Datolite

MINERALS INDEX

Datolite

Ca(BOH)SiO₄

Monoclinic

Forms
c(001), b(010), a(100), M(110), o(120), t(013), g(012), m(011), x(101), H’(308), y(104), v(103), x(102), n(111), L(112), W(114), l(113), e(112), g(111), Q(122), i(212), b(121), U(123), D(133), C(125), T (702), and k(4.7.10) Position and letters of Dana⁷.
[Web Ed. note: the symbol H’ was presented as a Germanic uppercase ‘H’ in the original.]

**Combinations on crystals of datolite**
	Forms	Illustrations
1	c, a, M, o, t, g, m, T, x, H’, y, n, L, W, b, U, d	Figure 140
2	c, a, M, t, g, m, x, H’, y, n, L, Q, b, D	Figure 141
3	a, M, m, n, L, e, g, D	Figure 142
4	c, a, M, m, n, D	Figure 143
5	c, M, g, m, n, e, g, D, Q, k	Figure 144
6	c, a, M, m, n, e, g, D
7	c, a, M, t, g, m, x, n, L, e, Q, i, b, g, C
8	c, a, M, m, x, n, C
9	a, M, m, v, C	Gordon (227), figure 2
10	c, a, b, o, m	Figure 145

The crystals are of a common datolite habit, the unit pyramid being dominant. Etching is common and has affected the unit pyramid chiefly, removing the parts of its surface adjacent to the base. The form D(133) is characteristic for the locality and is always seen either as large faces or as narrow truncations.

Figure 140 Plan of a part of a crystal of datolite showing the forms c(001), a(100), M(110), o(120), t(013), g(012), m(011), T(702), x(101), H’(308), y(104), n(111), L(112), W(114), b(121), U(123), and D(133). Parker mine
	Figure 141 Plan of a crystal of datolite showing the c(001), a(100), M(110), t(013), g(012), m(011), x(101), H’(308), y(104), n(111), L(112), Q(122), b(121), and D(133). Parker mine.
Figure 142 Doubly terminated crystal of datolite showing the forms a(100), M(110), m(011), n(111), L(112), e(112), g(111), and D(133). Parker mine.
	Figure 143 Crystal of datolite showing the forms c(001), a(100), M(110), m(011), n(111), and D(133). Parker mine.
Figure 144 Plan of a crystal of datolite showing the c(001), M(110), g(012), m(011), n(111), e(112), g(111), Q(122), b(121), D(133), and k(4.7.10). Parker mine.

⁷ In 1910 Palache (195) published a list of forms found on Franklin datolite. Ungemach (198) showed that the crystals had been incorrectly oriented and that of the new forms announced only the last two here listed were not already known. The present description is revised in accordance with Ungemach’s correction.

Occurrence
Datolite is one of the pneumatolytic minerals found in the Parker shaft at Franklin. It was reported first by Penfield (173) in his description of roeblingite and later by others, but invariably in small amounts. It is generally massive and glassy-white, filling openings between the other minerals of the aggregate. Some of the openings are 1 or 2 inches across, and the datolite does not fill all of them but only coats the surface of some with crystals. Most such crystals are imperfect through etching, but in one specimen in the Harvard collection, obtained by Wolff at the locality, the crystals are brilliant and measurable. The mass of the specimen consists of characteristically massive hancockite. In the cavity datolite and hancockite have crystallized together, the included needles of the hancockite tinging the datolite red.

The datolite has not been analyzed. It was carefully tested qualitatively for manganese, and as that element proved to be absent no complete analysis seemed to be needed.

In 1913 datolite with a different habit and association was found and was described by Gordon (227).

The specimens consisted of axinite crusted with pale-pink datolite crystallized in rosettes. The simple crystals showed the forms of combination 8. Their faces were rough, and sonic were clouded as if by the beginning of alteration. Later than the datolite is a deposit of barite, and small cavities are partly filled with needles of clear white calciothomsonite, otherwise unknown at Franklin.

In 1926 the variety of datolite called botryolite was found at Franklin in the form of a fibrous crust with a botryoidal surface that coats axinite and rhodonite, and also cahnite in some of the veins in which that mineral was first found. The datolite is snow-white and appears to be contemporaneous with the cahnite. It is interesting to find thus in the same veins three different boron minerals—axinite, cahnite, and datolite.

Still more recently a new habit of datolite was found at Franklin and was described by Berman (273). The crystals are tabular parallel to the clinopinacoid, with nearly equal development of the forms o(120) and m(021), which have approximately equal inclinations to the clinopinacoid. Thus a pseudo tetragonal symmetry is developed, as shown in figure 145. The determination of the mineral was confirmed by optical means and by a partial analysis.

Figure 145
Projection on the clinopinacoid of a crystal of datolite, tabular parallel to that face, showing the forms c(001), b(010), a(100), o(120), and m(011). Franklin.


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