A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. The word crystal is derived from the ancient Greek word κρύσταλλος (krustallos), which had the same meaning, but according to the ancient understanding of crystal. At root it means anything congealed by freezing, such as ice. The word once referred particularly to quartz, or "rock crystal".
The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as crystallization. In the old example referenced by the root meaning of the word crystal, water being cooled undergoes a phase change from liquid to solid beginning with small ice crystals that grow until they fuse, forming a polycrystalline structure. The physical properties of the ice depend on the size and arrangement of the individual crystals, or grains, and the same may be said of metals solidifying from a molten state.
Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. While the cooling process usually results in the generation of a crystalline material, under certain conditions, the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent intense warmth of fusion.
Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or upon crystallization from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization. Weak van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite.
Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can contain a profound effect on the properties of the materials.
Since the initial discovery of crystal-like individual arrays of atoms that are not regularly repeated, made in 1982 by Dan Shechtman, the acceptance of the concept and the word quasicrystal have led the International Union of Crystallography to redefine the term crystal to mean "any solid having an essentially discrete diffraction diagram", thereby shifting the essential attribute of crystallinity from position space to Fourier space. Within the family of crystals one distinguishes between traditional crystals, which are periodic, or repeating, at the atomic scale, and aperiodic (incommensurate) crystals which are not. This broader definition adopted in 1996 reflects the current understanding that microscopic periodicity is a sufficient but not a necessary condition for crystals.
While the term "crystal" has a precise meaning within materials science and solid-state physics, colloquially "crystal" refers to solid objects that exhibit well-defined and often pleasing geometric shapes. In this sense of the word, many types of crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and table salt are common examples of crystals.
Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect or the piezoelectric effect. Additionally, light passing through a crystal is often refracted or bent in different directions, producing an array of colors; crystal optics is the study of these effects. In periodic dielectric structures a range of unique optical properties can be expected as seen in photonic crystals.
Inorganic matter, if free to take that physical state in which it is most stable, tends to crystallize. There is no practical limit to the size a crystal may attain under the right conditions, and selenite single crystals in excess of 10 m are found in the Cave of the Crystals in Naica, Mexico.
Crystalline rock masses have consolidated from aqueous solution or from molten magma. The vast majority of igneous rocks belong to this group and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized, but many lavas were poured out at the surface and cooled very rapidly; in this latter group a small amount of amorphous or glassy matter is frequent. Other crystalline rocks, the evaporites such as rock salt, gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in arid climates. Still another group, the metamorphic rocks which includes the marbles, mica-schists and quartzites; are recrystallized, that is to say, they were at first fragmental rocks, like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution. The high temperature and pressure conditions of metamorphism have acted on them erasing their original structures, and inducing recrystallization in the solid state.
|Crystal||Particles||Attractive forces||Melting point||Other properties|
|Ionic||Positive and negative ions||Electrostatic attractions||High||Hard, brittle, good electrical conductor in molten state|
|Molecular||Polar molecules||London force and dipole-dipole attraction||Low||Soft, non-conductor or extremely poor conductor of electricity in liquid state|
|Molecular||Non-polar molecules||London force||Low||Soft conductor|
|Sidney Lanier composed this poem in 1880 in Baltimore, Maryland.|
At midnight, death’s and truth’s unlocking time,
When far within the spirit’s hearing rolls
The great soft rumble of the course of things—
A bulk of silence in a mask of sound,—
When darkness clears our vision that by day
Is sun-blind, and the soul’s a ravening owl
For truth and flitteth here and there about
Low-lying woody tracts of time and oft
Is minded for to sit upon a bough,
Dry-dead and sharp, of some long-stricken tree
And muse in that gaunt place,—’twas then my heart,
Deep in the meditative dark, cried out:
“Ye companies of governor-spirits grave,
Bards, and old bringers-down of flaming news
From steep-wall’d heavens, holy malcontents,
Sweet seers, and stellar visionaries, all
That brood about the skies of poesy,
Full bright ye shine, insuperable stars;
Yet, if a man look hard upon you, none
With total lustre blazeth, no, not one
But hath some heinous freckle of the flesh
Upon his shining cheek, not one but winks
His ray, opaqued with intermittent mist
Of defect; yea, you masters all must ask
Some sweet forgiveness, which we leap to give,
We lovers of you, heavenly-glad to meet
Your largesse so with love, and interplight
Your geniuses with our mortalities.
