Fluoborite

MINERALS INDEX

Fluoborite

6MgO.B₂O₃.3(F₂,H₂O)

Hexagonal

Character
Fluoborite is found as hairlike material in fluffy, loosely felted aggregates filling cavities in pyrochroite that contain crystals of mooreite, some of which enclose fibers of fluoborite. Associated and easily confused with it are fibrous white rosettes of willemite. It is found also in compact fibrous form intimately mixed with an ill-defined hydrous zinc carbonate, as veinlets in zincite. Some of the veinlets are fluorite.

Its crystallographic character cannot be determined directly, as the fibers are too small. The type material from Sweden is definitely hexagonal, however, and X-ray powder spectrographs of the material from both localities indicate that they have essentially the same crystal structure.

Fluoborite is optically uniaxial and negative. The fluffy material has a specific gravity of 2.88 and refractive indices w = 1.548 and e = 1.518; the material associated with zincite has a specific gravity of 2.92 ±0.01 and indices w = 1.547 and e = 1.522.

Composition
Fluoborite is a hydrous fluorine-bearing magnesium borate in which fluorine and water seem to be interchangeable. The fluffy material is so light that difficulty was experienced in obtaining enough for analysis, and both sorts of material are so mixed with other minerals that a pure sample could not be separated. The best analyses, however, after eliminating known impurities, give molecular ratios in close agreement with that of the type material and yielding the same formula, although the Sterling Hill material contains more fluorine than the type material from Sweden.

**Analyses of fluoborite**
	1	2	3	4	5	6	7
MgO	60.07	46.33	62.07	67.38	1.487*	0.007	1.538 = 6 x 0.256
ZnO	2.41	15.00	2.05		0.030*
MnO	1.93	4.13	2.09		0.028*
CaO	1.19				0.021	0.021
B₂O₃	17.25	10.87	18.68	17.40	0.246		0.246 = 1 x 0.246
F	17.60	9.83	17.14	17.00	0.463†		0.753 = 3 x 0.251
H₂O	5.22	10.62	1.29	5.37	0.290†
CO₂	1.23	5.22			0.028	0.028
SO₃		0.27
	106.90	102.27	103.32	107.15
O = F₂	7.41	4.14	7.22	7.15
	99.49	98.13	96.10	100.00

[* Figures reflected in the 0.007 value of column 6.]

[† Figures reflected in the 0.753 value of column 7.]

1. Material associated with zincite, about 97 percent pure, Sterling Hill. L. H. Bauer (267), analyst.

2. Material associated with mooreite, large carbonate impurity, Sterling Hill. L. H. Bauer (267), analyst.

3. Material of 2 after treatment with hydrochloric acid until effervescence ceased; deficiency in total presumably due to loss of water.

4. Theoretical composition computed from formula adopted.

5. Molecular equivalents of 1, based on F₂ for fluorine.

6. Molecular equivalents of carbonate impurity deducted.

7. Molecular ratio of remainder, leading to formula adopted.

In its relatively large content of fluorine, fluoborite seems to stand alone among the borates. In other respects it is rather similar in composition to sussexite and hambergite, neither of which, however, contains fluorine.

Occurrence
Fluoborite was discovered at Norberg, Sweden, by Geijer and described and named by him (245a) in 1926. It had not been found elsewhere until its discovery by Bauer and Berman (267) at Sterling Hill in 1929. It was first found there, associated with altered pyrochroite, rhodochrosite, and zincite, in a vein in normal calcite-franklinite-willemite ore. Later it was found also as veinlets in zincite, associated with carbonates, part of them being hydrous.


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