The crystal structures of three nepheline samples, (1) , from the Khibina-Lovozero complex, Kola Peninsula, Russia, (2) , from Monte Somma, Italy, have been refined to R 1 indices of 1.8, 2.7 and 1.7% using 1273, 1191 and 1364 unique observed (| F o | > 4F) reflections collected with a single-crystal diffractometer and MoK␣ X-radiation. No superstructure reflections were observed. Viewed down [001], the aluminosilicate tetrahedra occur at the vertices of a 6 3 net in which there are two distinct types of six-membered rings of tetrahedra: one quarter are nearly regular hexagonal rings centered on (0, 0, z), whereas the other three-quarters are flattened hexagonal rings centered on 0 ½ z. In both rings, the tetrahedra have the sequence ududud, and linkage up and down c forms a four-connected three-dimensional framework. The A site is located in the channel along 0 0 z and is [9]-coordinated by oxygen atoms between 2.97 and 3.04 Å; = 2.999, 3.009 and 3.008 Å for samples 1, 2 and 3, respectively. The A site is fully occupied by K in nepheline (1), and it is partly occupied by K (0.80 and 0.88) in samples (2) For the nepheline sample from the Khibina-Lovozero complex, the simplified chemical formula, K 2 Na 6 Si 8 Al 8 O 32 , corresponds to the ideal end-member formula of nepheline with the A site fully occupied by K, the B site fully occupied by Na, and Si and Al completely ordered at the T sites. This sample crystallized in a hyperagpaitic rock formed at the last stages of low-temperature pneumatolitic activity. Nepheline (2), also formed in low-temperature pegmatitic environment, shows very high Al-Si order at the T sites. The Monte Somma nepheline is of high-temperature origin. Previous refinements of the structure had indicated a fairly disordered Al-Si distribution for it. The present results indicate maximal Al-Si order, in accord with previous 29 Si MAS NMR results. Our results suggest that nepheline exhibits maximal Al-Si order irrespective of temperature of origin of the nepheline.Keywords: nepheline, cation ordering, end-member composition, Khibina-Lovozero, Russia, Bancroft, Canada, Monte Somma, Italy. SOMMAIRENous avons affiné la structure cristalline de trois échantillons de néphéline, (1) Mots-clés: néphéline, mise en ordre des cations, composition du pôle,

Megakalsilite, KAlSiO 4 , hexagonal, a 18.1111(8), c 8.4619(4) Å, V 2403.7(2) Å 3 , c:a = 0.4672, space group P6 3 , Z = 24, R = 0.039 (for 3255 observed reflections collected with a four-circle X-ray diffractometer), is a new mineral species from Mount Koashva, Khibina alkaline massif, Kola Peninsula, Russia. It is of pegmatitic origin. Associated minerals are K-feldspar, sodalite, cancrinite, natrolite, pectolite, aegirine, natrite, nacaphite, vitusite, fluorcaphite, belovite, umbite, lemmleinite-K, lomonosovite, lovozerite, phlogopite, sphalerite, and galena. The mineral was found in only one hand specimen of pegmatite rock as a corroded, irregularly shaped grain 2 ϫ 3 mm across, intergrown with cancrinite, sodalite, and natrite. It is transparent, colorless with a white streak, a vitreous luster, and fluoresces pale whitish green under ultraviolet light. Megakalsilite has a Mohs hardness of 6, is brittle with a conchoidal fracture, and has no cleavage. D meas is 2.58 (2)

