Varieties of Quartz: Part 1

Quartz occurs in a great number of varieties that differ in form and color. Quartz can occur as single crystals, crystals in druses, massive aggregates, and dense nodules. Quartz is colorless, if pure, but can assume any color because of inclusions of other minerals or because of the presence of built-in trace elements.

The more common varieties have been given their own names. The different varieties are commonly classified in two major groups:

One of these major groups is called macrocrystalline, which includes varieties that commonly form visible crystals. When people talk about quartz, they mostly think about macrocrystalline quartz. Some examples of macrocrystalline quartz are shown below:

Individual transparent crystals of euhedral quartz (= nice angular crystals), (largest crystal here is 2.3 cm height).

 

A cluster of transparent crystals of quartz (largest crystal 6 cm height). To get crystals like these, they must have free space to grow into.

7 cm height

4.5 cm height

The above two images are of transparent macrocrystalline crystals of quartz containing inclusions of the mineral rutile, thereby forming "rutilated quartz." The acicular (needle-like) crystals of rutile, a titanium oxide, form within the structure of the quartz crystal. The "needles" can be golden, black, or other hues.

Drusy quartz called "cactus quartz"(5.8 cm height) when it coats much larger, singular crystals of quartz.

Amethyst is the purple, violet, or lavender variety of quartz. The colors are the result of natural radiation emanating from the surrounding rock. This radiation affects any iron impurities that might be present in the quartz and consequently causes the coloration. This specimen is 9.5 cm in width.

Citrine is the yellow to orange variety of quartz, with the colors being the result of the presence of iron impurities. This specimen is 8.3 cm height.

Smoky quartz is the gray, brown, or black variety of quartz, with the colors being the result of exposure to radiation.

Rose quartz is the rose or pink variety of quartz. The color is due to nano fiber inclusions (see my May 31, 2016 post). Rose quartz does not form crystals or crystal faces, and it is never transparent. It is, however, classified as a macro crystalline variety of quartz, as it is made up of many inter grown crystals and subindividual crystals. This specimen is polished and 2.3 cm height.

Aventurine (unpolished [4 cm height] and polished pieces) is actually a quartzite (a metamorphic rock, not a mineral) composed on interlocking macro crystalline quartz grains and other color-imparting minerals. Aventure is commonly green but can be orange brown, gray, or blue. The color is due to the presence of small-sized reflective particles of the mineral mica or particles of iron. 

The next post will show some of the common varieties of the second group: cryptocrystalline (or microcrystalline) varieties of quartz.

Cleavage in minerals

Cleavage refers to the specific planes of weakness along which some minerals split. These planes (or directions) are smooth, shiny and parallel to the zones of weak atomic bonding inherent in the structure of the mineral. 

Cleavage is an important “tool” used in the identification of minerals. Unlike crystal faces, which can be also flat and smooth, cleavage is only evident when a mineral is broken.

Some of the more common types of cleavage are shown below:

PINACOIDAL = one direction of cleavage (like a sheet of paper), resulting in thin, flat sheets which can be peeled apart (e.g., mica).

Two views (oblique and side) of a sheet of biotite mica (maximum dimension 26.5 cm). 

RHOMBOHEDRAL = three directions of cleavage with the cleavage planes forming angles other than 90° (e.g., calcite).

rhombohedral calcite, 3 cm high

CUBIC = three directions of cleavage with the cleavage planes forming 90° angles; (e.g., halite).

cubic halite, 7.5 cm high; the lower right side has a piece broken off

OCTAHEDRAL = when a mineral breaks in the form of a diamond, resulting in eight nearly equal faces (octahedron); (e.g., octahedral fluorite).

octahedral fluorite, 3 cm high

DODECAHEDRAL = when a mineral breaks in the form of a polyhedron with 12 faces; (e.g., garnet).

dodecahedral garnet, 4 cm high

Not all minerals have cleavage (e.g., quartz), but all minerals show fracture, which is the tendency of a mineral to break along an irregular surface. In glass and some minerals, like quartz, the broken surface is called conchoidal fracture, and it does not follow any planar surfaces (i.e., crystal faces).

conchoidal fracture in quartz, specimen is 3 cm high

quartz crystal showing natural crystal faces, specimen is  4.5 cm high