Face-centered cubic (F)
The simple cubic system (P) consists of one lattice point on each corner of the cube. Each atom at the lattice points is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom (1⁄
8 × 8).
The body-centered cubic system (I) has one lattice point in the center of the unit cell in addition to the eight corner points. It has a net total of 2 lattice points per unit cell (1⁄
8 × 8
The face-centered cubic system (F) has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of 4 atoms per unit cell (1⁄
8 × 8 from the corners plus 1⁄2× 6 from the faces).
Attempting to create a C-centered cubic crystal system (i.e., putting an extra lattice point in the center of each horizontal face) would result in a simple tetragonal Bravais lattice.
Crystal classes
The isometric crystal system class names, examples, Schönflies notation, Hermann-Mauguin notation, point groups, International Tables for Crystallography space group number, orbifold, type, and space groups are listed in the table below. There are a total 36 cubic space groups.
Crystal class Example Schönflies
Hermann-Mauguin notation
point
groups # orbifold Type
Tetaroidal Ullmannite T 23 23 195-199 332 enantiomorphic Diploidal Pyrite Th 2/m3 m3 200-206 3*2 centrosymmetric
Gyroidal Petzite O 432 432 207-214 432 enantiomorphic
Hextetrahedral Sphalerite Td 43m 43m 215-220 *332
Hexoctahedral Galena Oh 4/m32/m
m3m 221-230 *432 centrosymmetric
Crystal class Cubic space groups Tetaroidal P23 F23 I23 P213 I213
Diploidal Pm3 Pn3 Fm3 Fd3 I3 Pa3 Ia3
Gyroidal P432 P4232 F432 F4132 I432 P4332 P4132 I4132
Hextetrahedral P43m F43m I43m P43n F43c I43d
Hexoctahedral Pm3m Pn3n Pm3n Pn3m Fm3m Fm3c Fd3m Fd3c Im3m Ia3d
Other terms for hexoctahedral are: normal class, holohedral, ditesseral central class, galena type.
Atomic packing factors and examples
One important characteristic of a crystalline structure is its atomic packing factor. This is calculated by assuming that all the atoms are identical spheres, with a radius large enough that each sphere abuts the next. The atomic packing factor is the proportion of space filled by these spheres.
Assuming one atom per lattice point, in a simple cubic lattice with cube side length a, the sphere size would be a⁄
2 and the atomic packing factor turns out to be about 0.524 (which is
quite low). Similarly, in a bcc lattice, the atomic packing factor is 0.680, and in fcc it is 0.740. The fcc value is the highest theoretically possible value for any lattice, although there are other lattices which also achieve the same value, such as hexagonal close packed and one version of tetrahedral bcc.
As a rule, since atoms in a solid attract each other, the more tightly-packed arrangements of atoms tend to be more common. (Loosely packed arrangements do occur, though, for example if the orbital hybridization demands certain bond angles.) Accordingly, the simple-cubic structure, with especially low atomic packing factor, is rare in nature, but is found in polonium. Thebcc and fcc, with their higher densities, are both quite common in nature. Examples of bcc include iron, chromium, tungsten, and niobium. Examples of fcc include lead (for example inlead(II) nitrate), aluminum, copper, gold and silver.
Multi-element compounds
Compounds that consist of more than one element (e.g. binary compounds) often have crystal structures based on a cubic crystal system. Some of the more common ones are listed here.
Interpenetrating simple cubic (caesium chloride) structure
A caesium chlorideunit cell. The two colors of balls represent the two types of atoms.
One structure is the "interpenetrating simple cubic" structure, also called the "caesium chloride" structure. Each of the two atom types forms a separate simple cubic lattice, with an atom of one type at the center of each cube of the other type. Altogether, the arrangement of atoms is the same as body-centered cubic, but with alternating types of atoms at the different lattice sites.
Examples of compounds with this structure include caesium chloride itself, as well as certain other alkali halides when prepared at low temperatures or high pressures. More generally, this structure is more likely to be formed from two elements whose ions are of roughly the same size (for example, ionic radius of Cs+= 167 pm, and Cl− = 181 pm) .
The space group of this structure is called Pm3m (in Hermann-Mauguin notation), or "221" (in the International Tables for Crystallography). The Strukturbericht designation is "B2".
The rock-salt crystal structure. Each atom has six nearest neighbors, withoctahedral geometry.
The coordination number of each atom in the structure is 8: the central cation is coordinated to 8 anions on the corners of a cube as shown, and similarly, the central anion is coordinated to 8 cations on the corners of a cube.
Rock-salt structure
Another structure is the "rock salt" or "sodium chloride" (halite) structure. In this, each of the two atom types forms a separate face-centered cubic lattice, with the two lattices interpenetrating so as to form a 3D checkerboard pattern.
Examples of compounds with this structure include sodium chloride itself, along with almost all other alkali halides, and "many divalent metal oxides, sulfides, selenides, and tellurides". More generally, this structure is more likely to be formed if the cation is slightly smaller than the anion (a cation/anion radius ratio of 0.414 to 0.732).
The space group of this structure is called Fm3m (in Hermann-Mauguin notation), or "225" (in the International Tables for Crystallography). The Strukturbericht designation is "B1". The coordination number of each atom in this structure is 6: each cation is coordinated to 6 anions at the vertices of an octahedron, and similarly, each anion is coordinated to 6 cations at the vertices of an octahedron.
Zincblende structure
A zincblende unit cell
Another common structure is the "zincblende" structure (also spelled "zinc blende"), named after the mineral zincblende (sphalerite). As in the rock-salt structure, the two atom types form two interpenetrating face-centered cubic lattices. However, it differs from rock-salt structure in how the two lattices are positioned relative to one another. Altogether, the
arrangement of atoms is the same as diamond cubic structure, but with alternating types of atoms at the different lattice sites.
Examples of compounds with this structure include zincblende itself, many compound semiconductors (such as gallium arsenide and cadmium telluride), and a wide array of other binary compounds.
The space group of this structure is called F43m (in Hermann-Mauguin notation), or "#216" in the International Tables for Crystallography. The Strukturbericht designation is "B3".
Sumber:
http://en.wikipedia.org/wiki/Cubic_crystal_system
http://www.scribd.com/doc/16168944/Kristal-Secara-Sederhana-Dapat-Didefinisikan-Sebagai-Zat-Padat-Yang-Mempunyai-Susunan-Atom-Atau-Molekul-Yang-Teratur
PRAKTIKUM
KRISTAL DAN MINERAL
Nama
: Meiliza Fitri
NIM
: 12109038
Shift
: Rabu Sore
Asisten
: 1. M. Nur Ilham (12107035)
2.
