Two mixites from Boss Tweed Mine, Tintic District, Juab County, Utah and Tin Stope, Majuba Hill, Pershing County, Nevada, USA, were analysed by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and by Raman spectroscopy. The SEM images show the mixite crystals to be elongated fibres up to 200 μm long and 2 μm wide. Detailed images of the mixite crystals show the mineral to be composed of bundles of fibres. The EDX analyses depend on the crystal studied, though the Majuba mixite gave analyses which matched the formula BiCu6(AsO4)3(OH)6.3H2O. Raman bands observed in the 880–910 cm−1 and 867–870 cm−1 regions are assigned to the AsO-stretching vibrations of (HAsO4)2− and (H2AsO4)− units, whilst bands at 803 and 833 cm−1 are assigned to the stretching vibrations of uncomplexed (AsO4)3- units. Intense bands observed at 473.7 and 475.4 cm−1 are assigned to the v4 bending mode of AsO4 units. Bands observed at 386.5, 395.3 and 423.1 cm−1 are assigned to the v2 bending modes of the HAsO4 (434 and 400 cm−1) and the AsO4 groups (324 cm−1). Raman spectroscopy lends itself to the identification of minerals on host matrices and is especially useful for the identification of mixites.

The combination or stacking of different traits or genes in plants is rapidly gaining popularity in biotech crop production. Here we review the existing terminology regarding gene stacking in plants, and its implications in relation to genetics, biosafety, detectability and European regulations. Different methods of production of stacked gene traits, as well as the status of their cultivation and approval, are reviewed. Related to the different techniques of transformation and production, including classical breeding, and to differences in global authorization and commercialization practices, there are many types, definitions, and perceptions of stacking. These include: (1) stacking of traits and (2) stacking of events, which are the most widely accepted perceptions of stacking, and (3) stacking of genes, which from the analytical and traceability point of view may be a more appropriate perception. These differences in perceptions and definitions are discussed, as are their implications for analytical detection and regulatory compliance according to (in particular) European Union (EU) regulations. A comprehensive terminology regarding gene stacking with regulatory relevance is proposed. The haploid genome equivalent is proposed as the prevailing unit of measurement at all stages throughout the chain, in order to ensure that terminology and definitions of gene stacks are adapted to analytical detection, traceability, and compliance with EU regulations.