The crystal structure of malinkoite, NaBSiO 4 , a 13.8964(4) Å, c 7.7001(2) Å, P6 3 , Z = 18, D x = 2.922 g cm -3 , from the Khibina-Lovozero complex, Kola peninsula, Russia, has been solved by direct methods and refined to an R index of 3.8% using 2094 unique observed (|F o | > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK␣ X-radiation. Malinkoite is a framework borosilicate with a structure consisting of six-membered rings of regularly alternating (SiO 4 ) and (BO 4 ) tetrahedra, and Na located in the large channels extending along [001]. There are three unique Si sites, each occupied by Si and each coordinated by four O atoms in a tetrahedral arrangement, with distances of 1.625, 1.622 and 1.622 Å, respectively. There are three unique B sites, each occupied by B and each coordinated by four O atoms in a tetrahedral arrangement, with distances of 1.476, 1.476 and 1.473 Å, respectively. There are five unique Na sites, each completely occupied by Na. Three of the Na sites are [9]-coordinated, with distances of 2.626, 2.642 and 2.630 Å, respectively; the remaining two Na sites are [6]-coordinated, with distances of 2.444 and 2.440 Å, respectively. The (SiO 4 ) and (BO 4 ) tetrahedra link to form six-membered rings of two different configurations based on the following sequences of up (U) and down (D) linkages: UDUDUD and UUDUDD in the ratio of 1:2. Nine-coordinated Na polyhedra are located in big cages formed by stacking of UDUDUD rings. Stacking of UUDUDD rings produces smaller cages occupied by [6]-coordinated Na. In accord with the ratio of two types of rings, there are twelve Na octahedra and six [9]-coordinated Na polyhedra per unit cell in malinkoite. The framework topology is related to that of beryllonite NaBePO 4 .In the crystal structure of lisitsynite, KBSi 2 O 6 , four-membered rings consist of two (SiO 4 ) and two (BO 4 ) tetrahedra, and are linked into zig-zag chains (puckered ladders) along [001]. There are two types of chemically distinct six-and eight-membered rings: (4Si + 2B) and (5Si + B); (5Si + 3B) and (6Si + 2B). Two different pairs of six-membered and eight-membered rings, (4Si + 2B) and (5Si + 3B), and (5Si + B) and (6Si + 2B), respectively, form sheets on (100) and (010). Stacking of sheets and zig-zag chains gives rise to a three-dimensional framework. There are one-dimensional seven-ring channels along the c direction. Lisitsynite, KBSi 2 O 6 , and synthetic "boroleucite", K(Si 2 B)O 6 , may be regarded as polymorphs with different patterns of Si-B order. SOMMAIRE Nous avons résolu la structure cristalline de la malinkoïte, NaBSiO 4 , a 13.8964(4), c 7.7001 (2) Å, P6 3 , Z = 18, D x = 2.922 g cm -3 , provenant du complexe de Khibina-Lovozero, péninsule de Kola, en Russie, par méthodes directes, et l'avons affiné jusqu'à un résidu R de 3.8% en utilisant 2094 réflexions uniques observées (|F o | > 4F), prélevées avec un diffractomètre pour cristaux uniques muni d'un détecteur CCD et avec rayonnement MoK␣. Il s'...

Nafertisite, (Na,K)3(Fe 2 + ,Fe 3 + , D)IO[Tiz(Si,Fe 3 + ,AI)12037](OH,O)6, is an alkali ne titanosilicate from the Khibina massif (Kola Peninsula, Russia). It is monoclinic A2/m with a = 5.353(4), b = 16.176(12), C = 21.95(2) A, ß = 94.6(2r, Z = 2. The comparison with bafertisite, Ba2(Fe,MnMTi2Si4017](OH,Oh, and astrophyllite, (K,Na)3(Fe,Mn)7 [TizSiR027](OH,O)4, allowed to obtain a structural model for nafertisite which has been tested against X-ray diffraction data obtained from a very poor crystal. On the basis of a bafertisite-like B module (A' o h(M D)4 [X2T4017](OHh and a mica-like M module (A, D)(MDh[T401O](OH)2, a polysomatic series BMn can be defined where bafertisite, astrophyllite, nafertisite and mica are the members with n = 0, 1, 2, 00, respectively. It is called heterophyllosilicate series because the crystal structures of the members consist of 2: 1 HOH layers where 0 is an octahedral sheet and H is a tetrahedral-like sheet which differs from the T sheet of the phyllosilicates for the insertion of Ti octahedra; these play a role similar to that of Si tetrahedra. A group of titanosilicates is shown to be derivatives of the bafertisite structure with different interlayer contents.

The present report contains recommendations on the nomenclature of the labuntsovite-group (LG) minerals based on a crystal-structural classification. The labuntsovite group includes hydrous titanium and niobium silicates, both orthorhombic and monoclinic, with a structure characterized by a framework consisting of chains of (Ti,Nb)O-octahedra, linked by four-member rings of Si,O-tetrahedra. This framework contains open cavities occupied by H 2 O molecules and extra-framework cations. Seven subgroups are distinguished, each corresponding to different structure types. In accordance with the nomenclature recommendations, LG minerals have different root names if they belong to different subgroups, or are characterized by different prevailing cations in the (Ti,Nb)-position. The species with doubled unit cell are distinguished by the prefix "para". In the case of structurally ordered members (space groups C2/m, I2/m), separate species within each subgroup are recognized by different root names (to distinguish the members with Ti 8 Nb or Ti ‹ Nb) and by a modifier (-K,-Mn,-Mg etc., to distinguish the members with differently occupied "key" positions). Unlike zeolites, a complex substitution of extra-framework cations is possible in monoclinic LG minerals: 2C → D(H 2 O) 2 where C = K, Ba; D = Fe, Mg, Mn or Zn. Only one of these coupled sites (C or D) can be more than 50 % occupied, whereas the other is more than 50 % vacant and is not considered as the "key" (species-forming) position. In the case of structurally disordered members (vuoriyarvite series) with space group Cm and a large number of extra-framework positions, separate species are recognized in which different extra-framework cations (K, Na, Ca, Sr, Ba) are the most abundant in atomic proportions.