L`qbg1//7Lhmdq`knesgdLnmsg9Rsdkkdqhsd This month’s mineral, stellerite, is a rare zeolite with a decidedly international flavor. It was first found in Russia, recognized as a mineral species by a Polish mineralogist, and named for a German naturalist. Our specimens were collected in India from the Deccan Traps, one of the world’s largest volcanic provinces and the source of the finest zeolite specimens. OGXRHB@K OQNODQSHDR Chemistry: Ca4(Al8Si28O72)A28H2O Hydrous Calcium Aluminosilicate (Hydrous Calcium Aluminum Silicate), usually containing some sodium and potassium Class: Silicates Subclass: Tectosilicates Group: Zeolite Subgroup: Heulandite Crystal System: Orthorhombic Crystal Habits: Dipyramidal, usually spherical as radiating, rounded nodules or sheaf-like aggregates of thin, platy crystals; also massive and fibrous. Color: Usually colorless or white, sometimes pale hues of yellow, brown, orange, or pink Luster: Vitreous Transparency: Transparent to translucent Streak: White Refractive Index: 1.484-1.497 Cleavage: Perfect in one direction Fracture: Uneven, brittle Hardness: 4.5 Specific Gravity: 2.2 Luminescence: None Distinctive Features and Tests: Best field marks are radiating form, thin crystals, and primary occurrence in hydrothermally metamorphosed volcanic environments. Stellerite is easily confused with the zeolite minerals stilbite-Ca [(Ca0.5,K,Na)9(Al9Si27O72)A28H2O] and barrerite [Na8(Al8Si28O72)A26H2O]. Positive differentiation between these species often requires laboratory methods. Dana Classification Number: 77.1.4.4 M @L D Stellerite, which is named for German naturalist Georg Wilhelm Steller (1709-1746), is pronounced just as it is spelled—STELL-er-ite. Stellerite was formerly thought to have two varieties, which were referred to as “calcium stellerite” and “sodium stellerite.” In contemporary European mineralogical literature, stellerite appears as “stellerit” and “stellerita.” BNL ONRHSHNM Stellerite is the fifth zeolite group mineral to be featured as our Mineral of the Month. Our other zeolites have included scolecite [Ca(Al2Si3O10)A3H2O] in May 1997, stilbite-Ca [Ca0.5,K,Na)9(Al9Si27O72)A28H2O] in May 1999, heulandite-Ca [(Ca0.5,Na,K)9(Al9Si27O72)A24H2O] in November 2002, and thomsonite-Ca [Ca2Na(Al5Si5O20)A6H2O] in November 2007. Okenite [Ca5Si9O23)A9H2O], our January 1998 featured mineral, and apophyllite [KCa4Si8O20(F,OH)A8H2O], our July 1996/July 2006 featured mineral, are often found with zeolite group minerals in India, so that many assume they also are zeolite minerals, but though they are closely related, they do not belong to the zeolite mineral group. Bnoxqhfgs 1//7 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 6 6 / N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7//,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 1 L`qbg1//7Lhmdq`knesgdLnmsg9Rsdkkdqhsd As shown by its chemical formula Ca4(Al8Si28O72)A28H2O, stellerite contains the elements calcium (Ca), aluminum (Al), silicon (Si), oxygen (O), and hydrogen (H). Stellerite’s molecular weight is made up of 5.68 percent calcium, 7.66 percent aluminum, 27.90 percent silicon, 56.76 percent oxygen, and 2.00 percent hydrogen. The cations (positively charged ions) in the stellerite molecule include four calcium ions, each with a +2 charge, and eight aluminum ions, each with a +3 charge. These provide a cumulative cationic charge of +32. The anion (negatively charged ion) in the stellerite molecule is the silicate radical 32- (Si28O72) , with the -32 charge accruing from the +4 charge of each silicon ion and the -2 charge of each oxygen ion. The cumulative anionic -32 charge balances the +32 cationic charge to provide the stellerite molecule with electrical stability. The “A28H2O” in stellerite’s chemical formula means that it is a hydrous (or hydrated) mineral with 28 attached molecules of water (H2O). Attached water molecules are collectively called “water of hydration” and consist of electrically neutral water molecules that do not affect the electrical balance of the parent molecule. Water molecules can attach themselves to other molecules because of their unusual atomic configuration in which two hydrogen ions are grouped together on one side of a large oxygen ion. These grouped hydrogen ions retain a small positive charge, while the opposite side of the molecule, dominated by the large oxygen ion, retains a small negative charge. This subsequent polarity enables water molecules to function as tiny dipole magnets that attach themselves to other molecules with a weak attraction called “hydrogen bonding.” Stellerite is a member of the silicates, the largest and most abundant of all mineral classes. In the silicates, silicon and oxygen are combined with one or more metals. The basic silicate structural unit is the 