Goyazite

Chemical formula: SrAl<sub>3</sub>(PO<sub>4</sub>)(PO<sub>3</sub>OH)(OH)<sub>6</sub>

Strontium phosphate from the crandallite group, forming small, rhombohedral or pseudocubic crystals, often found in diamond-bearing alluvium.

## Characteristics Goyazite is a hydrated strontium and aluminum phosphate belonging to the alunite supergroup. It typically forms small, but well-developed rhombohedral crystals, which often resemble cubes (they are pseudocubic). It also occurs as granular aggregates, radial aggregates, and as pebbles in alluvial sediments. ## Physical Properties This mineral has a hardness ranging from 4.5 to 5 on the Mohs scale, meaning it is harder than fluorite but can be scratched by a steel tool. Its density is relatively high, ranging from 3.16 to 3.33 g/cm³. The luster is most often vitreous, but can also be resinous, greasy, and pearly on cleavage surfaces. It is a transparent to translucent mineral. ## Colors and Varieties Goyazite is most often colorless, white, gray, or has yellowish and honey-yellow hues. Pink, light brown, or greenish specimens are rarer. There are no named commercial varieties. ## History and Name The mineral's name, given in 1884 by the Austrian mineralogist Eugen Hussak, comes from the Brazilian state of Goiás. It was originally thought that the first studied specimens came from there. Later, it was established that the type material was actually found in diamond-bearing deposits near Diamantina in the state of Minas Gerais, Brazil. ## Uses Goyazite has no industrial application. It is primarily an object of interest for collectors specializing in mineral systematics, phosphates, or minerals from specific locations, as well as scientists studying weathering processes and the genesis of secondary deposits.

Properties

Mohs hardness
4.5-5
Luster
Vitreous to Resinous, Greasy
Streak
White
Density
3.16-3.33
Cleavage
Good on {0001}
Fracture
Conchoidal to Uneven
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Helpful features in identifying goyazite include its characteristic, pseudocubic rhombohedral crystals, relatively high density, and hardness around 5 on the Mohs scale. The environment of occurrence is also an important indicator, especially its presence in alluvial sediments (particularly diamond-bearing) or in rocks rich in aluminum and phosphorus. ## Distinguishing from Similar Minerals Macroscopic differentiation of goyazite from other minerals of the crandallite group – such as crandallite (calcium), gorceixite (barium), or florencite (cerium) – is practically impossible without advanced chemical analyses (e.g., EDS). It can be confused with some carbonates (e.g., dolomite), from which it differs by its lack of reaction with hydrochloric acid. It differs from quartz by its lower hardness and different crystal form. ## Crystal Forms It most often forms rhombohedra, which can be flattened or elongated, often resembling a cube. Crystals rarely exceed a few millimeters. It also occurs as granular aggregates, spherical and radial aggregates, and as rounded grains and pebbles in river sediments.

Geological environment

## Genesis Goyazite is a secondary mineral. It forms as a result of low-temperature hydrothermal processes or the weathering of rocks rich in aluminum, strontium, and phosphorus. It forms in the oxidation zones of ore deposits, in pegmatites, carbonatites, and in aluminum-rich sedimentary and metamorphic rocks. Due to its hardness and chemical resistance, it often concentrates in alluvial deposits as a heavy mineral. ## Mineral Associations Minerals co-occurring with goyazite are most often diamond (in clastic deposits), other phosphates of the crandallite group (gorceixite, florencite), apatite, monazite, quartz, kaolinite, as well as herderite and augelite. ## Localities The most important and classic localities for this mineral are in Brazil, in the state of Minas Gerais (Diamantina, Ouro Preto), where it occurs in diamond-bearing alluvium. It is also known from the United States (Arkansas, New Hampshire, California), Canada (Yukon Territory), Germany (Bavaria), Russia (Kola Peninsula), and many other places in the world where it occurs under appropriate geological conditions.

Rarity

Not very common

For collectors

## Quality Criteria The most valued specimens by collectors are those with sharp, well-formed crystals with distinct luster and pure color (e.g., honey-yellow or colorless). Transparent crystals are highly prized, although they are rare. The size of the crystals is of great importance – specimens exceeding a few millimeters are already considered significant. Attractiveness is enhanced by its placement on a rock matrix or association with other rare minerals. ## Popular Localities The historical diamond-bearing deposits in the Diamantina region of Minas Gerais, Brazil, are considered the source of the world's best goyazite specimens. Specimens from this locality are considered classic and most desired by collectors.

Care and storage

## Cleaning Goyazite specimens should be cleaned gently, using a soft brush and distilled water. Due to good cleavage, ultrasonic cleaners should be avoided, as they could cause crystals to crack. ## What to Avoid The mineral is relatively soft, so it must be protected from scratching by harder minerals, such as quartz or corundum. As a phosphate, it may be sensitive to strong acids. It does not require special protection from light or temperature changes. ## Storage Collectible goyazite specimens are best stored in separate, padded boxes or on a soft surface to avoid abrasions and mechanical damage. It should be isolated from harder minerals in the collection.

Sources

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