USE OF CRYSTAL FORMS AND HABITS IN MINERAL IDENTIFICATION

The atoms within the crystal of a mineral are arranged in a regular fashion to form a lattice, and the crystal exhibits a shape with surface regularity which reflects its internal symmetry[Ref1]. The shape of a crystal is often typical of a mineral. and often typical the location where found; thus, crystal shape comprised of crystallographic forms modulated by crystal habit can be a useful tool in mineral identification. 

The crystals of all minerals fall into seven families defined by their required symmetries as given in the table in Figure 1[Ref2]. The hexagonal family comprises two crystal systems as seen in Figure 1. Planes and shapes which enclose space as shown in Reference 3 are the crystallographic forms which comprise the shapes of crystals exhibited by minerals. The basic forms exhibited by the seven crystal systems are shown in Figure 2. Environmental conditions during deposition can influence the both the forms present on the crystal and the habit of a crystal in influencing its shape [Ref5]. 

Figure 1. The crystal families, systems, and their required symmetries[Ref2].

Figure 2. Forms of the basic prisms exhibited by the six crystal systems[Ref4].

Gallery of Crystal Systems, Forms, and Habits

In order to introduce some of the forms and habits of crystals I’ll use examples of minerals we often have enjoyed seeing in the literature and at lapidary and mineral shows as shown in Figures 3-9.

CUBIC CRYSTAL SYSTEM

Figure 3. Skeletal or hoppered crystals of galena on sphalerite, Madan Ore Field, Rhodope Mountains, Bulgaria [Ref5,8].

TETRAGONAL CRYSTAL SYSTEM

Figure4. Tabular crystal of wulfenite, Los Lamentos Mountains, Chihuahua, Mexico[Ref5,9].

ORTHORHOMBIC CRYSTAL SYSTEM

Figure 5. Acicular crystals of mesolite on green hydroxyapophylite, Pashan quarries,
Pashan Pune District, Maharashtra State, India[Ref5,10].

HEXAGONAL CRYSTAL SYSTEM

Figure 6. Bipyramidal crystals of quartz paramorph after hexagonal beta quatz, with hematite crystals, Florence Mine, Egremont, Cumbria, England, UK[Ref3,11]. 
Figure 7. Crystal of Beryl var. emerald displaying faces of the hexagonal and dihexagonal prisms, of the hexagonal pyramid, and of the basal pinacoid, Muzo Mine, Muso Municipality, Boyaca’ Department. Colombia[Ref3,12].

TRIGONAL CRYSTAL SYSTEM

Figure 8. Phantomed schalenohedral crystals of calcite, Mariposa Mine, Terlingua District, Brewster County, Texas[Ref 6,13]
Figure 7. Rhombohedral crystals of calcite, Gonsen Mine, St. Gallen, Switzerland[Ref3,14].

MONOCLINIC CRYSTAL SYSTEM

Figure 8. Crystal of gypsum (selenite), with faces comprising  two domes and six pinacoids, Gilbralter Mine, Naica, Chihuahua, Mexico[Ref3,5,15].

TRICLINIC CRYSTAL SYSTEM

Figure 9. Crystal of Axinite-(Fe) with 7 pinacoidal faces, Pulva Mount. Tyumenskaya, Urals Region, Russia, Asia[Ref5.16].

REFERENCES

Ref 1. https://www3.nd.edu/~amoukasi/CBE30361/Lecture__crystallography_A.pdf

Ref 2. https://en.wikipedia.org/wiki/Crystal_system

Ref 3. https://www.tulane.edu/~sanelson/eens211/forms_zones_habit.htm

Ref 4. http://www.geologyin.com/2014/11/crystal-structure-and-crystal-system.html

Ref 5. https://en.wikipedia.org/wiki/Crystal_habit

Ref 6. http://www.galleries.com/minerals/property/habits.htm

Ref 7. http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi8.htm

Ref 8. https://www.youtube.com/watch?v=f_g3r79mG9s

Ref 9. https://www.pinterest.com/pin/369506344411688204/

Ref 10. https://www.mindat.org/photo-303305.html

Ref 11. https://www.irocks.com/minerals/specimen/42517

Ref 12. https://www.rockngem.com/uncommon-emerald-exhibit-opening-sept-26/

Ref 13. https://www.spiritrockshop.com/Calcite_Mariposa.html

Ref 14. https://www.fabreminerals.com/LargePhoto.php?FILE=Calcite-SH47AB1f.jpg&LANG=EN

Ref 15. https://www.irocks.com/minerals/specimen/38370

Ref 16. https://www.crystalclassics.co.uk/product/cc19390/

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