4- silica tetrahedron (SiO4) , in which a silicon ion is surrounded by four equally spaced oxygen ions positioned at the four corners of a tetrahedron (a four-faced polyhedron). In the silicates, silica anions and metal cations join together in repeating chains to form seven types of structures: independent tetrahedral silicates (nesosilicates); double tetrahedral silicates (sorosilicates); single- and double-chain silicates (inosilicates); ring silicates (cyclosilicates); sheet silicates (phyllosilicates); and framework silicates (tectosilicates). Stellerite is a framework silicate or tectosilicate, a classification that includes such common and familiar minerals as quartz [silicon dioxide, SiO2] and the feldspar group of complex aluminum silicates containing potassium, calcium, and/or sodium. But the stellerite crystal lattice is actually based on a modified 4- 5- tectosilicate structure in which silica tetrahedra (SiO4) alternate with alumina tetrahedra (AlO4) . This arrangement is reflected in stellerite’s chemical formula Ca4(Al8Si28O72)A28H2O, which expresses the 3+ 32- aluminum ions [Al ] as a structural part of the aluminosilicate radical (Al8Si28O72) . Each stellerite molecular unit consists of 14 silica tetrahedra and 4 alumina tetrahedra. Because of its alumina tetrahedra, stellerite is more accurately described as a modified tectosilicate or an aluminosilicate. Within the stellerite lattice, all silica and alumina tetrahedra covalently share their four oxygen ions to create a repetitive, three-dimensional structure of hollow cells separated by channels and large spaces. Because the charge of the aluminosilicate tetrahedron is -5, the stellerite molecular units carry an excess negative charge that must be balanced by additional cations. This is achieved by four calcium ions [4Ca2+] that bond ionically to each molecule. The channels and spaces within and between these molecules provide space for the attachment of 28 water molecules by hydrogen bonding. Stellerite’s hollow molecular cells and intermolecular channels, which are characteristic of all zeolite minerals, account for its low specific gravity (density) of only 2.2. Its perfect one-directional cleavage is explained by the alternate layering of silica and alumina tetrahedra in sheet-like arrangements. At Mohs 4.5, stellerite is among the softest of all silicate minerals. This softness is mainly due to the considerable interionic distances within the hollow molecular units that reduce the strength of the atomic bonds. Bnoxqhfgs 1//7 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 6 6 / N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7//,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 2 L`qbg1//7Lhmdq`knesgdLnmsg9Rsdkkdqhsd This unique structure of zeolites provides several unusual properties. First is the ability to lose and regain water of hydration without any alteration of the crystal structure. Another is that the network of hollow spaces and channels passes certain ions, atoms, and molecules while blocking others, and thus functions as a molecular filter. Finally, the weak cationic bonding within zeolite molecules is easily broken, enabling substitution by other metal cations in a phenomenon called “ion exchange.” All these zeolite properties have many industrial applications (see “Technological Uses”). Through complex metal-cation substitution, stellerite forms a partial solid-solution series with two other minerals, stilbite-Ca, [hydrous calcium potassium sodium aluminosilicate, (Ca0.5,K,Na)9(Al9Si27O72)A28H2O] and barrerite [hydrous sodium aluminum silicate, Na8(Al8Si28O72)A26H2O]. In the stilbite-Ca partial series, both potassium and sodium substitute for some of the calcium in stellerite. Because this complex substitution alters the silicate-aluminosilicate arrangement within the lattice, stilbite-Ca, unlike stellerite, crystallizes in the monoclinic system. But in the partial solid-solution series with barrerite, only sodium replaces some of stellerite’s calcium in a simpler substitution that does not alter the crystal lattice. Because both barrerite and stellerite crystallize in the orthorhombic system, they can be very difficult to distinguish. Stellerite is a rare mineral that occurs primarily in amygdaloidal cavities of basaltic volcanic rocks. These cavities form from gas bubbles during the solidification of silica-deficient (mafic) magma. In subsequent low-grade, hydrothermal metamorphism, percolating alkaline groundwater deposits zeolite minerals, often as well-developed crystals associated with other zeolite minerals, quartz, and calcite [calcium carbonate, CaCO3]. Stellerite occasionally occurs with other zeolites as a cementing material in sandstones, and in bedded sedimentary deposits where percolating alkaline