What is cacoxenite in amethyst?
Cacoxenite in amethyst labels golden sprays in quartz. Cacoxenite is an iron phosphate, 26% P2O5; goethite has no phosphorus; GIA identified goethite by EDS.
“Cacoxenite in amethyst” names golden to brown radiating sprays inside purple quartz. Cacoxenite is an iron phosphate, 26.04% P2O5, hardness 3 to 4, and its Handbook sheet names no quartz. GIA researchers identified radial goethite clusters in amethyst by EDS. Without an analysis showing phosphorus, the inclusion’s identity is not established.
In short
- What the label means. Golden, yellow-brown or orange radiating sprays inside amethyst, the purple variety of quartz. The amethyst itself is quartz, hardness 7, density 2.65; whether it is natural is the question on how to tell if amethyst is real. The inclusion name is the part that needs evidence.
- The phosphorus rule. Cacoxenite is Fe3+24AlO6(PO4)17(OH)12(H2O)75 on the IMA list: 26.04% P2O5, 11.37% phosphorus by mass, worked. goethite is FeO(OH), hematite Fe2O3 and quartz SiO2: no phosphorus in any of them. No phosphorus, no cacoxenite. An analysis that finds phosphorus in the spray does not prove cacoxenite either, but one that finds none rules it out.
- What GIA found. Maloney, Belley and Conliffe, Gems & Gemology, Fall 2024 list radial bladed to acicular goethite clusters up to 1 mm long, EDS-confirmed, in Newfoundland amethyst, and write that radiating aggregates of ochre-colored bladed to acicular goethite are common in amethyst from numerous sources, including geodes in basalt. They do not mention cacoxenite.
- The named-in-source check. Three GIA pieces were fetched on brown or golden inclusions in amethyst and quartz: Newfoundland amethyst (2024), multi-colour-zoned quartz from Brazil (2017) and trapiche-like amethyst from Brazil (2018). By script, cacoxenite appears 0 times in each; goethite in all three. The cacoxenite sheet itself names no quartz, amethyst or Brazilian locality.
- Documented failure: the eye cannot settle it. Both minerals grow as yellow to brown radiating needles; both sheets print acicular or radial habits. Inside quartz the spray cannot be streaked or scratched. GIA’s 2018 note called brownish inclusions in Brazilian amethyst likely goethite, based on appearance and occurrence in quartz matrix, while saying they could not be identified. That is the most a lens can do.
| Name | Formula (IMA) | Phosphorus, wt% | Habit (sheet) | Hardness | Density | 20.00 g piece in water | Streak (sheet) | Colour, lustre (sheet) | Named in the 3 GIA articles |
|---|---|---|---|---|---|---|---|---|---|
| Cacoxenite | Fe3+24AlO6(PO4)17(OH)12(H2O)75 | 11.37 (26.04 as P2O5) | Acicular along [0001]; compact concentric spherical to radial aggregates to 8 mm | 3–4 | 2.2–2.6 | 10.91–12.31 g | None printed | Yellow, golden yellow, brownish yellow, reddish or deep orange; silky | 0 of 3 |
| Goethite | FeO(OH) | 0 | Capillary to acicular crystals in divergent sprays; masses with radial fibrous internal structure | 5–5.5 | 4.28 | 15.33 g | Brownish yellow, yellow-orange, ocher-yellow | Blackish brown; yellowish to reddish brown in aggregates; silky when fibrous | 3 of 3 |
| Hematite | Fe2O3 | 0 | Rhombohedral, pseudocubic, prismatic; platy, in rosettes; radiating fibrous masses | 5–6 | 5.26 | 16.20 g | Cherry-red or reddish brown | Steel-gray, dull to bright red; metallic to dull | 2 of 3 |
| Quartz (the host, amethyst) | SiO2 | 0 | Prismatic crystals; colour from chemical or particulate inclusions | 7 | 2.65 | 12.45 g | White | Colourless, bluish violet, yellow to yellowish brown and others; vitreous | 3 of 3 |
What is cacoxenite in amethyst? A label for golden sprays
“Cacoxenite in amethyst” is a name given to amethyst with golden to brown radiating inclusions; the amethyst is quartz, and the inclusion name is an identification claim. Amethyst is the purple variety of quartz, SiO2, hardness 7, density 2.65, as on quartz and is quartz a mineral. The Handbook quartz sheet says quartz takes yellow to yellowish brown and other colours from chemical or particulate inclusions, without naming which minerals.
