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The chemical composition and morphology of solid material isolated from various Cactaceae species have been analyzed. All of the tested specimens deposited high-purity calcium oxalate crystals in their succulent modified stems. These deposits occurred most frequently as round-shaped druses that sometimes coexist with abundant crystal sand in the tissue. The biominerals were identified either as CaC 2 O 4 .2H 2 O (weddellite) or as CaC 2 O 4 .H 2 O (whewellite). Seven different species from the Opuntioideae subfamily showed the presence of whewellite, and an equal number of species from the Cereoideae subfamily showed the deposition of weddellite. The chemical nature of these deposits was assessed by infrared spectroscopy. The crystal morphology of the crystals was visualized by both conventional light and scanning electron microscopy. Weddellite druses were made up of tetragonal crystallites, whereas those from whewellite were most often recognized by their acute points and general star-like shape. These studies clearly demonstrated that members from the main traditional subfamilies of the Cactaceae family could synthesize different chemical forms of calcium oxalate, suggesting a definite but different genetic control. The direct relationship established between a given Cactaceae species and a definite calcium oxalate biomineral seems to be a useful tool for plant identification and chemotaxonomy.Calcium oxalate is considered to be the most commonly occurring biomineral in higher plants (for example, see reviews of Arnott and Pautard, 1970; Franceschi and Horner, 1980; Arnott, 1982). It has been demonstrated that crystal growth is a highly controlled intracellular process (Mann, 1989; Fraú sto de Silva and Williams, 1991; Baran, 1995). The cells in a plant tissue that produce the crystals are generally referred to as crystal idioblasts (Foster, 1956; Arnott, 1982).Calcium oxalate occurs in two hydration states in plants, as the monohydrate (whewellite) or as the dihydrate (weddellite; Frey-Wyssling, 1981; Arnott, 1982). A number of crystal habits have been found for both hydration states: raphids, prisms, styloids, druses, and crystal sand (Franceschi and Horner, 1980; Arnott, 1982). Other less common shapes could be considered as variations of the mentioned forms. Both the chemical nature and the morphology of these crystals, as well as their localization within the plant body, could be specific for a given species. Some higher plants may accumulate enormous quantities of inorganic material, and this is especially true for some members of the Cactaceae family (Franceschi and Horner, 1980). For example, as early as in 1938, a cactus species (Cactus senilis) was described as containing as much as 85% of its dry weight as calcium oxalate (Cheavin, 1938).We described recently the isolation and characterization of biominerals from two different Cactaceae species. This bioinorganic material appears in the form of highly pure and well-crystallized calcium oxalates that typically grow in the form of druses, i.e. s...

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Crystallographic data of vanadates, phosphates, chromates, sulphates and selenates belonging to the CrVO 4 (b-CrPO 4 ) structure type and of some of its most important polymorphs, are reported. Characteristic structural peculiarities and the effect of pressure on the phase transformations are discussed. A definitive stability field for this structure type could be established. Synthetic procedures for the preparation of the different compounds adopting this and closely related structures are presented and its thermal, spectroscopic and magnetic properties are discussed. A brief overview on the most important characteristics of materials adopting the a-CrPO 4 structure is also presented.