Selenite, satin spar, desert rose, and gypsum flower are four varieties of the
mineral gypsum; all four varieties show obvious crystalline structure. The four
"crystalline" varieties of gypsum are sometimes grouped together and called
selenite. All varieties of gypsum, including
selenite and alabaster, are composed of calcium sulfate dihydrate, with the
chemical formula CaSO4·2H2O. Selenite contains no significant selenium; the
similarity of names comes from both substances being named from the Ancient
Greek word for the Moon. History and etymology
The etymology of selenite is through Middle English selinete, from Latin
selenites, from Greek selēnitēs, literally, moonstone or stone of the
moon, from selēnē. The ancients had a belief that certain transparent crystals
waxed and waned with the moon. From the 15th century, "selenite" has referred
specifically to the variety of gypsum that occurs in transparent crystals or
crystalline masses. Identification of crystals as gypsum
All varieties of gypsum are very soft minerals. This is the most important
identifying characteristic of gypsum, as any variety of gypsum can be easily
scratched with a fingernail. Also, because gypsum has natural thermal
insulating properties, all varieties feel warm to the touch.
Varieties Though sometimes grouped together as
"selenite", the four crystalline varieties have differences. General
identifying descriptions of the related crystalline varieties are:
= Selenite= most often transparent and colorless: it
is named after Greek σεληνη= "the moon". if selenite crystals show translucency,
opacity, and/or color, it is caused by the presence of other minerals including
druse druse is the crust of tiny, minute, or
micro crystals that form or fuse either within or upon the surface of a rock
vug, geode, or another crystal = Satin spar=
most often silky, fibrous, and translucent; can exhibit some coloration
the satin spar name can also be applied to fibrous calcite – calcite is a harder
mineral – and feels greasier, waxier, or oilier to the touch.
= Desert rose= rosette shaped gypsum with outer druse
of sand or with sand throughout – most often sand colored
the desert rose name can also be applied to barite desert roses – barite is a
harder mineral with higher density = Gypsum flower=
rosette shaped gypsum with spreading fibers – can include outer druse
the difference between desert roses and gypsum flowers is that desert roses look
like roses, whereas gypsum flowers form a myriad of shapes
Use and history Because of the long history of the
commercial value and use of both gypsum and alabaster, the four crystalline
varieties have been somewhat ignored, except as a curiosity or as rock
collectibles. Crystal habit and properties
Crystal habit refers to the shapes that crystals exhibit.
Selenite crystals commonly occur as tabular, reticular, and columnar
crystals, often with no imperfections or inclusions, and thereby can appear water
or glass-like. Many collectible selenite crystals have interesting inclusions
such as, accompanying related minerals, interior druse, dendrites, and fossils.
In some rare instances, water was encased as a fluid inclusion when the
crystal formed. Selenite crystals sometimes form in thin
tabular or mica-like sheets and have been used as glass panes as at Santa
Sabina in Rome. Selenite crystals sometimes will also
exhibit bladed rosette habit often with accompanying transparent, columnar
crystals. Selenite crystals can be found both attached to a matrix or base rock,
but can commonly be found as entire free-floating crystals, often in clay
beds. Satin spar is almost always prismatic
and fibrous in a parallel crystal habit. Satin spar often occurs in seams, some
of them quite long, and is often attached to a matrix or base rock.
