Uralite isn’t a mineral. It’s the term for a pseudomorph (“false form”) of an amphibole, usually actinolite-tremolite, replacing a pyroxene, usually augite or diopside.
The photos here are of specimens from a famous uralite locality, the Calumet Mine in the Turret Mining District, Chaffee County, Colorado USA.
The term pseudomorph covers a variety of origins, from one mineral encrusting another and taking its shape (some would call that a perimorph, or if the original material has been removed, an epimorph), to a true replacement at a level from molecular to the rock as a whole. Replacement might happen soon after the first mineral formed, or much later when conditions changed.
Uralite is one of the replacement varieties, one that probably essentially represents a chemical and structural change while the minerals were crystallizing.
Pyroxenes typically crystallize from molten material at higher temperatures than amphiboles, so pyroxenes usually form earlier, in a hotter melt. This relationship is part of Bowen’s Reaction Series, which predicts the general order of formation of minerals in typical rocks as they cool.
But it is not necessarily a discrete, clear-cut separation, with pyroxenes forming, their formation stopping, and then amphiboles forming. As crystals develop in molten rock, the composition of the remaining melt changes, and sometimes the early solid crystals may interact with that surrounding melt. When intrusive magmas interact with the rocks they intrude to the extent of exchanging chemicals, it’s called metasomatism (meaning “changed body,” in contrast to metamorphism, which is just “changed form,” without introduction of new chemicals).
The process of “uralitization” seems to be effectively one of metasomatism, but within a continually changing melt, leading some to call the process “autometasomatism,” meaning the chemical changes in the melt and their reactions with the early-formed pyroxenes produce new minerals, in this case amphiboles. Some might prefer the phrase “deuteric alteration,” which just means secondary alteration produced by the still-molten material itself, and not by some external cause.
Lower temperatures in the melt allow more water to be included in the crystallizing minerals, and indeed amphiboles like tremolite, Ca2Mg5(Si8O22)(OH)2, contain water (as OH), whereas pyroxenes like diopside, CaMgSi2O6, do not, even though both are fundamentally calcium-magnesium silicates.
The change from pyroxenes to amphiboles is common in crystallizing magmas, and has been known for a long time, albeit not usually in the fine form the crystals from the Calumet Mine display.
The chromolithograph above, drawn by George H. Williams (1886, USGS Bulletin #28, “The gabbros and associated hornblende rocks occurring in the neighborhood of Baltimore, Maryland”), shows brown cores of hypersthene, a magnesium-iron silicate in the enstatite (magnesium) to ferrosilite (iron) series of pyroxenes; the name hypersthene is now discredited but has a long and frequent use historically. The brown hypersthene is surrounded by rims of colorless to green hornblende, a catch-all term for minerals in a calcium-magnesium-iron-aluminosilicate amphibole group. The field of view for this thin section rendering is about 3 millimeters.
The rock here is from near Pikeville, Maryland USA, northwest of the city of Baltimore, where it was found in a zone of metamorphosed gabbro, the coarse-grained equivalent of basalt. Both are dark, iron- and magnesium-rich rocks, but fine-grained basalt crystalized relatively quickly, while coarse gabbro took longer to cool so its crystals grew larger. The origin of the word “gabbro” is from an occurrence near the village of Gabbro, Tuscany, Italy.
The gabbro in Williams’ chromolithograph is part of the Piedmont Province, a linear zone of Precambrian to early Paleozoic rocks that comprise the eastern part of the Appalachians from Alabama to New Jersey (and perhaps as far as Newfoundland). The tectonic history is complex, but most of the Piedmont near Baltimore probably formed during collisions about 470 to 440 million years ago as part of the Taconic Orogeny (named for rocks in the Taconic Mountains of eastern New York and western New England). The Taconic Orogeny (mountain-building episode) represents the collisions between ancestral eastern North America and large volcanic island arcs, together with other terranes including probable back-arc basin sediments and possible oceanic crust.
The Piedmont and its rolling hills are the roots of the high mountains that were here 440 million years ago.
“Uralite” is from occurrences in the Ural Mountains, Russia. “Hypersthene” is from words meaning “exceeding” and “strength,” because it is stronger than hornblende; “hornblende” comes from German words meaning “horn,” referring to a crystal projection, and “blende,” to blind, in the sense of “dazzle,” implying it deceives, since it was a common dark mineral that contained no metallic ores.