Thus unto thee, O sweetest Shakespeare sole,
A hundred hurts a day I do forgive
(’Tis little, but, enchantment! ’tis for thee):
Small curious quibble; Juliet’s prurient pun
In the poor, pale face of Romeo’s fancied death;
Cold rant of Richard; Henry’s fustian roar
Which frights away that sleep he invocates;
Wronged Valentine’s unnatural haste to yield;
Too-silly shifts of maids that mask as men
In faint disguises that could ne’er disguise—
Viola, Julia, Portia, Rosalind;
Fatigues most drear, and needless overtax
Of speech obscure that had as lief be plain;
Last I forgive (with more delight, because
’Tis more to do) the labored-lewd discourse
That e’en thy young invention’s youngest heir
Besmirched the world with.
Father Homer, thee,
Thee also I forgive thy sandy wastes
Of prose and catalogue, thy drear harangues
That tease the patience of the centuries,
Thy sleazy scrap of story,—but a rogue’s
Rape of a light-o’-love,—too soiled a patch
To broider with the gods.
Thou dear and very strong one, I forgive
Thy year-worn cloak, thine iron stringencies
That were but dandy upside-down, thy words
Of truth that, mildlier spoke, had mainlier wrought.
So, Buddha, beautiful! I pardon thee
That all the All thou hadst for needy man
Was Nothing, and thy Best of being was
But not to be.
Worn Dante, I forgive
The implacable hates that in thy horrid hells
Or burn or freeze thy fellows, never loosed
By death, nor time, nor love.
And I forgive
Thee, Milton, those thy comic-dreadful wars
Where, armed with gross and inconclusive steel,
Immortals smite immortals mortalwise
And fill all heaven with folly.
Brave Aeschylus, thee I forgive, for that
Thine eye, by bare bright justice basilisked,
Turned not, nor ever learned to look where Love
So, unto thee, Lucretius mine
(For oh, what heart hath loved thee like to this
That’s now complaining?), freely I forgive
Thy logic poor, thine error rich, thine earth
Whose graves eat souls and all.
Yea, all you hearts
Of beauty, and sweet righteous lovers large:
Aurelius fine, oft superfine; mild Saint
A Kempis, overmild; Epictetus,
Whiles low in thought, still with old slavery tinct;
Rapt Behmen, rapt too far; high Swedenborg,
O’ertoppling; Langley, that with but a touch
Of art hadst sung Piers Plowman to the top
Of English songs, whereof ’tis dearest, now,
And most adorable; Caedmon, in the morn
A-calling angels with the cow-herd’s call
That late brought up the cattle; Emerson,
Most wise, that yet, in finding Wisdom, lost
Thy Self, sometimes; tense Keats, with angels’ nerves
Where men’s were better; Tennyson, largest voice
Since Milton, yet some register of wit
Wanting;—all, all, I pardon, ere ’tis asked,
Your more or less, your little mole that marks
You brother and your kinship seals to man.
But Thee, but Thee, O sovereign Seer of time,
But Thee, O poets’ Poet, Wisdom’s Tongue,
But Thee, O man’s best Man, O love’s best Love,
O perfect life in perfect labor writ,
O all men’s Comrade, Servant, King, or Priest,—
What ‘if’ or ‘yet’, what mole, what flaw, what lapse,
What least defect or shadow of defect,
What rumor, tattled by an enemy,
Of inference loose, what lack of grace
Even in torture’s grasp, or sleep’s, or death’s,—
Oh, what amiss may I forgive in Thee,
Jesus, good Paragon, thou Crystal Christ?”
(Ezek 1:22, with the epithet "terrible," as dazzling the spectators with its brightness). The word occurs in Rev 4:6; Rev 21:11; Rev 22:1. It is a stone of the flint order, the most refined kind of quartz. The Greek word here used means also literally "ice." The ancients regarded the crystal as only pure water congealed into extreme hardness by great length of time.
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