The thermal transformation of a Brazilian antigorite heated dry from 600° to 1300°C, was studied by high resolution electron microscopy and selected area electron diffraction (SAD). Below 600°C, the antigorite particles did not change morphologically or crystallographically, as evidence by the presence of the superlattice in the SAD patterns and lattice images. At 650°C, traces of antigorite remained, although the superlattice structure had disappeared and the first indications of forsterite formation were observed in the same SAD. Diffraction patterns of the transition phases showed various topotactical relationships between antigorite and forsterite. At 800°C, the crystallization to forsterite was much more pronounced. At 900°C, the individual particles retained their original shapes, but they were actually pseudomorphs, for their SAD patterns showed only forsterite. Between 1000° and 1300°C various topotactical relationships were observed. Although the overall transformation to forsterite appears to be similar for both chrysotile and antigorite, the transformation of antigorite took place at a slightly higher temperature (~50°C) with a larger number of topotactical relationships than for chrysotile. Enstatite was formed at 1300°C, but it was impossible to determine the topotactic relations between enstatite and forsterite, which was possible with chrysotile.

Alteration of the crystal structure of Mulhacén antigorite caused by dry, vibration grinding was investigated by X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analyses (TG), grain-size distribution, and transmission and analytical electron microscopy (TEM, AEM). Grinding for 1 min reduces particles to a size ideal for IR and TG. With prolonged grinding, XRD and electron diffraction patterns showed that the crystal structure was affected mainly along the c axis, causing a partial loss of crystallinity. TG analyses revealed that vibration grinding modified mineral dehydration, accelerating the dehydroxylation process and transforming the structural OH to adsorbed water in the resulting matrix. IR spectra and AEM showed that grinding affected the tetrahedral sheet to a lesser extent than the octahedral sheet. Partial release of Mg by preferential destruction of the octahedral sheet after 10 min grinding produced an increase in the Si/Mg ratio in semi-crystalline particles, whereas the amorphous material product after 120 min showed the same composition as the initial antigorite. TEM and grain-size distribution results revealed that grinding led to a general decrease in particle size at the beginning of the experiment followed by the agglomeration of ultrafine particles as grinding proceeded.

Physical alteration of magnetite-bearing antigorite grains is investigated in this study using Mössbauer, visible/near-infrared (VNIR) and mid-IR spectroscopy coupled with scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses. An expected decrease in grain size with grinding is observed using SEM. The HRTEM images illustrate that the nanophase-sized grains which adhere to larger grains have 7 Å antigorite patterns. Mössbauer spectroscopy shows the presence of antigorite, magnetite and an amorphous phase. Visible/near infrared spectra exhibit features common in serpentine. These spectra also show an increasing continuum slope with grinding, an effect which is characteristic of thin coatings or tiny grains on surfaces. Mid-IR spectra indicate the formation of fine-grained Si-OH and carbonate in these samples with grinding.

The second occurrence of the asbestiform silicate carlosturanite is reported. The mineral occurs in Taberg, Sweden, and it was previously reported as ‘picrolite’, that is fibrous antigorite. Carlosturanite from Taberg is non-pleochroic, green in colour and has positive optical elongation. The unit cell parameters are a = 36.55, b = 9.31, c = 7.27Å, β = 101.05°. Based on the determined formula, (Mg20.47Fe0.59Mn0.06Cr0.01Ti0.01) (Si11.68Al0.28)O27.81(OH)34.19H2O carlosturanite can be expected to crystallize within the pure MgO-SiO2-H2O system as a low-grade metamorphic mineral.

Because there are close similarities in structure and properties of carlosturanite and the picrolite variety of antigorite, their distinction requires careful attention to X-ray and electron diffraction patterns and chemical compositions. The present identification of the Taberg specimen as carlosturanite puts a lower limit of 33.7 Å to the known range of a parameters of antigorite.

The low-grade contact metamorphic decomposition of antigorite serpentine to olivine and talc has been studied using TEM. The reaction was more complex than has been assumed by previous workers in that (i) FeO played a significant role in controlling the temperature of reaction, and (ii) minor amounts of an aluminous phase (chlorite) were probably produced. The reaction occurred over a range of temperatures within which the assemblage antigorite + talc + olivine was stable and the antigorite became progressively more Mg-rich as the reaction proceeded. Oriented nucleation of talc occurred within the antigorite. However, the olivine breakdown product did not require nucleation; rather it appears to have grown on pre-existing olivine grains. Communication between the sites of reaction was probably facilitated by an ubiquitous grain-boundary fluid.

A critical examination of the reference X-ray powder-diffraction patterns of the serpentine minerals in the Powder-Diffraction File (PDF) database has revealed an unsettling situation. Most of the patterns in, or previously in, the PDF database are inaccurate, misidentified, or of poor quality. The PDF database is not a dependable tool for identifying the serpentine minerals, and has not been since the mid-1960s. This has serious implications for studies on serpentine minerals that have depended on the PDF database, particularly those by nonmineralogists doing health and environmental studies of chrysotile asbestos. In the current PDF database, lizardite-1T, carlosturanite, some amesite, and possibly some antigorite (but with inappropriate polytype symbols) can be identified. Only one of the many multilayer lizardites can be identified. The current pattern for chrysotile-2Mc1 (clinochrysotile) is of reasonable quality, but not the best, however the earlier patterns still in the database are so problematic that any chrysotile-2Mc1 identification must be considered suspect. Chrysotile-2Oc1 (orthochrysotile), and any mixture of serpentines cannot be identified using the PDF database. Until the reference serpentine patterns are corrected the PDF database cannot be considered a reliable identification tool. High-quality powder-diffraction patterns of the serpentine minerals have been published and can be rapidly introduced into the PDF database.© 2000 International Centre for Diffraction Data.