FMF - Friends of Minerals Forum, discussion and message board

[Josele]

Pericline is defined in mindat as A variety of albite occurring as white, elongated crystals, generally in alpine-type clefts.
My old but confiable Klockmann-Ramdohr defines pericline as a plagioclase whose crystals are elongated in the b axis direction, specifying the direction of elongation and leaving open the possibility of a certain percentage of Ca in his composition.

After all pericline is just a variety and is not defined by the IMA, but, could someone give a proper definition of pericline?

Thanks for your comment!

[Jesse Fisher]

Pericline, like cleavelandite and adularia are varieties of feldspar, defined solely by habit, and as such are not "officially" recognized.minerals. Deer, Howie, and Zussman (Rock Forming Minerals, 1963) say about pericline:

"Pericline is the sodium feldspar counterpart of adularia, and it has a similarly restricted paragenesis and a characteristic habit." (page 3)

"Albite crystals are often tabular parallel to (010) but they are sometimes elongate parallel to Y, as in the "pericline" habit." (page 20)

That's about it. Basically a specific habit of a plagioclase feldspar that falls within the traditional albite compositional range (An 0-10)

[Pete Modreski]

There are probably those much more expert on the feldspars than I, but I will just try to state what I know, or think I know, and see what others may add.

Being as you say a name for a variety (morphological) of albite, I think one might say that pericline has a somewhat "fuzzy definition--I think this is analagous to the concept of "fuzzy logic" in mathematics, where it is defined not simply as yes or no (as in 1 or 0) according to one parameter, but is based on a "degree of match" to several parameters.

Thus, along the lines of what you say, pericline is defined as having crystals elongated on b, occuriing as free-growing crystals in Alpine-cleft metamorphic environments, And I think one might add, being white or near-white in color, being relatively pure (but not necessarily 100%) albite in composition, and often (always? I really do not know) exhibiting pericline-law twinning.

A good way to supplement/complement the text definition is to look at pictures of those specimens that have been described as pericline; for example, the photos posted on mindat; the are 118 such photos. Collectively, those photos constitute a good visual representation of what has been called pericline.

I tried (without immediate success) to look up and see how much of a composition range is represented by what has called "pericline"; but I've not immediately found any papers with chemical analyses to show this. Some more serious literature search would probably be required.

I don't know how much this helps, if any. I think there is clearly much material that "anyone would agree" should or could be called pericline; but there are probably examples of the fringe of the expected characteristics, in the "gray area", for which the correct nomenclature would be uncertain.

[Pete Richards]

I also find pericline a fuzzy concept! It is often described as a twin "according to the pericline law", which involves twinning by rotation about the b-axis and a composition plane called the "rhombohedral section" - I don't even know what that is!

Apparently not all pericline is twinned, though, if pericline means a general crystal shape only.

I agree that it is typically found in the Alps, is usually white, and is elongated along the b-axis.

Some things I have seen suggest that it is mostly microscopically twinned, like the plagioclase feldspars, but in a different direction, leading to cross-hatch twinning seen mostly or entirely in thin sections. Is this really a petrologists' twin? Does it really have a manifestation as a twin between two discrete individuals?

Maybe I should look at the Mindat pictures....

[alfredo]

Both Mindat and Mineralienatlas - https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Periklin
- seem to have only a brief fuzzy description of pericline, basically just distorted albite habit found in alpine-type veins, with no mention of which axis is elongated, nor anything about twinning.

[Josele]

Thanks for your inputs!
Although mindat nor mineralienatlas do not specify the elongation direction it seems that we are in accord that pericline crystals are elongated in the b axis direction, opposed to albite-type whose main elongation is in the direction of c axis.

Also it seems to be agreement in the hydrothermal environment, limited to alpine-type fissures where the temperature is relatively low compared to others albite deposits. That may be the reason of some authors to call it "the adularia of sodium feldspars", added to the fact that adularia habit is also elongated in the b axis direction.

Relating chemical composition, here I'm still confused. Klockmann-Ramdohr seem to say that that pericline-law twinning is done exclusively in pericline-type crystals while albite-law twins are done only in albite-type crystals.
But many authors along history have tried to deduce the Na-Ca% from the rhombic section of pericline twins and associated striation on (010).
The effect of composition and structural state on the rhombic section and pericline twins of plagioclase felspars.
Or pericline twinning can occur also in albite type crystals of any plagioclase, or pericline habit can be done also in anortite, or I have not understood anything of nothing.

Comments will be very welcome!

[Bob Carnein]

Pete Richards: Pericline twinning is polysynthetic, just like albite twinning but in a different direction. Thus, there are many individual crystals involved. Both types of twinning occur together in microcline, leading to the "cross hatched" or plaid twinning seen in microcline in thin section.

[Roger Warin]

Here is an example of polysynthetic albite twinning in a plagioclase crystal in a basalt of Vesta asteroid (eucrite meteorite).
The central part always intrigued me. Is the twin of pericline?
Roger.

[Bob Carnein]

I'm no expert on meteoritic plagioclase, but your photo appears to show combined albite and pericline twinning. Both can occur together in plagioclases of various compositions, including the very high-calcium plagioclase of the Vesta meteorites. From my reading, the pericline twinning is likely to be subordinate.

Some other interesting features in your photo: the albite twinning on the right side of the photo appears to fade into the zone of pericline (?) twinning; and the albite-twin lamellae don't appear to be continuous across this zone.

[Roger Warin]

Hi Bob,
Indeed, the process of twin interrupted the crystal phases, after its growth. It was the first time I watched this twin of pericline in a plagioclase crystal.
Roger.

[Roger Warin]

Hi,
Here is an albite specimen decorated with many crystals of pericline barely visible because colorless.
First picture of the specimen with crystals of albite.
Second photo: a detail of the many secondary crystallization, pericline.
Third picture: a third unknown crystallization.
What do you think about it?
Roger.

[Bob Carnein]

Nice pictures, Roger! How was the pericline identified?

[Roger Warin]

With an old museum label, but I'm not satisfied, because I'd like to see a sketch of the ideal crystal of this specimen.

[Roger Warin]

Pericline is a variety of albite, alkali feldspar. It was recognized in the crystalline state by the ancient crystallographers. However, this habit is much rarer in the macroscopic state that the twin of the same name. The pericline law has [010] as the twin axis. So much so that many people believe that the only pericline is a twin law characterizing alkali feldspar.
As noted on this forum, albite is sometimes elongate parallel to Y. It's the "pericline" habit.
Here is a good specimen of pericline crystal twinned. Pericline sketch from A. Lacroix.
Roger.