By Bill Shelton
It seems as though it might be a simple matter to deal with this topic. Well, perhaps it is but I have the idea that mineral collectors may not exactly know what is involved. Any source reveals that there are seven systems and 32 crystal classes where the system is subdivided based on characteristic symmetry for that system. Here, we shall look at the cubic system and the five classes within the system. Each class is named for a certain form that characterizes the class; also, we find a customary reference to the class where numbers and letters will also describe the same class. This is also called the point group. The full symbol and an abbreviated symbol are commonly used for the same purpose. Below, I have arbitrarily listed examples that I consider to be more or less important for collectors. Species that are not included are the ones I think many collectors do not seem interested in as well as rarely found samples that will be hard to collect and/or purchase.
The first class is the tetrahedral pentagonal dodecahedral; also noted as 23. We find the name tetartohedral used as well. References may call this class the ullmannite type. Some older sources say cobaltite belongs here. Actually, it has been moved to the orthorhombic, pseudcubic division where we can see crystals that look like pyritohedra but are not. Kostov and Kostov (1999) indicate that it can and does resemble pyrite which is in a higher symmetry class, 2m3. Sadly, this is the most notable member of this class. So, who’s left? Well, we have ullmannite and I believe it is of little interest so this class is of minor concern in my opinion for we collectors. Ullmannite fits in P23 space group and will display octahedral, pyritohedral and tetrahedral crystal forms as well as twins.
The next class (2m3 or m3) is the di(akis) dodecahedral which is also labeled pentagonal hemihedral, dipliodal, didodecahedral and pyritohedral class. References may call it the pyrite type. I choose to refer to four species as the prime examples for us to consider. This class is important and the members are likely to be familiar and present in your collections. Sperrylite, Pa3 space group, resembles pyrite especially regarding morphology and variation of crystal habits. It is less common among collector material than the others listed here. Smaltite, a name found in older sources has been discredited and is considered as a variety of skutterudite. The Handbook of Mineralogy (1990) shows analytical data for CoAs2 (more like the material previously classified as smaltite and data for CoAs3 that is akin to the original skutterudite. Space group Im3 is for this mineral; cubic, octahedral and cubo-octahedral forms are commonly present. Bixbyite, Im3 space group, exhibits cubic forms and often has twins. Pyrite, the namesake here, is a member of Pa3 space group. Pyritohedral, dodecahedral, cubic, and octahedral forms are common as are twins. It is the most prevalent example for this class.
Third among the five classes, we find bar43m. This is the hexa(kis) tetrahedral, hextetrahedral, of tetrahedral hemihedral class. The names tetrahedral class and tetrahedrite type are also used in some sources. Members more or less noted in collections and numerous by comparison to some other classes, include sphalerite. It is Fbar43m space group and an exhibit tetrahedral, cubo-octahedral, dodecahedral forms and twins. This very common mineral is no doubt well-known to you. Tetrahedrite, Ibar43m space group, shows tetrahedral forms and twins. Tennantite, also Ibar43m space group, looks essentially the same in general as tetrahedrite. Last, we find helvite, space group Pbar43m, with tetrahedral forms and twins.
Class 432 also noted as 43 is the plagiohedral. I have chosen to say no more - it has no mineral representatives anyway. Previously, cuprite was placed here. The Handbook says it is m3m and space group Pn3m – so look for it in the next class.
Finally, the major class, called the normal class or galena type, is represented as 4m3m2 which is abbreviated as m3m. The name hexoctahedral or holohedral may be found. A lot of collector type minerals are placed here. I include thirteen examples below but there are nearly 30 very well-known species in this class.
Species Space Group Forms twins?
Copper Fm3m cube, octahedral, dodecahedra,tetrahedra. twins
Gold Fm3m octahedral, dodecahedra, cube twins
Platinum Fm3m cube (often isoferroplatinum and then twins, Pm3m. twins?
Halite Fm3m cubic
Galena Fm3m cube, octahedral, cobo-octahedra twins
Fluorite Fm3m cube, octahedral, dodecahedra twins
Spinel Fd3m octahedral, cube, dodecahedra twins
Diamond Fd3m octahedral, dodecahedra, cube twins
Grossular Ia3d dodecahedra, trapezohedra (garnet grp)
Sodalite P43n dodecahedra twins
Analcime Ia3d trapezohedra twins
Microlite Fd3m octahedral
Pollucite Ia3d cube, dodecahedra, trap. (rare)
Well, now we should list a few others. Silver, lead, iron, alabandite, magnetite, argentite (acanthite pseudos), gahnite, franklinite, chromite, pyrochlore, and uraninite all belong here. Also, all the garnet group like almandine, andradite, pyrope, spessartine and uvarovite are best included here. So, the best group for further study is the so-called normal group since we find so many minerals we all enjoy and probably have in our collections. Based on the source I chose to use, I think the data here is more or less correct and up to date.