THE MONOCLINIC SYSTEM

If you followed the above discussion, then you have a good handle on the three classes possible in the monoclinic system. The lowest symmetry class has a mirror plane and lacks the two fold axis. The other low symmetry class has the two fold rotational axis but lacks the mirror plane. Which is lower in symmetry depends on the outcome of the argument: is a mirror a higher symmetry operation than a two fold rotation? The highest monoclinic symmetry class is the 2/m class with both operations being present. The 2/m class is a very popular class as more minerals form a structure with this symmetry than any other. By comparison, the other two classes of the monoclinic system are found lacking with a significant decrease in minerals representing them.

The Prismatic Class



GYPSUM
• Class: 5th
• Symmetry: 2/m
• Symmetry Elements: One two fold axis with a perpendicular mirror plane
• Axes: None are equal
• Angles: Alpha and beta angles = 90 degrees but gamma does not.
• Common Forms: The monoclinic prism and the pinacoid.
• Most Common Minerals Known to this Class: This is the largest symmetry class in terms of the number of minerals with almost eleven hundred recognized members. Some of the more noteworthy members are Acanthite, actinolite, aegirine, allanite, annabergite, arfvedsonite, arsenopyrite, augite, aurichalcite, azurite, bannisterite, beryllonite, biotite, borax, boulangerite, brazilianite, brochantite, butlerite, calaverite, carnotite, catapleiite, caledonite, celsian, chalcocite, charoite, chondrodite, chrysotile serpentine, clinochlore, clinoclase, clinoptilolite, colemanite, cookeite, cornwallite, creedite, crocoite, cryolite, cryptomelane, cummingtonite, datolite, diopside, dufrenite, edenite, epidote, erythrite, esperite, euclase, fluorrichterite, gadolinite, gaylussite, gibbsite, glauberite, glauconite, graftonite, gypsum, harmotome, hedenbergite, hessite, heulandite, hodgkinsonite, hornblende, howlite, hubnerite, hydroboracite, hydromagnesite, hydrozincite, illite, jadeite, jamesonite, jordanite, kernite, kidwellite, kieserite, kinoite, kottingite, kovdorskite, ktenasite, lamprophyllite, lanarkite, langbanite, laumontite, lazulite, leadhillite, legrandite, leucosphenite, linarite, liroconite, ludlamite, malachite, manganite, melanterite, monazite, montmorillonite, muscovite, olivenite, orpiment, orthoclase, palygorskite, papagoite, pargasite, phillipsite, phlogopite, polybasite, polylithionite, pseudomalachite, psilomelane, pyrrhotite, realgar, richterite, riebeckite, romanechite, rosasite, roselite, sanidine, sauconite, semseyite, sklodowskite, sphene, spodumene, staurolite, stilbite, stringhamite, sussexite, sylvanite, synchysite, tainiolite, talc, tenorite, thomsenolite, titanite, tremolite, trona, vermiculite, veszelyite, vivianite, volborthite, whewellite, whiteite, wohlerite, wolframite, xonotlite, zinnwaldite, zippeite and zirconolite-2M.

The Sphenoidal Class



MESOLITE

• Class: 4th
• Symmetry: 2
• Symmetry Elements: One two fold axis of rotation
• Axes: None are equal
• Angles: Alpha and beta angles = 90 degrees but gamma does not.
• Of Note: Crystals can be left or right handed, enantiomorphic, and hemimorphic, having different tops and bottoms.
• Common Forms: The sphenoid, pinacoid and pedion.
• Most Common Minerals Known to this Class: Amicite, boltwoodite, franklinfurnaceite, goosecreekite, halotrichite, joaquinite-(Ce), mesolite, miargyrite, pickeringite, remondite-(Ce), rinkite, uranophane, wollastonite-2M and many organic chemicals crystallize in this class including sugar and tartaric acid. There are roughly 70 minerals belonging to this class.

The Domatic Class



NEPTUNITE

• Class: 3rd
• Symmetry: m
• Symmetry Elements: One mirror plane of symmetry
• Axial Lengths: None are equal
• Angles: Alpha and beta angles = 90 degrees but gamma does not.
• Common Forms: The dome, pinacoid and pedion.
• Most Common Minerals Known to this Class:Alamosite, antigorite (serpentine), clinohedrite, natron, neptunite and scolecite Roughly 40 minerals belong to this symmetry class.