THE MINERAL STURMANITE
- Chemistry: Ca6(Fe, Al, Mn)2(SO4)2(B(OH)4)(OH)12 - 26H2O, Hydrated Calcium Iron Aluminum Manganese Sulfate Tetrahydroxoborate Hydroxide.
- Class: Sulfates
- Group: Ettringite
- Uses: Only as mineral specimens.
Specimens
Its chemistry is really interesting. Compared with its close cousin, ettringite, Ca6Al2(SO4)3 (OH)12 - 26H2O, one of the sulfate ion groups has been replaced by the rare ion group, (for lack of a shorter term) tetrahydroxoborate, B(OH)4 with a negative one charge. In addition, four out of every five atoms in this mineral is either a part of a water molecule or an hydroxide and that does not count the hydroxides in the B(OH)4 ion group. It's almost all water! This fact is reflected in its very low specific gravity of only 1.8+, that's less than twice the specific gravity of water.
Some mineral references will list sturmanite's chemistry as Ca6Fe2(SO4)2(B(OH)4)(OH)12 - 26H2O. This formula is written without the manganese and aluminum which do substitute for the iron to an appreciable extent.
It is difficult under ordinary means to distinguish ettringite from sturmanite. Both are members of the Ettringite Group and have similar crystal habit, density, luster and will often share the same bright yellow color.
PHYSICAL CHARACTERISTICS:
- Color is a bright yellow.
- Luster is vitreous.
- Transparency: Crystals are transparent to translucent.
- Crystal System is trigonal; bar 3 2/m.
- Crystal Habits include hexagonal prisms terminated by an hexagonal pyramid or a pinacoid, more commonly by both. Terminations can be rounded or dome-like but many have nice flat faces. Acicular, platy and fibrous forms are also seen.
- Cleavage is poor and rarely seen.
- Fracture is uneven.
- Hardness is 2.5
- Specific Gravity is approximately 1.8+ (well below average)
- Streak is pale yellow.
- Other Characteristics: Crystals are usually rather small, typically less than 1/4 inch long.
- Notable Occurrences include the Kuruman District, South Africa.
- Best Field Indicators are crystal habit, density, hardness, lack of cleavage, streak and color.
 Amethyst Galleries' Mineral Gallery MINERALS |
STURMANITE specimen stu-1
$ 125.00
$ 125.00
Dims: 2.4" x 1.6" x 0.8" (6.1 x 4.1 x 2.0 cm)
Wt: 1.47 oz. (41.6 g)
N'Chwaning Mine, Hotazel, Kuruman, South Africa
Several bright yellow Sturmanite crystals rest on the host rock of this specimen. They have the standard hexagonal prismatic form of their kind, and though their form is not perfect, it is still definite, and only one of the crystals shows any serious damage. The largest of these crystals measures 0.4 x 0.6 x 0.5" and has probably the best form of the cluster. All have a vitreous luster on their prism faces and a pearly luster on their termination faces, and have an internally fractured but transparent coating around a translucent core. The host rock on which the Sturmanites rest is difficult to identify, but contains many tiny, colorless, transparent hexagonal crystals. They seem to be quartz crystals, but they have a few oddities in their structure that make me doubt just a little bit.
stu-1 ($125.00)
N'Chwaning Mine, Hotazel, Kuruman, South Africa
STURMANITE specimen stu-2
$ 60.00
$ 60.00
Dims: 0.6" x 0.4" x 0.3" (1.5 x 1.0 x 0.8 cm)
Wt: 9.7 g w/ specimen box
N'Chwaning Mine, Hotazel, Kuruman, South Africa
This specimen had me fooled at first; I thought it was a splendid hexagonal prismatic sulfur crystal. However, the documentation defined it as a Sturmanite crystal, finer than most that I have seen. Its color is bright yellow, of course, and it has exceptional form and no damage that I can see. Its prismatic form is topped by a shallow pyramidal termination that is truncated by a basal pinacoid. It has a vitreous luster and is transparent to a degree, but is very heavily internally fractured, so that clarity is almost nonexistent. As the crystal was broken off at its base, there is no host rock. It is affixed to the inside of a specimen box for display and protection.
