- rate of cooling
- amount of silica
- amount of gases
In underground situations, magma can take years, even thousands of years to cool. That gives atoms and molecules plenty of time to move around, eventually finding homes in slowly growing crystals. There is not a huge number of individual crystals, so there is open space between the growing crystals, especially during early stages of cooling. Eventually, as the crystals grow larger and larger, they will bump into one another, or late-forming minerals will grow in the spaces between the early-forming crystals. Of course, it is liquid (magma) that occupies the space between crystals. The chemistry of the magma will change, inevitably, as growing minerals remove certain atoms and molecules from the magma, leaving a changing proportion of remaining atoms and molecules. Thus, we say that magmas evolve.
Near the surface, where rocks are generally cool or just slightly warm, and especially on the surface, where it is only "room temperature," magma cools more rapidly. A tremendous number of super-tiny crystals nucleate (begin to grow from just a few starting atoms). These crystals are not able to grow very large, because there are so many of them, that before too long they bump into one another. And, there is less space between all those tiny crystals. In the extreme, when cooling is so rapid that magma is quenched (as in drinking water to quench your thirst), minerals don't even get a chance to grow, and the material solidifies to form volcanic glass. Did you catch that? There are no minerals in glass, just atoms and molecules. The atoms and molecules are not in any particular order or arrangement, and we say the structure is amorphous (without structure).
A for not, phan for visible, or apparent. The crystals in an aphanitic texture are too small to be seen with the naked eye (without using a microscope). Aphanitic textures, as per the discussion above, are associated with igneous rocks formed from fast-cooled magma. If the magma has a large amount of gas, gas bubbles can be trapped within the magma as the crystals grow, and when it is all solid, the gas bubbles can form a vesicular texture, but the rock itself, around the holes, still has an aphanitic texture. Extrusive (volcanic) rocks often have aphanitic textures. Examples include rhyolite, andesite, and basalt.
Phan for visible, or apparent. As you might expect, a phaneritic texture is one where the crystals are large enough to be seen with the naked eye. These textures are, of course, associated with slower cooling rates. And, generally, the larger the crystals, the slower the cooling rate indicated. Intrusive (plutonic) igneous rocks often have phaneritic textures. Examples include granite, diorite, and gabbro.
The name actually means purple, but that is just because volcanic rock with this texture, when first described, had this color. This texture simply refers to a mixture of crystal sizes. There are a couple of terms, however. There are often large crystals, surrounded by a matrix of small crystals. The large crystals are called phenocrysts. The matrix (surrounding volume) of smaller crystals is called the groundmass. A porphyritic rock can just be called a porphyry.
As discussed above, when describing the effects of fast cooling, volcanic glass, called obsidian, doesn't contain mineral crystals, but an amorphous solidification of atoms and molecules. But, the odd thing is that, technically, glass is not a proper solid, but an amorphous (structureless) solid, having an internal arrangement of atoms akin to that of a liquid. Because of this amorphous state, glass has been described as an extremely viscous (refers to a slow flow rate) liquid, but the flow rate of glass is effectively zero. See http://en.wikipedia.org/wiki/Glass for a debunking of the notion that glass panes in old churches are thicker at the bottom because of slow flow [A debunking I welcome, after repeating this urban myth!]. Regarding viscosity, in everyday usage we can say that molasses is "thicker" than water. In scientific usage, we would say that molasses is more viscous than water. Higher viscosity means more sluggish movement, more "gooey." Less viscosity means more fluid, more "runny." Glass forms from very viscous magmas. The viscosity, the "gooeyness," is part of the reason why crystals don't form, because it is difficult for atoms and molecules to move through the fluid, even if there were growing crystals to reach. Be careful with obsidian, because, as you know, glass breaks into sharp-edged pieces that can cut you. Also, such viscous magmas trap gas bubbles very well, and can form a glassy material called pumice, which can have so many trapped bubbles that it will actually float on water (Try it).
Pyro, for fire. Clastic for particles, fragments, grains, clasts. A pyroclastic texture is one containing pieces of material blown out of a volcano, to settle out of the air nearby, or, in the case of the super-fine-grained volcanic ash, even many miles away from the volcano. A deposit of volcanic ash is called tuff. A deposit of ash that was still hot when it landed, and the sticky ash particles welded together, is called welded tuff. Pyroclastic material isn't always fine-grained ash, however, as it includes large blocks of rock or blobs of magma blown out of a volcano. Igneous rocks with pyroclastic textures are usually associated with volcanoes fed by silica-rich magma. The increased amount of silica makes the magma more viscous and, as a consequence, more effective at trapping dissolved gases and holding back pressure, causing the flow system to plug up. A plugged up volcano is not a good thing, as eventually it will let out a blast of pent up pressure, fragmenting rock and partially solidified magma, and sending it up and away. Such volcanic eruptions can be dramatically explosive.
This texture refers to the extreme in large size for crystals. The magmas forming pegmatites have much water in them, which makes for a very fluid magma. Atoms and molecules are able to easily move through the liquid, such that growing crystals get a steady supply of needed chemical elements. As a result, the crystals can grow very large, even to the point of being ridiculously large, the size of a house almost. Most pegmatites are essentially very coarsely crystalline granites.