What cacoxenite is. cacoxenite is a hydrated iron aluminium phosphate, Fe3+24AlO6(PO4)17(OH)12(H2O)75 on the September 2026 IMA list: hexagonal, typically in compact concentric spherical to radial aggregates to 8 mm, yellow to golden or orange, silky, hardness 3 to 4. Its Handbook sheet puts it in oxidized magnetite and “limonite” iron ores, Fe,Mn-bearing novaculites, and iron-rich sediments and soils; by script, the sheet’s text contains no “quartz”, “amethyst”, “Brazil” or “Minas”.
So the question has two halves. Is the purple host amethyst? That is answerable at home with a refractometer and a scale; the method is on how to tell if amethyst is real. Are the sprays cacoxenite? That is a mineral identification inside another crystal, and needs an analysis. The rest of this page is about the second half.
Is cacoxenite in amethyst real? The phosphorus rule
No phosphorus, no cacoxenite: an inclusion that shows no phosphorus on analysis cannot be cacoxenite. Worked on standard atomic weights, the IMA formula weighs 4632.95, of which 17 phosphorus atoms make 11.37%; as oxides, 26.04% P2O5, the figure the Handbook prints in its ideal column. Iron is 28.93%, aluminium 0.58%. Goethite, FeO(OH), is 62.85% iron and contains no phosphorus; hematite, Fe2O3, none; quartz, SiO2, none.
What the rule can and cannot do. The rule only excludes. Phosphorus in a spray would be consistent with cacoxenite but also with other iron phosphates; cacoxenite’s own sheet names dufrénite, rockbridgeite, beraunite and strengite as associates, all phosphates. A positive identification needs structure as well as chemistry. GIA’s Newfoundland study identified its inclusions with a scanning electron microscope and energy-dispersive X-ray spectrometers (EDS), with reference hematite and goethite mounted in the same polished epoxy puck, which is the kind of comparison the question needs.
The rule in practice. A lens cannot read phosphorus. Hardness (3 to 4 against 5 to 5.5) and streak (none printed against brownish yellow) would separate cacoxenite from goethite on an exposed crystal, but not on a spray sealed inside quartz of hardness 7. How hardness is tested, and why not on a finished stone, is on how to test mineral hardness properly.
What are the golden inclusions in amethyst? What GIA identified
In the GIA work we could fetch, radiating golden-brown sprays in amethyst are goethite. Maloney, Belley and Conliffe, Gems & Gemology, Fall 2024 studied amethyst from the La Manche vein system, Newfoundland. Their inclusion list includes radial bladed to acicular goethite clusters up to 1 mm long, EDS-confirmed. They write that radiating aggregates of ochre-colored bladed to acicular goethite are common in amethyst from numerous sources, and that the inclusions are known in amethyst from epigenetic hydrothermal veins (hematite and goethite), geodes in basalt (goethite only), pockets in pegmatites (hematite only) and miarolitic cavities (hematite only).
Brazil. GIA Gems & Gemology, Fall 2017 describes multi-colour-zoned quartz from Brazil whose last pale purple amethyst layer was filled with bundles of brown goethite needles up to about 3 mm. GIA Gems & Gemology, Summer 2018 describes trapiche-like amethyst from Brazil with brownish tiny inclusions in its arms that could not be identified but were likely goethite, based on appearance and occurrence in quartz matrix.
The named-in-source check. Method: the three articles were saved as text and searched by script, case-insensitive. Cacoxenite: 0, 0 and 0 mentions. Goethite: present in all three; hematite in two; phosphorus in none. That is a count over three fetched articles, not over the literature, and it is not proof that cacoxenite never occurs in quartz. It is what the evidence we could fetch says.
What is and is not established
Established, by the sources on this page. Cacoxenite is a phosphate with 26.04% P2O5 (IMA formula, Handbook). Its sheet does not name quartz, amethyst or Brazil. Radiating goethite sprays are documented in amethyst, by EDS in at least one GIA study and described in two more. Goethite contains no phosphorus.
Not established. Whether any particular stone labelled cacoxenite in amethyst contains cacoxenite. We found no analysed source we could fetch that identifies cacoxenite as an inclusion in amethyst, and none that rules it out for every stone. Mindat refused the fetch, and dealer and forum pages were found by search but are not sources and are not used here. A laboratory report naming the inclusion, with its method, settles a given stone.
Related questions. Colour loss in amethyst is on why is my amethyst losing color; growth-zone phantoms, another inclusion feature, on what is a phantom crystal. The rest is indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.