Sunday, November 22, 2015


There seems to be a bit of confusion among many introductory geology students, some of the general public, and even rockhounds about “chlorite minerals.”  I often confused many of my introductory geology students (not on purpose) by asking them to name a common chlorite group mineral.  The most common answer was halite or salt, a sodium chloride (NaCl).  Halite, and a variety of other minerals, contains the element chlorine (Cl), a member of the halogen group.  In the free-state chlorine is a gas; however, it has a very high electron affinity and often gains an electron and becomes the negatively charged anion.  This ion then is able to combine with the positively charged cations and forms a number of compounds (like halite). 

However, the chlorite minerals (name from the Greek chloros or khloros meaning green) do not contain the element chlorine (normally a green gas and also named for chloros or khloros)) and hence the common confusion between chlorite and chloride.  The second confusing fact is that “chlorite” is not a recognized mineral!  But, it is often used, in a generic sense, for any soft, commonly green, micaceous, clay-like mineral. I certainly use the term---as in chlorite schist or quartz with chlorite inclusions.

The general formula for the Chlorite Group is X4-6Y4O10(OH,O)8 where X represents one or more of aluminum, iron, lithium, magnesium, manganese, nickel, zinc or rarely chromium. The Y represents aluminum, silicon, boron or iron but mostly aluminum and silicon. ( notes the occurrence of 12 different Chlorite Group minerals.  Most are rare or uncommon with clinoclore (Mg, Fe) pennantite (Mn, Al) and chamosite (Fe, Mg) being the most common types.  Cookeite (Li, Al), described in the previous post, is rare to uncommon.

Clinochlore, the subject of this small post, is interesting in that a particular variety, seraphinite, is polished and used as a semi-precious gemstone, and the red to pink variety known as kammereite forms beautiful specimens. 
Clinochlore [(Mg,Fe++)5Al2Si3O10(OH)8], a magnesium ferrous iron aluminum silicate is magnesium rich (more than the iron) and forms a solid solution series with chamosite, the iron-rich end member [(Fe++,Mg)5Al2Si3O10(OH)8]. My specimen is labeled “Clinochlore collected from the Barenfall Dam, Pinzgau, near Salzburg, Austria.”  That particular locality produces clinochlore rather than chamosite (, otherwise my identification of one mineral or the other would be dubious! 

Pseudohexagonal crystals of stacked tabs of dark greenish-brownish-black clinochlore.  Width of specimen ~9 mm.

Clinochlore has most of the characteristics of the other Chlorite Group minerals:  a variety of colors ranging from white/clear (no iron) to green to blackish-green (lots of iron).  One particular pink to crimson red variety is kammereite where chromium is the chromophore.  Unfortunately, I have not yet acquired a specimen! 

In addition, clinochlore is soft at ~2.0-2.5 (Mohs), has a mostly white streak, a pearly sort of luster, translucent in thin sheets although the lighter colors may form transparent flakes.  My specimen has tiny pseudohexagonal tabular crystals but other forms are massive, fibrous or granular.  There is perfect basal cleavage and this character produces the mica-like specimens; however, the individual cleaved tabs are elastic (bends) but are not flexible (bends and returns to original shape).

Photomicrograph of above specimen.  Note that the tin tabs lose the dark color and become translucent.  Width about 5 mm.
Clinochlore is the result of metamorphic and hydrothermal alteration of iron- and magnesium-rich silicates such as amphiboles and pyroxenes, and especially of oceanic basalts.  Weathering of clinochlore produces the silicate mineral vermiculite and several clay minerals belonging to the Smectite Group.  In fact, some authors classify Chlorite Group minerals as “clay minerals”. 

I mentioned above a particular variety of crimson red to pink clinochlore called kammereite with chromium chromophores.  Although too soft (~2.5 Mohs) for faceting and cabbing, the specimens are quite collectible.  The other collectible variety of clinochlore is seraphinite, a fine grained and quite dense mixture of dark clinochlore and silvery muscovite often creating an optical reluctance (if highly polished).  The name comes from the resemblance of the clinochlore inclusions to the “feathery” wings of the biblical seraphim angles.  It is rather uncommon variety and seems limited to a locality or two in eastern Siberia where some limestones were metamorphosed by intrusive granite.

Cut, but not polished, slab of seraphinite—note angle wings.  Total width ~2.3 cm.

As a couple of side notes, I have a fairly clear, terminated quartz crystal collected from southern Oklahoma in the Ouachita Mountains—see Posting July 21, 2014.  Upon inspection it is easy to observe numerous green inclusions of a Chlorite Group mineral; the exact mineral is unknown to me.  

Terminated clear quartz crystal with inclusions of a Chlorite Group mineral.  Collected from Ouachita Mountains, southeastern Oklahoma. Height of crystal ~2.1 cm.

At a small rock and mineral show several years ago I was visiting with a dealer near the end of the event and purchased a few specimens.  The dealer threw in a larger perky box and said it was nothing but an ugly chlorite schist that no one wanted to purchase.  In reality, the specimen is sort of interesting with not only the micaceous Chlorite Group mineral arranged in platy alignment but numerous tiny green, prismatic crystals of epidote and a few nicely formed magnetite octahedrons.  It was labeled “Missouri” and my guess is the Silvermine District in Madison, County.

Magnetite octahedron in “chlorite schist.”  Maximum width of exposed face ~5 mm.

Photomicrograph: Group (~7 mm) of micaceous and platy aligned Chlorite Group mineral in a “chlorite schist” from Missouri.

Photomicrograph: Group (2 mm) of tiny epidote crystals in the “chlorite schist.”     

If nothing else, these last two posts have introduced me to the complex mineralogy of Chlorite Group minerals and taught me that the adjectives chlorite, chloritic or chloritoid should be used in a generic sense.