Crystal habits of galena, PbS, principal ore mineral of lead

--- samples from Bulgaria and Peru

[290 kb]

Figure 1. Galena from the Krushev Dol mine, Madan ore field, Rhodope Mountains, Smolyan oblast, southern Bulgaria. Sample 67x66x29 mm. Note the twinned and truncated crystals of coarse galena with rounded margins to the crystal faces. The lead sulphide is associated with a fine-grained black sphalerite. This sample is from Minerals Bulgaria, Ilford, England.


"Rock of the Month #214, posted for April 2019" ---

Galena, the common ore of lead:

soft, dull grey, heavy and distinctive, galena has been known since antiquity. TO BE CONTINUED..


Table 1. Some cubic crystal forms

Table 1. Some cubic crystal forms
Crystal habit NotesGeneral Miller index
Cube Six equal square faces at 90 degrees {100}
Octahedron Eight triangular faces {111}
Dodecahedron Twelve rhombohedral faces {110}
Tetrahexahedron 24 faces: four-sided pyramids atop each face of a cube {015}

TO BE CONTINUED.

There are currently five other "rocks" in the Rock of the Month index, that have galena as a principal or sole ore mineral. Two come from the Madan ore field in southern Bulgaria, and the Viburnum Trend of southeastern Missouri, and are sufficiently coarse-grained to be worth another look in the context of crystal habit.


[286 kb]

Figure 2. Coarse galena (with less-obvious sphalerite) from the Huanzala mine, Huallanca, Ancash department, Peru. Sample from David K. Joyce.


A mineral species is defined as a naturally-occurring substance that has a definite crystal structure and a composition that is either fixed or varies within known limits. Minerals are mostly abiogenic, though organisms living and dead, of all sizes, may play a part in their development by providing chemical elements and/or favourable sites for their nucleation and growth.

These pages tend to focus more on a mineral's composition than its structure, but the arrangement of atoms in a mineral is fundamental to many aspects of each mineral, including physical and optical properties. Determination of crystal structure, most often by x-ray methods, is a key method by which mineralogists and crystallographers identify and characterize a mineral species.

The crystal habit of a mineral refers essentially to to the development of certain crystal faces in preference to others. This determines the shape and general appearance of a mineral in a given specimen. Galena is a textbook member of the cubic crystal system, the simplest of structures,in which each crystal ideally displays three axes that are mutually perpendicular and of equal length. The classic shape or habit is of course a cube, as seen in the Viburnum sample on these pages. Another is the octahedron, and a third the dodecahedron. Galena, halite (common salt), fluorite and diamond are all species of mineral that display variations of cubic crystal development. The members of the garnet family of silicates are also cubic. However, some minerals display certain habits more often than others: galena, halite and fluorite often develop as perfect cubes, but many variants are possible.

References (n=xxx)

xxx. TO BE ADDED.

Graham Wilson, 15,27 November 2018, 05,12 December 2018

Visit the "Rock of the Month" Archives!

or the "Rock of the Month Index"!