Tephroite |
| (Mn,Mg,Zn)2SiO4 |
| Orthorhombic |
Forms
a(100), b(010), c(001), m(110), n(210),
s(120), r(130), j(270), d(101), w(103),
h(011), k(021), e(111), f(121), l(131),
and q(122)
[Combinations on crystals of tephroite]
Crystallographic measurements
Accurate crystallographic data on tephroite from Franklin were first published
by Gordon (222). The following table contains the results of measurements
of 8 excellent crystals obtained by the author in 1915 but not yet published:
The axial ratio was calculated from 40 readings on terminal faces of 8 crystals, which yielded the values:
| po = 1.278, and qo = 0.5899 ; or a : b : c = 0.461 : 1 : 0.589. |
Gordon's axial ratio, derived from the measurement of four crystals, is practically identical and is a : b : c = 0.4606 : 1 : 0.5899.
The form w(103) is new for tephroite.
Habit
Crystals of tephroite are rare, and the mineral is commonly found in coarse
granular form, breaking into rectangular blocks with rudely striated faces.
The cleavage is distinct parallel to the base and to the brachypinacoid and
is therefore rectangular. The hardness is 5.5 to 6, and the specific gravity
is 3.87 to 4.1. The crystals are bluish-green by daylight and pink by artificial
light, resembling in this respect the closely related mineral glaucochroite.
The color of the granular material is ash-gray, brown, red-brown, and flesh-red
and is darker or black oil weathered surfaces. Some of the color is due to
included zincite. The luster is
vitreous to greasy. Tephroite is easily mistaken
in some of its forms for willemite, as it has the same luster and color range
as the variety troostite. Its better and distinctly rectangular cleavages
are a definite means for its identification.
Contact twins on h(011), the common twinning plane for this group of minerals, were described by the author (257) and are shown in figure 103.
| Figure
103 Projection on the macropinacoid of a crystal of tephroite, twinned on the brachydome h(011), showing the forms a(100), m(110), s(120), and f(121). Franklin. |
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Optical properties
Tephroite is biaxial and negative; X = b and Z = c,
2V = 60° ; r > v (perceptible, nearly colorless in section); a
= 1.77, b = 1.807,
g = 1.825, all
±0.001 (Berman).
Composition
Tephroite is essentially an orthosilicate of manganese, commonly containing
more or less magnesium, zinc, iron, or calcium, or several of those metals.
The following analyses of Franklin tephroite show its range in composition
in that respect. It gelatinizes easily with acids and fuses at 3.5 to a black
mass; magnesian varieties are less easily fused.
In all the analyses there is sensible conformity to the accepted ratios for the orthosilicate formula—different amounts of magnesium and zinc and, to a less extent, iron and calcium taking the place of part of the manganese. Brush was of the opinion that the zinc found in all analyses is present in included zincite, but that this is not invariably true is shown by the fact that the molecular ratios in some analyses more nearly satisfy the orthosilicate formula when zinc is regarded as essentially a part of the mineral rather than as a constituent of mechanical inclusions.
Occurrence
At Franklin tephroite is in places an important constituent of the ores, being
found in considerable abundance mixed with zincite willemite and franklinite
in a calcite gangue. In 1905 and again in 1911, it was thus seen by the author
in large masses in ore being taken from the west leg of the ore body in the
Buckwheat open cut. Such material is illustrated in plate
12, B.
The crystals described above form part of the filling of a thin vein. They were obtained by Mr. Cahn, but the locality where they were found is unknown. The largest was nearly an inch long; the others were much smaller. Their bluish color caused them to be mistaken for glaucochroite, which they were supposed to be until Gordon's similar error was discovered. (See page 80.) The crystals are singly terminated and either stand free in the vein or are embedded in granular calcite. The main filling of the vein is reddish manganesian garnet partly coated with dark-red friedelite in stalactitic form. Crystals of tephroite and clear green willemite are implanted on the friedelite. Later there was deposited a fibrous brown mineral somewhat doubtfully determined as friedelite.
| Figure
104 Crystal of tephroite showing the forms c(001), a(100), b(010), m(110), s(120), n(210), w(103), d(101), h(011), k(021), e(111) f(121), and l(131). Franklin. |
 |
Almost all the free crystals of tephroite are marked by a later growth in parallel position of pale-brown pyramidal crystals, forming a serrate edge along the outer margin of their terminal faces. These pyramids are evidently orthorhombic and are strictly parallel to the tephroite beneath and to one another. Their faces are dull but give poor measurements, with an average value for six faces of f = 66° 7' and r = 73° 2'.
