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Hotsonite, Al5(PO4)(SO4)(OH)10•8H2O, is a hydrated aluminum-phosphate-sulfate which was discovered over 30 years ago in a geologically dynamic area known as the Bushmanland Group near Pofadder, South Africa. This area experienced a continental collision as well as intense metamorphic activity, creating a complex landscape for hotsonite’s formation. Hotsonite is a secondary mineral appearing as white, chalky compact masses encrusted in zaherite, Al12(SO4)5(OH)26•20H20 and found in metamorphic schists. Due to hotsonite’s fine-grained crystals and earthy texture, its structure has been difficult to ascertain. Modern analytical techniques should be utilized to further explore the unknown crystal structure and unique chemical properties of hotsonite.

Composition Chemical analysis of hotsonite was performed in Koenabib, South Africa in 1984 using wave-length dispersive x-ray fluorescence spectrometry (Beukes et al. 1984b). The main components are Al2O3, SO3, and P2O5 with minor quantities of trace minerals. When Beukes first studied hotsonite, the ideal formula was determined to be Al11(PO4)2(SO4)3(OH)21•16H2O (Beukes et al. 1984b). Further testing of samples from the Blyavinskij mine in the Urals, Russia yielded the ideal formula of Al5(PO4)(SO4)(OH)10•8H2O (Jambor et al. 1991). See Table 1 for a comparison of the analyses. Hotsonite is chemically similar to sanjuanite, Al2(PO4)(SO4)(OH)•9H2O, and kribergite,  Al5(PO4)3(SO4)(OH)4•4H2O. Hotsonite, kribergite and sanjuanite are the only known phosphate-sulfate minerals containing aluminum as the only cation (Colombo et al. 2011). (a)	(b)	 	(a)	        (b) SO3		16.80	13.73	CaO	0.24	0.89 P2O5	9.85	 9.06	Na2O	0.21 SiO2	0.35	 0.16	H2O+		31.29 TiO2		 0.01	H2O-		4.28 Al2O3	39.15	39.60	H2O	33.30 Fe2O3	0.03	0.18	insol. 0.33	 MgO	0.28	0.41	____________________ Total:	100.21	99.94 (a) Koenabib, South Africa (b) Blyavinski mine, Russia Table 1. Chemistry of hotsonite (Beukes et al. 1984b; Jambor et al. 1991) Structure Hotsonite belongs to the triclinic crystal system and fits within the Point Group 1 or *1 (Beukes et al. 1984b). The angles measure 112º32´, 107º32´ and 64º27´ for α, β and γ, respectively. The unit cell dimensions for hotsonite are a=11.288(59), b=11.658(60), and c=10.550(67), with the standard errors of estimate noted by parentheses (Beukes et al. 1984b). When examined by x-ray diffraction, the pattern of hotsonite resembles sanjuanite, but it is notably different than kribergite (De Bruiyn et al. 1989). Single-crystal studies have not been performed on hotsonite as its cryptocrystals have average grain dimensions of only 1 to 15 um (Colombo et al. 2011; Frost et al. 2011). EVERything else: Hotsonite appears as white, chalky veins and encrustations with lathlike or acicular crystals with average grain dimensions of 1 to 15 um. Hotsonite has an earthy fracture and a hardness of Mohs’ 2.5. Density was measured at 2.060-2.068g/cm3. The luster is dull to silky and appears colorless under 500x magnification with slightly elongated aggregates (Beukes et al. 1984b). Hotsonite is located in biotite-sillimanite schists in the Hotson Formation which overlays the Wortel Formation (Schoch et al. 1985; De Bruiyn et al. 1994). These formations belong to the Bushmanland Group, a metavolcano-sedimentary succession. Much of the stratigraphy is broken, making the interpretation of the geology of the area difficult to ascertain at first sight. The Hotson and Wortel Formations have experienced four known metamorphic events which created involved folding, thrusting, faulting, fracturing and shearing, with the strongest event occurring from 1.02 Ma to 1.4 Ma (Lacassie et al. 2006). During Rodinia’s collision, beginning approximately 1250 Ma, a process of intense folding and thrusting dramatically deformed the crust, creating the uplifted Namaqua Belt. Sillimanite and higher metamorphic rocks were formed in the Namaqua Belt during this period. After a geologically dormant period of approximately 150 Ma, around 1000 Ma, more folding and shearing occurred again (Colliston et al. 2013). Finally, weathering and hydrous environments set the stage for the chemical reactions necessary for hotsonite to develop. Hotsonite has a unique paragenesis. In most instances, hotsonite transforms through three distinct steps from sillimanite to natroalunite to zaherite to hotsonite. First, aided by sodium in local feldspar, hotsonite transforms from sillimanite to natroalunite. Second, through a hydrous or weathering occurrence, sodium and some sulfate may be removed, altering natroalunite to zaherite. Alternatively, sillimanite may transform directly to zaherite, consuming no sodium. It should be noted that zaherite is the rarest of these minerals. Finally, the conversion of hotsonite occurs, removing sulfate and water (Schoch et al. 1985).Hotsonite gets its name from the Hotson Farm where it was discovered in small abandoned open-cast sillimanite mines. The Hotson Farm is approximately 65 km west of Pofadder, South Africa (Beukes et al. 1984a). This area possesses the largest source of lead and zinc metal in South Africa as well as many other deposits such as sillimanite, gypsum, halite and bismuth (Bailie et al. 2006; Lacassie et al. 2006).