Angles of cyrstals and the law of constancy of interfacial angles

In any crystals these are threee types of angles 

1. Plans angle 

2. Dihedral interficial angle 

3. Polyhedral or solid angle 

The law of constancy of angle states: if crystals of the same substance can have different habits depending on the number and size of faces, but the angle between the corresponding faces remained constant.

Measurements of interficial angle 

This is done with goniometer there are two types of goniometer

1.contact goniometer

2. Reflecting goniometer

Note: The interficial angle is the most important and is the angle between any two faces a of a Crystal. It is the angle between two normal to two faces or planes. In the diagram below is the interficial angle where as A is the plane angle between F and F1.

Crystal habit is the tendency for specimens of a to repeatedly grow into characteristic shapes. These shapes are influenced by the atomic structure of the mineral, but they can also be influenced by the environment of crystal growth. Regardless of influence, crystal habit shapes are characteristic of the mineral and displayed by many specimens of that mineral. Crystal habit is an external shape displayed by an individual crystal, but more often it is an external shape displayed by an aggregate of crystals. Crystal habit names are often adjectives that help convey the shape of a crystal or a group of crystals.

Bladed, cubic, fibrous, granular, prismatic, and radiating are names of crystal habits that quickly convey a generalized geometric appearance. Other crystal habit names are less familiar, but once a person learns the name it can easily be applied to mineral specimens to which that habit applies.

Many minerals have characteristic habits which can serve as clues to their identification. However, many minerals do not have a characteristic external shape, and the name of this habit is “massive”. In addition, most specimens of any mineral do not display a characteristic habit.

In rare instances, one mineral might replace crystals of another mineral to assume its characteristic habit. Crystals formed in this situation are known as pseudomorphs.

What is a Crystal form

“Crystal form” is a concept similar to “crystal habit”. A crystal form is a solid crystalline object that is bounded by a set of flat faces that are related to one another by symmetry. Euhedral crystals are the best representations of a crystal form. Examples include:

 Cubicrystals of halite or fluorite

Octahedral crystals of diamond or fluorite

Dodecahedral crystals of garnet

Hexagonal crystals of quartz or corundum

Pisolitic minerals occur in crystalline aggregates that are rounded and about the size of peas. Individual pisolites are made up of many tiny radiating mineral crystals. They often develop a concentric structure formed when crystalline aggregate layers grow to enlarge the pisolites. Similar to oolitic, but pisolites are much larger than oolites. Mineral example:  The specimen of bauxite in the photo is about six centimeters across and contains numerous pisolites that are about the size of peas.

Columnar crystals are long prisms with enough width that the name acicular (or needle-like) does not apply. A single "column" might contain multiple parallel crystals. Mineral examples: calcite, tourmaline, and gypsum. These enormous crystals of selenite gypsum have a columnar habit. They are in the "Cave of the Crystals" cavern, Chihuahua, Mexico (a person in the lower-right quadrant of the photo serves as scale). These are some of the largest well-formed crystals in the world. The photo is a Wikimeida OTRS image by Alexander Van Driessche.

Cubic crystals of pyrite. Fluorite and halite are two common minerals with a cubic shape. Cubes have six square faces and four-fold rotational symmetry around three axes. The photo shows cubic crystals of pyrite from Navajún, Rioja, Spain, that have grown in a marlstone. Specimen is approximately 4 inches (9.5 centimeters) across. Image by Carles Millan and used under a Creative Commons license.

Fibrous is a habit name used when minerals occur in very fine fiber-like crystals. They are often so fine that they look like fine hair. The habit also includes aggregates made up of a large number of parallel or radial fibers. Mineral examples: actinolite, chrysotile, serpentine, and tremolite. The actinolite crystals on this rock have a fibrous habit. Because of their fibrous shape (a roughly 1:20 aspect ratio) and properties, fibrous crystals of actinolite are regulated as asbestos. This photo is a Creative Commons image published by Didier Descouens.

Hopper crystals are partially formed crystals that have experienced more rapid growth on their outer edges than in the center of the crystal. This causes them to be well-developed on the outer edges but less developed or "hollow" in the center. Halite (shown above) is one of the best-known examples of a mineral that sometimes displays the hopper crystal habit. Other mineral examples: galena and ice.