Kaolinite aggregates are the major alteration product in the strongly deformed areas of a shear zone developed in contact-metamorphosed slates from the Southern Iberian Massif near Jaén (SE Spain). Muscovite and Fe-rich chlorite in strain shadows of broken andalusite crystals were observed in slates from less deformed areas. The presence of phyllosilicate packets nearly perpendicular to each other and veinlets parallel to {110} of andalusite reveal the importance of cleavage-controlled fracturing of andalusite to the mineral reactions. The compositional similarity between kaolinite and andalusite suggests a direct transformation of andalusite to kaolinite in the strongly altered area. The absence of clear preferred orientation of phyllosilicates relative to andalusite, and the compositional differences of andalusite with muscovite and chlorite from the less altered samples, indicate andalusite dissolution and chlorite-muscovite precipitation during a first mineral-growth stage. Muscovite-kaolinite and chlorite-kaolinite intergrowths indicate a later retrograde replacement stage.

Unusual corundum-bearing rocks occur in the Bond Range, northern Tasmania, in a hydrothermally altered Cambrian quartz porphyry. The assemblage exhibits quartz and corundum in mutual contact, a rare phenomenon, in association with andalusite, pyrophyllite, diaspore and other minerals. This metastable assemblage apparently resulted from advanced argillic alteration at moderate temperature and low pressure, followed by rapid depressurisation accompanying boiling of hydrothermal, granite-derived fluids. This corundum occurrence appears to be unrelated to the sapphires in placer deposits in Tasmania.

Manganian andalusite occurs abundantly as porphyroblasts in manganiferous metasediments subjected to contact metamorphism under hornblende hornfels facies at the contact of a picrodolerite dyke near Manbazar, Purulia District, India. The Mn2O3 content of andalusite varies from 13.2% to 19.17%, corresponding to 14.8 and 21.14 mole per cent of ‘Mn2SiO5’ respectively. Based on the analysis showing maximum amount of Mn2O3 in andalusite, the mineral formula may be represented as follows:

Other minerals in the assemblage are muscovite, manganophyllite, spessartine, piemontite, quartz, braunite, hematite and rutile. The manganian andalusite is completely fresh and appears to have formed at the expense of spessartine, piemontite and braunite during contact metamorphism. The manganian andalusite probably formed at about 600°C at around 3 kbar pressure. This is another rare example of andalusite with very high Mn2O3 (and Fe2O3) as well as that of an occurrence of abundant manganian andalusite.

Subsilicic sodium gedrite has been found in a hydrothermal vein together with sekaninaite (Fe-cordierite), andalusite, plagioclase, topaz, sillimanite, quartz, biotite, magnetite, ilmenite, hercynite, wolframite, pyrrhotite, chalcopyrite and pyrite. This vein is part of a hydrothermal vein system consisting predominantly of quartz-feldspar veins, some of which contain andatusite, sekaninaite, biotite, muscovite, fluorite and accessory oxides and sulphides, which was formed during the later stages of crystallization of a high-level anorogenic Svecofennian granite. Petrographic observations suggest the following crystallization sequence for the quartz-feldspar veins: plagioclase-quartz-andalusite-sekaninaite-microcline-biotite-albite-oxides and sulphides-muscovite-fluorite, and for the subsilicic sodium gedrite-bearing vein: andalusite-sekaninaite-subsilicic sodium gedrite-biotite-quartz-albite-sillimanite-topaz-oxides and sulphides. Electron microprobe analysis revealed that all subsilicic sodium gedrite is relatively homogeneous with only the following compositional variation: Na(A) 0.57–0.81, Aliv 2.31–2.57, XMg 0.15–0.21. The temperature (T) for the formation of the hydrothermal vein system is estimated at 550–600°C and the pressure (P) is estimated to be less than 3 kbar.

Many evolved peraluminous granite plutons contain either andalusite (Al2SiO5) or topaz (Al2SiO4(OH,F)2), but some plutons contain both of these A/NK [mol. Al2O3/(Na2O+K2O)] = ∞ phases. We present the results of experiments conducted to locate the andalusite-topaz boundary in water-saturated peraluminous haplogranite melts in T-P-Xspace, where T = 700—650°C, P = 200 MPa. andX(A/NK = 1.1, 1.2, 1.3, 1.4, and F = 0, 1, 2, 4 wt.%).The starting materials are synthetic K-Na-Al-Si gels, with added H2O and AgF, with seeds of natural andalusite and topaz, and with run times of 5—7 days. Phase identification is by electron microprobe. All experimental run products contained quartz. For all values of bulk A/NK, F concentrations ⩽ 1 wt.% favour andalusite stability, whereas F concentrations ⩾ 1 wt.% favour topaz stability. For bulk A/NK ⩽ 1.2 and intermediate F concentrations, no A/NK = phase is present; for bulk A/NK ⩾ 1.3, quench or metastable mullite formed. Topaz reaction rims on naturally occurring andalusite, frequency of naturally occurring assemblages, general chemical considerations, and experimental evidence all suggest that some topaz is the product of a peritectic reaction between an early primary magmatic andalusite and a late F-enriched melt. The appearance of such topaz is an important mineralogical expression of F activity in the melt, and may be an indicator for high field-strength Sn-W-U-Mo polymetallic mineralization.

The unit-cell parameters of andalusite and sillimanite have been measured by high-T powder X-ray diffraction up to 1000°C at ambient pressure. Within the temperature range investigated, all the unit-cell parameters varied smoothly, indicating no phase transition. The volume-temperature data were fitted with a polynomial expression for the thermal expansion coefficient (αT = a0 + a1T + a2T-2). yielding a0 = 2.55(2) × 10–5K–1, al = 0 and a2 = 0 for andalusite, and a0 = 1.40(4) × 10–5K–1a1 = 7.1(8) × 10–9K–2 and a2 = 0 for sillimanite. Using the new thermal expansion data determined in the present study and compressional data from the literature, the P-T phase relations of the kyanite-andalusite-sillimanite system were calculated thermodynamically, with the invariant point located at ∼523°C and 3.93 kbar.

Graphitic, quartz-rich black slate within the Late Ordovician Abercrombie Beds, Lachlan Fold Belt, southeast Australia, has undergone contact metamorphism adjacent to the Siluro-Devonian Wyangala Batholith. This produced porphyroblasts of andalusite and cordierite, and smaller grains of pale green phengitic mica. Later regional metamorphism caused complete retrogression of cordierite and partial retrogression of andalusite, with margarite replacing some andalusite.

The aluminous minerals andalusite, margarite and phengite all contain V and Cr substituting for Al. Andalusite has up to 1.39% V2O3 and 1.09% Cr2O3, margarite has up to 1.07% V2O3 and 0.37% Cr2O3, and phengitic mica has up to 6.93% V2O3 and 1.52% Cr2O3. This mica also has BaO contents of up to 1.96%.

Chemical analyses reveal very high SiO2 contents for these rocks (about 89%), carbon contents of about 2%, and extremely low CaO, FeO, MgO and Na2O. Although V and Cr are prominent in aluminous minerals, their concentrations in the rock are only about average for black shales. However, Ba values range from 2000 to 6000 p.p.m., well above average for black shales. It is suggested that V and Cr probably precipitated from sea water, but Ba may have been concentrated by planktonic organisms such as radiolaria.