The crystal habit of a mineral describes its visible external shape. It can apply to an individual crystal or an assembly of crystals.
In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.
Factors influencing a crystal's habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies like growth striations. Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the same mineral; corundum.
Some minerals may replace other existing minerals while preserving the original's habit: this process is called pseudomorphous replacement. A classic example is tiger's eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
The names of crystal habits are derived from:
Predominant crystal faces (prism - prismatic, pyramid - pyramidal and pinacoid - platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).
Read more about Crystal Habit: List of Crystal Habits
Famous quotes containing the words crystal and/or habit:
“Madame, ye been alle beautee shrine
As fer as cercled is the mapemounde:
For as the crystal glorious ye shine,
And like ruby been youre cheekes rounde.”
—Geoffrey Chaucer (1340?–1400)
“Much unhappiness comes from walking alone. When there are several, it’s somewhat different. I must get into the habit of listening to others, for what the others say concerns me, too.”
—Alfred Döblin (1878–1957)