ROUGH DRAFT authorea.com/81377

# Introduction

The mineral gmelinite is the most common member of the gmelinite family, a rare group of zeolites. Gmelinite was once described as one mineral, $$\mathrm{(Na_{2},Ca,K_{2})_{4}Si_{16}Al_{8}O_{48} \cdot 22H_{2}O}$$, but now is comprised of three endmembers: $$\mathrm{Na_{4}Si_{8}Al_{4}O_{24} \cdot 11H_{2}O}$$, $$\mathrm{K_{4}Si_{8}Al_{4}O_{24} \cdot 11H_{2}O}$$, and $$\mathrm{Ca_{2}Si_{8}Al_{4}O_{24} \cdot 11H_{2}O}$$. The $$\mathrm{Na^{+}}$$form is the most prevalent of the three. Gmelinites are found in basalts, one locality being the Antrim basalt in eastern Ireland where they are found only in a small portion of the basalt, but are abundant in that location (Walker 1959). Gmelinite occurs in other various locations such as in Kazahkstan, parts of the United States, Nova Scotia and others (Paukov 2001, Chiyoda 2000) with other sodic aluminozeolites such as analcime.

This study looked into the shifts in the Raman spectra, as well as optical changes, for various forms of gmelinite, $$\mathrm{Na^{+}}$$, $$\mathrm{K^{+}}$$, $$\mathrm{Rb^{+}}$$, and $$\mathrm{Cs^{+}}$$.

## Gmelinite Structure

\label{subsection:gmelstructure}

The gmelinite framework is made of 6MRs interconnected to one another. The 6MRs then form a cage structure when layered together, also forming a 6MR double ring. These 6MRs reside in between 12 MRs, as seen in Figure \ref{fig:structure}. There is no Si or Al order in the 6MR suggesting that the channel pathways are likely to be disrupted (IZA Commission on Natural Zeolites 2005, Benco 2003). Extra-framework cations can be found in two different localities within the gmelinite framework often labeled as C1 and C2 (Figure \ref{fig:structure}). Two C1 sites can be found in the 6MR cage, and are nearly 100% occupied. The 12MR makes the channel pathway structure where there are 6 possible C2 sites that are typically less than 35% occupied (IZA Commission on Natural Zeolites 2005, Benco 2003).

\label{fig:structure} A structural model of gmelinite. The extra-framework cations can be found at either the C1 (orange) or C2 (yellow) cites. C1 sites are found in the 6MR cages bonded to framework $$\mathrm{O^{2-}}$$or to $$\mathrm{H^{+}}$$as $$\mathrm{H_{2}O}$$. The C2 cites are found in the 12MR. The blue circles denote bond locations (IZA Commission on Natural Zeolites 2005).

# Previous Work on Gmelinite

Gmelinite is a member of the P63/mmc space group, with its aluminosilicate structure discussed in Secti