silicate mineral
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
that has the general formula of . Nickel, E., and Nichols, M. (1980) IMA/CNMNC List of Mineral Names. Official IMA-CNMNC List of Mineral Names, 247. It was approved as a mineral by the International Mineralogical Association in 1980. Its name comes from the Sacrofano Caldera in the
Monti Sabatini
The Monti Sabatini is a geologic region in Lazio, central Italy, a remnant of intense volcanism which started ca. 600,000 years ago, mainly with pyroclastic and maar forming eruptions which continued until 100,000 years ago. The mou ...
from which it was discovered in
Latium
Latium ( , ; ) is the region of central western Italy in which the city of Rome was founded and grew to be the capital city of the Roman Empire.
Definition
Latium was originally a small triangle of fertile, volcanic soil ( Old Latium) on w ...
,
Italy
Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...
.
Structure and optical properties
It has a
hexagonal crystal system
In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crysta ...
, meaning crystallographically it has three horizontal axes of equal length that make an angle of 120° with their positive ends, with a long vertical axis running perpendicular to them. The mineral is anisotropic, meaning the velocity of light through a mineral will vary depending on the direction that the light is going. It has a moderate relief ranging from 1.486 to 1.505. It has a birefringence of 0.019, with birefringence being the decomposition of a light ray into two different rays when light hits a mineral. Sacrofanite is uniaxial negative, meaning it has one optic axis and that the index of refraction for the ordinary ray is greater than the index of refraction for the extraordinary ray.Nelson, Stephen, 25 October 2010, "Uniaxial Minerals, Uniaxial Indicatrix, Optic Sign, and Ray Path." http://www.tulane.edu/~sanelson/eens211/uniaxial_minerals.htm. Accessed 1 December 2010.
Occurrence
Sacrofanite is only found in the Sacrofano Caldera in Italy, yet it represents one of many new minerals found in Italian volcanoes. It can be found on display in The Mineralogical Museum at the University of Rome.
It occurs as a crystal coating within a
vug
A vug, vugh, or vugg (
) is a small- to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, ...
in a rock ejected from the volcano. Other minerals occurring in the rock include
sanidine
Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal sys ...
,
andradite
Andradite is a mineral species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.
Andradite includes three varieties:
* ''Melanite'': Black in color, referred to as "titanian andradite".fassaite,
leucite
Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in ...
and
hauyne
Hauyne or haüyne, also called hauynite or haüynite ( ), is a tectosilicate sulfate mineral with endmember formula . As much as 5 wt % may be present, and also and Cl. It is a feldspathoid and a member of the sodalite group. Hauyne was fir ...