On the dimorphy between the variscite and clinovariscite group: refined finestructural relationship of strengite and clinostrengite, Fe (PO 4 The isodimorphy of the ortho-and clino-minerals in the variscite group has been suggested by Strunz & von Sztrókay [6], but, so far the common stereo-chemical patterns within the monoclinic and orthorhombic modifications had not been recognized. Structure refinements of the hydrated phosphate minerals strengite and clinostrengite, which are re-presentative members of the variscite and clinovariscite group (Strunz [7]), are presented, and by appropiate axis transformations their structural relationship is illustrated.

The space group of rhodizite has been cgnfirmed to be 4+3_ with the cell constant a -7.31910.001 A. A new analysis and determination of the density of the Manjaka (Madagascar) material by Frondel. and Ito suggested to them the formula CsB 1 1 Al 4 Be 4 (01)4095'Intensities of 504 of the 760 reflections of the asymmetric Friedel unit were measured using filtered MoKo( radiation and a scintillation detector. After these were corrected .or Lorenta, polarization and absorption factors, a threedimensional Patterson function was computed. The structure deduced from this and three-dimensional electron-density and electron-density difference maps was refined torl2bj. This structure has an ideal composition GsBl2Be 4 A14O28. The nonalkali part, B 12 BgA1 4 0 28 , consists of close-packed oxygen atoms held together by Be and B atoms in tetrahedral coordination and Al in octahedral coordination, all forming a connected network, The omission of 4 oxygens from the closepacked set provides a hole which houses the alkali. The sum of the valences of the atoms of the network suggests that it is formally neutral; if so, the alkali would appear to be un-ionized. Alternatively, the alkali may contribute one electron to a possible metallic bond between the Al atoms. Such a metallii bond is suggested by the short Al-Al distance, which is 2.95 A.

The mineral rhodizite could not be assigned a chemical formula on the basis of chemical analyses. The solution of the crystal structure reported here reveals that CsB(12)Be(4)Al(4)O(28) is the ideal formula. The oxygen atoms occupy all but four sites per cell of cubic close packing whose period is (1/2)a. The boron, beryllium, and aluminum atoms occupy interstices of this array, the first two in tetrahedral coordination, the last in octahedral coordination. Together these atoms form a continuous network whose composition is B(12)Be(4)Al(4)O(28), which should be neutral. The alkali atom is enclosed in a cage in this network and presumably is not behaving as an ion.

The volume changes of cesium under pressure are discussed with respect to the packing density of the atoms and valence. The element is univalent in densely packed Cs I and Cs II. Valence increases in Cs III (packing density q = 0.973), in Cs IV (q = 0.943), in Cs V (q ≈ 0.99), and in close packed Cs VI. The diminuition of volume beyond ≈15 GPa is caused by this increase only which implies that electrons of the fifth shell act as valence electrons.

The volume changes of solid iodine under pressure are discussed with respect to the packing density of the atoms and to valence. The packing density of solid iodine which is 0.805 under ambient pressure increases to 0.976 in monoatomic iodine-II, 0.993 in iodine-III, and 1 in fcc iodine-IV. Simultaneously, the valence increases from 1 in the free molecule to 1.78 in the crystal structure under ambient pressure, 2.72 - 2.81 in iodine-II, 2.86 - 2.96 in iodine-III, and 3 in fcc iodine-IV. The valence then remains constant up to about 180 GPa and rises moderately to 3.15 at the highest investigated pressure of 276 GPa. Parameters for calculating bond numbers, valences and atomic volumes of densely packed halogens, hydrogen, oxygen, and nitrogen are given.