DESCRIPTIVE CRYSTAL HABITS?
The following terms are more descriptive than the
technical crystallographic terms and are therefore easier to understand.
There are basically two types: single crystal forms and aggregate forms.
The single crystal terms are used to describe individual crystals and so terms like platy or prismatic are used.
Aggregate terms are for minerals whose crystals come in groups of at times literally thousands of crystals and form a unique shape.
Individual crystals in these aggregates are usually hard to discern.
Terms such as dendritic and botryoidal are used to describe these.
All these terms are subjective and most minerals form more than one.
Some habits differ only in the slightest of ways and differntiation between these is therefore not very diagnostic.
However there are some minerals that seem to always be found in one habit.
Some examples of habits are pictured and each habit has a link to a mineral that has been known to be found in this habit, but remember few minerals are limited to just one habit.
These are some of the more common descriptive terms used to portray certain crystal habits of both individual and aggregate crystal varieties:
Individual crystal varieties:
- Long and needle-like, thinner than prismatic but thicker than
Natrolite crystals can be good examples of acicular crystals.
- Elongated and flattened like a blade of grass.
More elongated than platy and thinner than
Kyanite forms crystals that are a good example of bladed crystals.
- Rectangular and box-like, but not necessarily with flat sides.
More elongated than equant but less elongated than
prismatic and thicker than
Oligoclase forms crystals that are a good example of a blocky habit.
- Any three perpendicular axis through the crystal are more or less equal.
Can be used to describe rounded as well as angular crystals.
Fluorite forms crystals that are a good example of equant crystals.
- Thinner than acicular crystals in either individual crystals or in a tight compact almost cloth-like mass.
Okenite forms crystals that are a good example of the fibrous habit.
- Flattened and thin crystals (like plates) but wider than bladed and thinner than tabular.
Crystals of wulfenite generally show good examples of the platy crystal habit.
- Phantomed crystals occur when a crystal stops growing and then for some reason continues to grow.
During the hiatus in growth, small microcrystals of other minerals or even the same mineral may grow on the surface of the crystal.
Once the main crystal begins to grow again, it grows out and around these small crystals trapping them as a layer inside.
This layer has the shape of the crystal at the time it stopped growing and often has a ghostly look to it, hence the name phantom.
quartz are two minerals that forms phantomed crystals more often then others.
- One of the most common of crystal habits.
Prismatic crystals are "pencil-like", elongated crystals that are thicker than needles (see acicular).
Indicolite (a variety of
elbaite) forms good examples of prismatic crystals.
- A pseudomorph
(which mean false shape in Latin)
is a crystal that has replaced another mineral's chemistry or structure with its own without changing the outward shape of the original mineral.
Transformations from one mineral to another are not unusual in nature, but preserving the outward shape of the original mineral is!
The end result is that the crystal appears to be one mineral but is actually another.
The quartz specimen above was once
- Sceptered crystals occur when a crystal stops growing and then for some reason continues growth on the upper portion of the crystal but not the lower portion.
The result on prismatic crystals is a shape that appears like a jeweled royal scepter of kings and queens.
Scepters are quite popular and rare!
Smoky quartz, a variety of
quartz, forms sceptered crystals.
- Slightly more elongated than
equant but not as elongated as
prismatic and possibly more rounded than
Topaz forms crystals that are a good example of stubby crystals.
- Book-like (tablets) that are thicker than platy but not as elongated as
Wulfenite forms crystals that are a good example of tabular crystals.
Aggregate Crystal Varieties:
- Branching, tree-like clusters similar to dendritic.
Some of the best examples of the arborescent crystal form are found in many specimens of
- Resembling grape bunches with interlocking rounded masses.
Formed from acicular or bladed crystals growing from a common site for each rounded mass.
The tops of the crystals are smooth and blend so that individual crystal edges are indiscernible except from broken edges.
Botryoidal is similar to globular and
mammillary; but more of an aggregation of rounded masses.
Sub-botryoidal has more discernible crystals.
both form aggregates that are good examples of this form.
- A branching growth of crystals usually on a surface or as an inclusion that forms plant-like patterns similar to "Jack Frost" on windows and similar to
arborescent, but less tree-like.
Sal ammoniac forms aggregates that are a good example of this form.
- Outwardly oriented crystals usually lining the inside of a geode, but is also applied to other outwardly oriented crystal coatings.
Amethyst, a variety of
quartz, forms aggregates that are a good example of this form.
- Thin crust of crystalline material over host rock.
Sometimes forms from evaporation, efflorescence or simple precipitation.
Uranocircite forms aggregates that are a good example of this form.
- A hollow stone embedded in a layer of rock mineralogically different from the mineral composing the outer shell of the geode (see vugs).
Celestite is found in good examples of this form.
- Bubbly, rounded masses, similar to
mammillary but less coherent.
Gyrolite forms aggregates that are a good example of this form.
- Crystals of a small size, less than 1 cm across, that exhibit no really discernable crystal form.
Usually applied to many tiny crystals separated and disseminated throughout a host rock.
Franklinite forms crystals that are a good example of this habit.
- Hopper Crystals
- Crystals were their edges grew faster than their faces forming a "skeletal" like structure.
Minerals that form this type of habit usually grow quickly and crystals grown artificially,
see bismuth, often show this form.
Halite often forms crystals demonstrating this habit.
- Layered masses like sheets of paper.
Muscovite forms aggregates that are a good example of this form.
- Rounded, literally "breast-like" masses, similar to
botryoidal and globular but more rounded, larger individual masses.
kidwellite forms aggregates that are a good example of this form.
- Indiscernible masses of crystals usually too fine to see.
Lazurite forms massive examples.
- Flaky to platy crystals compacted together in sparkling masses.
Similar to lamellar but with smaller crystals.
Muscovite, the most common member of the Mica Group, forms aggregates that are a good example of this form.
- A stone of concentric growth around a center usually composed of mostly one mineral.
Concretions fall under this term.
If broken, may show concentric rings.
Forms nearly spherical balls, flat round "suns" to oddly shaped concretions.
Pyrite forms good examples of this form.
- Rounded pebble to sand sized nodules in a compact mass formed in sedimentary environs.
Hematite forms masses that are a good example of this form.
- Rounded marble to pebble sized nodules in a compact mass formed in certain sedimentary environments.
Nodules are larger than oolitic forms.
Gibbsite forms masses that are a good example of the this form.
- Distinct crystals arranged in an orientation outward from a common point.
Atacamite forms aggregates that are a good example of this form.
- Petal-like crystals arranged in a flattened radial habit around a central point.
Barite roses are good examples of the rosette form.
- A concretionary growth sometimes around a hollow tube, producing long, slowly tappering, rounded masses.
Cross-sections usually have circular rings like the rings of a tree.
Usually formed in caves or other voids in rocks from the precipitation of a mineral from an evaporating fluid.
malachite form aggregates that are attractive examples of this form.
- Similar to botryoidal, but with more crystalline components in the interlocking rounded masses.
typically form aggregates that are a good example of this form.
- A void in a rock whose "shell" contains the same mineral as the host rock (see
A vug can be thought of as a hole created by dissolving the host rock whereas a geode is formed by precipitating a hollow nodule in the host rock.
The cavity is sometimes filled with an assortment of minerals, often different than the host rock however.
A variety of Quartz known as Herkimer Diamonds are found inside of limestone vugs.