Metamorphic Gemstones

Most zeolites form in one or more of six geologic/hydrologic environments:
saline alkaline lakes, saline alkaline soil and land surfaces, deep sea sediments, open hydrologic systems, hydrothermal alteration zones and burial metamorphic occurrences (Hay, 1977).

Minerals that make up sedimentary rocks are of two principal types— detrital and authigenic.
Detrital minerals, such as grains of quartz and feldspar, survive weathering and are transported to the depositional site as clasts. Authigenic minerals, like calcite, halite, gypsum, and zeolites, form in situ within the depositional site in response to geochemical processes.


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Saline alkaline lake deposits (also known as closed hydrologic system deposits) form where saline alkaline lake waters alter volcanic ash deposited into the lake, Saline alkaline lakes typically occur in arid climates in block faulted or rifted terrains surrounded by igneous or metamorphic bedrock without significant pyritic shales or evaporites (Surdam, 1977). The combination of saline, alkaline waters and highly reactive volcanic material seems to be very favorable for zeolite growth. Zeolites in these deposits may be traced horizontally for several kilometers and are often laterally zoned according to the salinity gradient in the original lake water, Mineral zonation from the shoreline toward the center of the lake consists of unaltered glass; a zone of one or more of the following zeolites: chabazite, erionite, phillipsite, mordenite or clinoptilolite; an analcime zone and finally, at the center of the lake, a K-feldspar zone (Sheppard and Gude, 1969). Mineral zonation may not be apparent in rift valley settings because the long, narrow geometry of the valley compresses the mineral zones. Nonetheless, rift valley settings contain some of the purest, mono-mineralic concentrations of commercial zeolites known. High-grade beds are usually a few centimeters to a few meters thick, Saline alkaline soil and land surface deposits form through the interaction of alkaline brines with tuffaceous and non-
tuffaceous sediments such as claystones. Meteoric water percolating through sediments enriched in sodium carbonate-bicarbonate (by evapo-transpiration) transform sediments to zeolites.
Phillipsite, chabazite, natrolite and analcime are known to occur in these types of deposits (Hay, 1977).

Deep sea sediment deposits may form through the low-temperature alteration of volcanic glass, biogenic silica or poorly crystalline clays by pore water, although the exact mode of formation and required precursor minerals is still a matter of conjecture (Bass, 1976; Boles, 1977a). Major zeolites formed in deep sea sediments are phillipsite and clinoptilolite; analcime is a minor component and erionite, natrolite and mordenite occur only occasionally. Zeolites can constitute as much as 80% of some deep sea drilling cores. The type of zeolite encountered varies with the age of the sediments; phillipsite is more common in late Tertiary and younger sediments; clinoptilolite is most common in early Tertiary and Late Cretaceous sediments while analcime becomes more prevalent in Early Cretaceous sediments. Authigenic clays often occur with the zeolites (Hay, 1977).

Open hydrologic system deposits form by percolation of meteoric water through nonmarine or shallow marine tuffaceous sediments. These occurrences can be several hundred meters thick and be traced laterally for tens of kilometers. These deposits show a vertical mineral zonation with zones consisting, in descending order, of volcanic glass and clay minerals; zeolites such as clinoptilolite, mordenite, and minor phillipsite; and finally, analcime and/or potassium feldspar.
Contacts between zones are either horizontal or slightly inclined and cut bedding planes (Hay and Sheppard, 1977).


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Hydrothermal alteration zone deposits occur in areas of hydrothermal activity where hot fluid alters rock along its migration route. Zeolites are zoned according to the temperature of the altering fluid in the hydrothermal system. Clinoptilolite and mordenite occur in cooler, shallower parts of the system while analcime, heulandite, laumontite and wairakite occur in deeper, hotter areas of the system (Hay, 1977).


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Ferrierite Sample, Lovelock Nevada - Trabits Group: Project Photographs


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Ferrierite Outcrop Sample, Lovelock Nevada - Trabits Group: Project Photographs

Burial diagenetic deposits (also known as low-grade metamorphic deposits) originate by authigenic growth of zeolites due to increased temperatures associated with burial in sedimentary basins. Zeolites and other authigenic minerals in these deposits show vertical zonation with increasing alteration corresponding to increasing depth of burial. Volcanic glass or calcic plagioclase is a precursor to the zeolite.
Clinoptilolite, heulandite, analcime and laumontite predominate although in Tertiary and younger rocks mordenite, erionite, chabazite, and phillipsite are significant. Together, zeolites may comprise up to 25 percent or more of the rock volume in non-tuffaceous, metamorphosed rocks and much higher proportions in altered tuffs. Associated minerals include authigenic quartz, albite, adularia, calcite, phyllosilicates and occasionally sphene, prehnite and pumpellyite (Boles, 1977b).

Successful zeolite prospecting since the 1950s has centered on altered Tertiary volcaniclastic deposits in the western United States. While geologists have identified clays, feldspars,
feldspathoids and gels as parent materials for zeolites, volcanic glass shards are the main precursor for authigenic zeolite formation. Chemicals in ground water or surface water alter glass
to zeolites in a short period of time, forming zeolite beds.

These zeolite beds can be white, yellow, grey,green or brown. They tend to be more resistant than surrounding unaltered tuff, often forming overhanging ledges in drainages. The zeolite beds do not necessarily have to be very thick to be of commercial interest. For example, the Bowie deposit, an unusually pure chabazite deposit in Arizona, is mined despite being only six inches thick and oyerlain by up ro 30 feet or over-burden. Zeolitically altered tuffs may also contain glass shards, quartz, clays, K-fe|dspar, opal, chalcedony, calcite, cristobalite, evaporates, mafic minerals, searlesite, fluorite, dawsonite, and other authigenic minerals.

Zeolites transform to other minerals under weathering, hydrothermal alteration or metamorphic conditions.Some examples:
Silica-rich volcanic rocks commonly progresses from: Clay ➫ quartz ➫ mordenite–heulandite ➫ epistilbite ➫ stilbite ➫ thomsonite–mesolite-scolecite ➫ chabazite ➫ calcite.

The sequence of silica-poor volcanic rocks commonly progresses from:Cowlesite ➫ levyne–offretite ➫ analcime ➫ thomsonite–mesolite-scolecite ➫ chabazite ➫ calcite.