stu-2 ($ 60.00)
N'Chwaning Mine, Hotazel, Kuruman, South Africa
STURMANITE specimen stu-3
$ 24.00
$ 24.00
Dims: 0.7" x 0.5" x 0.3" (1.8 x 1.5 x 0.8 cm)
Wt: 1.4 g w/ foam base
New N'Chwaning Mine, Black Rock Cape, South Africa
This little thumbnail specimen is rather odd as far as Sturmanite goes. It occurs as a cluster of small hexagonal dipyramidal crystals that are intergrown, and thus slightly warped. They have a beige color and dull, waxy luster, unlike the bright yellow color and vitreous-to-pearly luster of most specimens I have seen. All are opaque, but one can see glimpses of yellow coloration and translucence in one or two spots when the specimen is rotated in a bright light. There is no host rock, and the specimen is affixed to a foam pad that fits inside a plastic specimen box. It is certainly an interesting little specimen- at first I thought that the crystals were a variety of beta-quartz that was heavily included with impurities.
stu-3 ($ 24.00)
New N'Chwaning Mine, Black Rock Cape, South Africa
STURMANITE specimen stu-4
$ 60.00
$ 60.00
Dims: 1.3" x 0.7" x 0.7" (3.3 x 1.8 x 1.8 cm)
Wt: 11.9 g w/ base
N'Chwaning Mine #2, Kuruman, South Africa
This small thumbnail specimen consists of at least 20 hexagonal prismatic Sturmanite crystals growing off of a rather rusty-looking host rock. They are generally in excellent condition, showing almost no damage, and measure up to 0.4" (1.0 cm) in length. All are long and thin, and show well-defined edges and smooth faces whose luster range from pearly to vitreous. Their color is a deep, somewhat dull yellow, and all are translucent. Upon close examination, though, one can see that each crystal has a very cloudy "core" that is surrounded by a layer of dimly transparent material. The specimen is hot-glued onto an acrylic base.
stu-4 ($ 60.00)
N'Chwaning Mine #2, Kuruman, South Africa
STURMANITE specimen stu-5
$ 85.00
$ 85.00
Dims: 2.1" x 0.3" x 0.3" (5.3 x 0.8 x 0.8 cm)
Wt: 2.9 g
N'Chwaning Mine #2, Kuruman, Black Rock Cape Province, South Africa
This specimen represents one of the largest Sturmanite crystals that I have yet seen. It shows excellent hexagonal prismatic form, and the only visible damage on it is restricted to a few tiny, partially intergrown crystals. It is double-terminated, with the less-formed termination shown in our close-up image. Its edges are straight and very well-defined and all but two of its prism faces are smooth and clean, showing a waxy-to-pearly luster. It has a moderate orange-brown coloration and is translucent, though patches of dim transparence are visible. It has no attached host rock or any other material.
stu-5 ($ 85.00)
N'Chwaning Mine #2, Kuruman, Black Rock Cape Province, South Africa
STURMANITE specimen stu-6
$ 120.00
$ 120.00
Dims: 2.1 x 1.0 x 0.4" (5.4 x 2.5 x 1.0 cm)
Wt: 0.5 oz. (15 g)
Kuruman, South Africa
This hand specimen consists of a partial crust of compact and fibrous (nearly massive) Sturmanite. It shows no crystal form and has the standard bright yellow coloration and dull waxy luster of the specie. It is essentially translucent, though it does show patches of dim transparence. There is no host rock present.
stu-6 ($120.00)
Kuruman, South Africa
STURMANITE specimen stu-7
$ 90.00
$ 90.00
Dims: 2.4 x 1.7 x 0.4" (6.1 x 4.2 x 1.1 cm)
Wt: 0.8 oz. (23 g)
Kuruman, South Africa
This interesting Sturmanite hand specimen is made up of a piece of crust that shows a compact, fibrous that appears nearly massive. Its bright yellow color and dull, waxy luster are standard for the specie, and the crust is dimly transparent within, though its surfaces are generally frosted. Only tiny bits of a black host rock are present.
stu-7 ($ 90.00)
Kuruman, South Africa