SMALL SPECIMEN; TINY ZEOLITES FAUJASITE-Na AND PHILLIPSITE-K

 I came home from the Bridling Estate sales with several micromounts stuffed into plastic containers holding 25 boxes. Some of the boxes contained pretty ordinary mounts of minerals like calcite, dolomite, and pyrite. However, several of the mounts contain interesting, and sometimes rare minerals.  I am uncertain who set the zeolite mount described today: faujasite with phillipsite.

Zeolites have always been sort of confusing to me (mineralogically) although the group is quite large in numbers, many members are quite common, and they are very important as industrial minerals.  In addition, zeolites like chabazite and heulandite are usually seen at almost every rock and mineral show. Some zeolites, such as the acicular/prismatic crystals of mesolite, natrolite, and scolectite, make interesting museum specimens, as well as beautiful objects de art. 

I would have trouble defining zeolites to my fellow rockhounds.  So, I leave that task to Lauf (2014): the zeolites are a large and highly diverse group of (generally) aluminosilicate minerals in which SiO₄, AlO₄, or other tetrahedra are arranged into a three-dimensional structural framework having cavities and channels that can host ‘extra-framework” cations and water molecules.  The important item in this definition are the words about cavities and channels: 1) these open areas are able to hold a variety of cations which may be exchanged with other cations found in contacting solutions. For example, in water softeners/purifiers where zeolites are the "salts" the sodium in the zeolites may be exchanged for magnesium and calcite in the water (the “hardeners”); 2) the channels and cavities also may act, in industrial chemistry, as molecular sieves to remove specific sized molecules.  Zeolites are incredibly important in various industries.  Today there are over 200 synthesized zeolites; however, natural zeolites are usually found in cavities of volcanic rocks deposited via post-depositional contact of the rock with alkaline groundwater.

Faujasite is a rare zeolite, or actually a series of minerals with three endmembers named after the dominant cation: calcium, sodium or magnesium.  MinDat writes the chemical formula as:  M_3.5[Al₇Si₁₇O₄₈]-32H₂O where M is Ca, Na₂ or Mg present in different percentages while potassium and or strontium may partially substitute for one of the cations. Faujasite-Na is the most common composition with the type example from Sasbach, Germany; faujasite-Ca has a type example from Ilbeshausen, Vogelsberg, Hessen, Germany; and faujasite-Mg has the only known example from an “old museum specimen” collected from Sasbach, Kaiserstuhl, Germany (Faujasite Series, 2005).  

The specimen in my collection came from the Type Locality of faujasite-Na in the Linberg Quarries (seven of them),  Kaiserstuhl Volcanic Complex, Southwest Germany.  The volcanic rocks at the quarries are Miocene in age, ~19-16 Ma, and are a type termed limbergite: dark-colored rock resembling basalt but with an absence of feldspar. The glassy groundmass is composed of augite and olivine often with another generation of euhedral augite crystals. The zeolites usually appear in vugs of the volcanic rocks and are post depositional.

The crystals of faujasite are octahedrons, colorless to gray to white in color, transparent to translucent, vitreous and often are intergrown with other zeolites such as phillipsite and offretite.

Radial aggregates of phillipsite-K along with a mass of individual crystals. Width of vug with crystals ~2 mm.  Width of largest aggregate(middle left) is ~0.3 mm.

A second zeolite present in my specimen is phillipsite-K.  Originally there was a single phillipsite, essentially the potassium variety although the sodium species was the original Type. In 1997 phillipsite was divided into species with names based on the dominant channel cation. The type examples for the new phillipsite species compositions are as follows: phillipsite-Na has the composition from the original phillipsite type locality at Aci Castello, Sicily, Italy; phillipsite-K, Capo di Bove, Rome, Italy; and phillipsite-Ca, Lower Salt Lake Tuff, Puuloa Road, Oahu, Hawaii (Phillips Series, 2005).

MinDat.org writes the chemical formula for the potassium species as: (K,Na,Ca_0.5,Ba_0.5)_4-7[Al_4-7Si_12-9O₃₂] - 12H₂O.  So there is a mixture of potassium, sodium, and calcium cations combined with aluminum silicate and lots of water.  Also notice the barium (Ba) in the formula, an element present in all of the phillipsite series minerals.  In fact, if barium is the chief cation, the mineral is known as harmotome.

Phillipsite-K crystals are colorless and transparent to white, prismatic, vitreous, hard (4-5 Mohs), and often terminated.  They occur as terminated individual crystals or spherical radiating aggregates—both are visible in my specimen and often are intergrown with faujasite and another zeolite named offretite.

Phillipsite-K is the most common of the species and is found  in cavities of basaltic lavas and as a diagenetic alteration product in volcaniclastic sediment. The species of phillipsite formed is usually controlled by the type and mineral composition of the host rock, and the nature of the contacting solution Phillipsite Series, 2005).

  REFERENCES CITED

Faujasite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

Lauf, R.J., 2014, Collector's guide to the zeolite group: Schiffer Earth Science Monographs Volume 17.

Phillipsite Series, 2005, Commission on Natural Zeolites: IZA Commission on Natural Zeolites (iza-online.org).

The photomicrographs below are at the very limit of my skills with the digital microscope.  Many are somewhat fuzzy.  As you can observe these crystals are less than 1 mm in size. 

 

Aggregate of radiating phillipsite-K crystals with large faujasite octahedrons on either side of the mass.  Width of phillipsite ~0.3 mm.  

 

Radial aggregate of phillipsite-K water-clear crystals (pointing arrow).  The arrow bisects a large fuzzy crystal of faujasite. The aggregate is embedded in a mass of tiny, terminated phillipsite-K crystals along with nondescript faujasite.  Length of aggregate ~0.8 mm.      

Two "larger" octahedrons of faujasite situated in a vug on a layer of what appears to be an earlier generation of faujasite crystals.  Width: small, see below. 

 

Black, or darkest green, euhedral crystals of augite embedded in the limburgite groundmass.  The two vugs to the right hold distinct individual octahedrons of faujasite on an earlier generation of what I assume is faujasite. The photomicrograph above is an enlargement of the lower right quadrant of the complete vug. Length of vug ~ 2.7 mm.

Another vug containing faujasite octahedrons (F) and a sphere of perhaps offertite (O), another rare zeolite.  I am way out on a limb here with that designation, however, under my microscope it appears similar to those identified in MinDat.  Width of vug ~ 3.0 mm 

This is an interesting vugs with groundmass containing euhedral augite, numerous octahedral faujasite crystals and in the middle there are numerous small spheres that are most likely Opal-AN.  Also scattered around are individual prismatic crystals of phillipsite-K.  The orange mineral surrounding the vug is weathered olivine.  Width of vug ~4.0 mm.