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: M3.5[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,Ca0.5,Ba0.5)4-7[Al4-7Si12-9O32]
- 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.
