Rock Cycle - Minerals (3A)

  • Analyzing characteristic shapes of minerals.
  • Comparing mineral shapes.
  • cubic
  • dipyramid
  • prism
  • rhombohedron
  • tabular

Students determine shapes of minerals.



Minerals are pure substances composed of one or more elements. Internally, a mineral has a repeating atomic structure or crystalline pattern. This structure is similar to the mathematical term "tessellation" which refers to polygons that repeat themselves in a pattern. The visible crystal shape of a mineral is due to this repeating atomic pattern. If space is available, a mineral will grow in its characteristic crystalline shape. If a growing crystal is constrained, it will take on the shape of the space.

Some minerals break in a characteristic pattern, called "cleavage," which is also caused by alignments in the mineral’s atomic structure. Gemologists sometimes use cleavage patterns to cut gems into faceted shapes.

It is sometimes difficult to tell whether a specimen is a crystal or a broken mineral. This lab will allow students to touch and see different mineral shapes. It is important that they begin to identify the shapes, not whether they can identify whether it is a crystal or cleaved specimen.

  1.  In the module you have ten specimens including quartz, amethyst, pyrite, calcite, halite, fluorite, feldspar, mica, gypsum, and citrine. You may want to include your own specimens.
  2. Review the shapes of a cube, dipyramid, tabular (3 dimensional rectangle), rhombohedron, and six-sided prism. Use the pictures below to help students identify different shapes.  In the Post Lab, students will look at more shapes.  Draw a rhomohedron, cube, dipyramid, six-sided pyramid (like a pencil top), and tabular (3-d rectangular, book-shaped). Have the students draw the shapes on their worksheets. Explain that some of the specimens are complete crystals, which may match the shapes they have drawn, while others may only show fragments of the shapes.  Do not worry about whether they are cleaved or crystals. 
  3.  When the students  are done, discuss the characteristics of the each of the specimens, as listed below. Be sure to go over the shapes. 

    QUARTZ - found in quartz watches, one of the most common minerals, crystals form 6 sided prisms, when it breaks it produces a conchoidal or rounded break similar to broken glass
    - purple-colored quartz,  six-sided crystal shape
    - yellow- to brown-colored quartz,  six-sided crystal shape
    FLUORITE - a source of fluorine, dipyramid (octahedron) or cubic (depending on specimen),  specimen in module is cleaved
    - often called fools’ gold, used to make sulfuric acid, cubic crystal shape
    - common table salt, cubic crystal shape
    - commonly called iceland spar, can see double image, rhombohedral  cleaved surface in module, crystal shape resembles a "dog tooth"
    - used in making some ceramics, rhombohedral cleaved specimen
    -  crystals are tabular or prismatic, but these specimens show "sheet" cleavage
    - rosette or tabular crystal shapes, they also cleave in tabular pieces; this specimen is a crystal

[Back to Rock Cycle Grid]   [Back to Minerals (3)]