There was too little of this material to yield more than a tiny fragment from which the presence of manganese could be proved.
If it is assumed that these crystals are also tephroite, in parallel position to the older tephroite, computation shows that the pyramid (221) of tephroite (a form not known) would have the position f = 65° 18', r = 70° 37'. The agreement is poor, but as the measurements were very poor it is possible that the discrepancy is not too great to be allowed.
Gordon’s crystals of tephroite likewise lined veins in massive ore together with greenish crystals of willemite. From specimens of this sort in the Harvard collection it is clear that the locality is different from that of the material just described. The veins, some of them 2 inches thick, consist chiefly of massive brown garnet, which is crystallized on the walls of what appear to be solution cavities. On the garnet are implanted numerous crystals of bluish tephroite, many of their faces, especially the terminal ones, being dull through etching. The largest crystal seen was scarcely one-third of an inch long. Later than the tephroite are scalenohedral crystals of calcite and slender crystals of willemite, white or pale green, with darker terminations, and brilliantly crystallized with third-order rhombohedrons. In a few cavities is a still later deposit of very dark brown friedelite in wormlike stalactitic aggregates.
This tephroite was analyzed, its shown in analysis 13. Except for the earliest (and hence unreliable) analyses of tephroite, this is the nearest to pure manganese orthosilicate yet found. The alumina is probably to be assigned to intermixed garnet, though its amount is puzzling. The optical properties given on page 76 are for this pure mineral.
Individual crystals of tephroite of unusual size have been found at Franklin on the picking table. Figure 105 was drawn from a fragment, measuring 1.2 by 3.5 by 3.5 inches, which is broken on three sides and obviously was once much larger.
| Figure
105 Large crystal of tephroite showing the forms c(001), b(010), s(120), k(021), and d(101). Franklin. |
 |
It is embedded in coarse limestone and is quite isolated. Others of similar dimensions have been found.
At Sterling Hill, besides being in the granular ore, tephroite has been found in rough isolated crystals embedded in limestone. These crystals have commonly been regarded as chondrodite by the local collectors, and certainly they resemble that mineral superficially. Their form, as shown in figures 107 and 108, is like that of olivine.
| Figure
107 Simple crystal of tephroite showing the forms b(010), m(110), and d(101). Sterling Hill. |
 |
 |
Figure
108 Prismatic crystal of tephroite showing the forms b(010), a(100), m(110), d(101), and e(111). Sterling Hill. |
Analysis 10, made by Jenkins and Bauer on a crystal of this type from the collection of T. Lang, is the highest in magnesium of all the analyses of tephroite except no. 4, the source of which was doubtful. Thus it is highly probable that the figure of tephroite given by Des Cloizeaux (85), reproduced in figure 106, with which is given the analysis by Damour of a magnesian tephroite, represents one of these crystals from Sterling Hill.
| Figure
106 Pseudocubic crystal of tephroite showing the forms b(010), d(101), and e(111). Sterling Hill. This figure follows the description of Des Cloizeaux (87). |
 |
They are also illustrated in plate 12, A.
Historical notes
Tephroite was first analyzed by Thomson (23)
under the name "silicate of manganese",
the material having been supplied by Torrey from Franklin and reported "not
scarce." Breithaupt (16) had given the name "tephroite" to
material from Sterling Hill obtained from a private collector in Dresden,
and this material was later analyzed by Rammelsberg (35),
who recognized its identity with Thomson's
mineral. For many years thereafter the mineral was quite overlooked by Franklin
collectors, until Brush (87) again called attention to its abundance at Sterling
Hill.
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