Desert roses are most often bladed, exhibiting the familiar shape of a rose,
and almost always have an exterior druse. Desert roses are almost always
unattached to a matrix or base rock. Gypsum flowers are most often acicular,
scaly, stellate, and lenticular. Gypsum flowers most often exhibit simple
twinning; where parallel, long, needle-like crystals, sometimes having
severe curves and bends, will frequently form “ram’s horns”, "fishtail",
"arrow/spear-head", and "swallowtail" twins. Selenite crystals can also
exhibit “arrow/spear-head” as well as “duck-bill” twins. Both selenite
crystals and gypsum flowers sometimes form quite densely in acicular mats or
nets; and can be quite brittle and fragile. Gypsum flowers are usually
attached to a matrix or base rock. = Color=
Gypsum crystals are colorless, white, gray, brown, beige, orange, pink,
yellow, light red, and green. Colors are caused by the presence of other mineral
inclusions such as, copper ores, sulfur and sulfides, silver, iron ores, coal,
calcite, dolomite, and opal. = Transparency=
Gypsum crystals can be transparent, translucent, and opaque. Opacity can be
caused by impurities, inclusions, druse, and crust, and can occur in all four
crystalline varieties. = Luster=
Both selenite and satin spar are often glassy or vitreous, pearly, and silky –
especially on cleavage surfaces. Luster is not often exhibited in the rosettes,
due to their exterior druse; nevertheless, the rosettes often show
glassy to pearly luster on edges. Gypsum flowers usually exhibit more luster than
desert roses. = Play of color=
Fibrous satin spar exhibits chatoyancy. When cut across the fibers and polished
on the ends, satin spar exhibits an optical illusion when placed on a
printed or pictured surface; and is often called and sold as the “television
stone”. Print and pictures appear to be on the surface of the sample.
Some selenite and satin spar specimens exhibit fluorescence or phosphorescence.
= Tenacity= All four crystalline varieties are
slightly flexible, though will break if bent significantly. They are not
elastic, meaning they can be bent, but will not bend back on their own.
All four crystalline varieties are sectile in that they can be easily cut,
will peel, and like all gypsum varieties, can be scratched by a
fingernail. The rosettes are not quite as soft due to their exterior druse;
nevertheless, they too can be scratched. Selenite crystals that exhibit in either
reticular or acicular habits, satin spar, in general, desert roses that are
thinly bladed, and gypsum flowers, particularly acicular gypsum flowers,
can be quite brittle and easily broken. = Size=
See also: Cave of the Crystals All four crystalline varieties can range
in size from minute to giant selenite crystals measuring 11 meters long such
as those found in the caves of the Naica Mine of Chihuahua, Mexico. The crystals
thrived in the cave's extremely rare and stable natural environment. Temperatures
stayed at 58 °C, and the cave was filled with mineral-rich water that drove the
crystals' growth. The largest of those crystals weighs 55 tons, is 33 feet
long, and is over 500,000 years old. Occurrence
Gypsum occurs on every continent and is the most common of all the sulfate
minerals. Gypsum is formed as an evaporative
mineral, frequently found in alkaline lake muds, clay beds, evaporated seas,
salt flats, salt springs, and caves. Gypsum, also, is frequently found in
conjunction with other minerals such as, copper ores, sulfur and sulfides,
silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum
has been dated to almost every geologic age since the Silurian Period 443.7 ±
1.5 Ma. In dry, desert conditions and arid
areas, sand may become trapped both on the inside and the outside of gypsum
crystals as they form. Interior inclusion of sand can take on shapes
such as, an interior hourglass shape common to selenite crystals of the
ancient Great Salt Plains Lake bed, Oklahoma, USA. Exterior inclusion occurs
as embedded sand grains on the surface such as, commonly seen in the familiar
desert rose. When gypsum dehydrates severely,
anhydrite is formed. If water is reintroduced, gypsum can and will reform
– including as the four crystalline varieties. An example of gypsum crystals
reforming in modern times is found at Philips Copper Mine, Putnam County, New
York, USA where selenite micro crystal coatings are commonly found on numerous
surfaces in the cave and in the dump. Whereas geology, mineralogy, and
rockhounding groups, clubs, and societies as well as museums usually
date, photograph, and note location of minerals, much of the retail mineral and
jewellery trade can be somewhat casual about dates, locations, and descriptive
claims. Images
See also Great Salt Plains Lake and Great Salt
Plains National Wildlife Refuge, Oklahoma, USA
Lake Lucero, White Sands National Monument, New Mexico, USA
Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA
Peñoles Mine, Naica, Chihuahua, Mexico References
External links Cavern of Crystal Giants Cueva de los
Cristales, Peñoles Mine, Naica, Chihuahua, Mexico
Mindat.org - scientific description of gypsum plus list of localities
Mindat.org - satin spar localities Pilar de Jaravia, Spain - large columnar
selenite crystals in massive geode at Quien Tal Pensara Mine
Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico, USA -
speleothems and gypsum flowers 1927 article on the giant crystals of
Naica Image of huge crystals of selenite