## 1 Introduction  
Jarosite, a secondary iron sulfate mineral is indirectly synthesized by oxidation of sulfide minerals. Minerals belonging to the jarosite series [1] comprise various isostructural compounds that generally occur in acidic environments such as mines, at the surface or near-surface conditions [2]. Their general formula is A\({}_{1,2}\)(H\({}_{3}\)O)\({}_{3}\)Fe\({}^{3+}\)\({}_{3}\)\({}_{2}\)(OH)\({}_{6-3y}\) (H\({}_{2}\)O)\({}_{3y}\), where the A could be (K, Na, H\({}_{3}\)O\({}^{+}\), NH\({}_{4}\), Ag and Tl) or Pb cation [2]. One of the rare natural compounds in the jarosite minerals group contains hydronium ion [2, 3]. Jarosite minerals have been synthesized using different methods that include chemical synthesis [4] and the biological oxidation of Fe [5], with acidity and temperature being shown to be significant factors affecting hydronium jarosite formation [6]. Unfortunately, phyto-engineering has not been explored enough in the synthesis of any jarosites, either as a mineral compound or at the nano-scale range. Phyto-engineering that employs plant materials to produce safe and biocompatible nanomaterials has received considerable interest in recent years due to the low cost, non-toxic, environmentally friendly, timeous, and 'one-pot' synthesis methodology employed during nanomaterial manufacturing [7]. Plant phytochemicals or photo-active including phenolic compounds have also been shown to serve as excellent reducing and chelating agents in the synthesis of smart nanomaterials harboring unique physico-chemical properties [8]. An example of a plant material that harbors a rich source of phytochemical compounds includes agro-waste material such as _Persea americana_ Mill. (avocado) seeds [9]. When the avocado seed extract was used as a chelating and reducing agent for iron sulfate, hydronium jarosite sheets coated with nanoparticles were obtained. This study further elucidated the electrochemical and magnetic properties of the avocado seed-mediated hydronium jarosite nanomaterials [10].  
The biomedical applications of hydronium jarosite nanomaterials synthesized using green methods have, however, not been investigated. We are also not aware of any studies on the synthesis of hydronium jarosite nanomaterials using specifically the husk of the avocado seed. Avocado seed husk is particularly rich in phytocompounds with high antioxidant potential which may potentially mediate the cytotoxicity of a nanomaterial when the seed husk is used in green nanochemistry [11]. Furthermore, since it is known that hydronium jarosites exhibit antimicrobial properties [12], hydronium jarosite nanoparticles may exhibit antimicrobial potential due to their unique physico-chemical makeup.  
Here, we report on the synthesis of a rare type of mineral--hydronium jarosite--in the form of a nanomaterial by using _P. americana_ seed husk aqueous extract as a reducing and chelating agent for the reduction of iron (II) sulfate heprahydrate. We further report on the structural characterization, cytotoxicity, and antimicrobial studies of the nanoproduct against murine macrophage cells (Raw 264.7) cell cultures and common infectious bacteria such as Gram-negative _Escherichia coli_, Gram-positive (_Staphylococcus aureus_), and methicillin-resistant _Staphylococcus aureus_ (MRSA). This study may have unique implications for the use of an agro-waste product--avocado seeds--as a sustainable source of nano-antibacterial.